Growth, Structure, Thermal Properties and Spectroscopic Characteristics of Nd3+-Doped KGdP4O12 Crystal

Tongqing Sun; Yu Zhang; Pai Shan; Zichang Zhang; Shaolin Chen; Yongfa Kong; Jingjun Xu

doi:10.1371/journal.pone.0100922

Abstract

A single crystal of Nd³⁺-doped KGdP₄O₁₂ was successfully grown with the top-seeded solution growth and slow cooling (TSSG−SC) technique. It crystallizes in space group C2/c with cell parameters a = 7.812(2) Å, b = 12.307(3) Å, c = 10.474(2) Å, β = 110.84(3)° and Z = 4. The IR and Raman spectra also indicated that the phosphoric polyhedra of Nd:KGdP₄O₁₂ has a cyclic symmetry. The chemical composition of the crystal was analyzed and the distribution coefficient of Nd³⁺ was calculated. The crystal morphology of KGdP₄O₁₂ was identified using X-ray diffraction. The compound has good thermal stability to 920°C. Its specific heat and thermal conductivity were determined for potential applications. The spectral properties of Nd:KGdP₄O₁₂ indicates that it exhibits broad absorption and emission bands, which are attributed to low symmetry of the crystal. The broad absorption band around 798 nm has a full-width at half-maximum (FWHM) of 14.8 nm and is suitable for AlGaAs laser diode pumping. Moreover, 5 at% Nd³⁺-doped KGdP₄O₁₂ crystal has a long luminescence lifetime of 300 μs and a high quantum efficiency of 96%.

Citation: Sun T, Zhang Y, Shan P, Zhang Z, Chen S, Kong Y, et al. (2014) Growth, Structure, Thermal Properties and Spectroscopic Characteristics of Nd³⁺-Doped KGdP₄O₁₂ Crystal. PLoS ONE 9(6): e100922. https://doi.org/10.1371/journal.pone.0100922

Editor: Andreas Hofmann, Griffith University, Australia

Received: January 21, 2014; Accepted: June 2, 2014; Published: June 26, 2014

Copyright: © 2014 Sun et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Funding: This work is supported by the National Natural Science Foundation of China (Grant No. 21271109) and the Fundamental Research Funds for the Central Universities of China (65122011). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: The authors have declared that no competing interests exist.

Introduction

With the development of the diode-pumped solid-state (DPSS) lasers based on neodymium-doped crystals, research on new laser host materials has gained much interest [1]. Nd:YAG crystal is still the most common Nd³⁺ laser material because of its good physical and laser properties, but it is limited to low Nd³⁺-doped concentration and narrow absorption bandwidth near the AlGaAs diode emission wavelength of 808 nm. Because the emission wavelength of the laser diode increases at 0.2−0.3 nm·K⁻¹ of the laser device, the temperature stability of the laser diode needs to be tightly controlled. Therefore, it is necessary to explore new and more efficient crystals for DPSS lasers. Furthermore, the absorption band of laser crystals close to the laser output of AlGaAs (λ = 808 nm) needs to have a large full-width at half-maximum (FWHM).

Recently, some Nd³⁺-doped laser crystals with broad absorption bands have been reported. They include host crystals with disordered structures such as NaLn(WO₄)₂ (Ln = Y or Gd, I4₁/a) [2], [3], SrLaGa₃O₇ () [4], Ca₃La₂(BO₃)₄ (Pnma) [5], Li₃Ba₂Ln₃(MO₄)₈ (Ln = Y, La or Gd, M = W or Mo, C2/c) [6−11], KBaGd(MO₄)₃ (M = W or Mo, C2/c) [12], [13] and CaNb₂O₆ (Pbcn) [14]. These species can induce inhomogeneous broadening of optical spectra from a disordered crystal lattice. Mixed crystals such as garnet (Lu_xY_1−x)₃Al₅O₁₂ [15] and rare earth oxyorthosilicate Ln₂SiO₅ (C2/c) (LuYSiO₅, GdYSiO₅ and LuGdSiO₅) [16−18] can enhance the structure disorder. Many of the host crystals above have low symmetry, especially the monoclinic system C2/c space group. Due to the strong anisotropy of their physical properties and biaxiality, host crystals with low symmetry may enrich spectroscopic properties of doped active ions [19].

The title compound, KGdP₄O₁₂, belongs to the broader family of condensed phosphates including those double phosphates of alkali and lanthanide ions with the general formula M^ILn^III(PO₃)₄. The structure of M^ILn^III(PO₃)₄ is highly dependent not only on the sizes of the alkali and the lanthanide ions but also on the crystallization environment. Double tetra-metaphosphates of potassium and gadolinium have low symmetry (monoclinic system) including three structural types with different space groups: P2₁, P2₁/n and C2/c. The first two types are written as KGd(PO₃)₄ in general and are double polyphosphates. Here, the phosphoric anions have a long-chain geometry of . The C2/c is a cyclotetraphosphate, and these phosphoric anions have a cyclic geometry of [P₄O₁₂]⁴⁻; hence its formula is usually written as KGdP₄O₁₂. The growth and structure of KGd(PO₃)₄ (P2₁) crystal was reported by Parreu et al [20], [21] and it is isostructural to KNd(PO₃)₄ [22]–a famous Nd stoichiometric laser material. The KGd(PO₃)₄ (P2₁) can be used as a self-frequency doubling (SFD) laser host material because of its noncentrosymmetrical structure and has attracted more attention [23−26]. KGdP₄O₁₂ crystallized in space group C2/c and KGd(PO₃)₄ in P2₁/n were successively synthesized in phosphoric acid by Naïli et al [27], [28].

Compared to other double phosphates of alkali and lanthanide ions with the general formula M^ILn^III(PO₃)₄, KGdP₄O₁₂ has the following characteristics as a new laser host material. First, Gd³⁺ is much closer to Nd³⁺ in size; hence KGdP₄O₁₂ is easily substituted with Nd³⁺. Second, raw materials for preparing KGdP₄O₁₂ are easily available and very cheap in comparison with double metaphosphates of cesium or rubidium. Third, the spectroscopic spectrum of doped active ions may be broadened because of its low symmetry in the space group C2/c. Finally, the KGdP₄O₁₂ crystal has a cyclic geometry that is difficult to cleave in contrast with compounds having long chain geometry.

To the best of our knowledge, the crystal growth of KGdP₄O₁₂ has not yet been reported. Here, we have grown a single crystal of Nd³⁺-doped KGdP₄O₁₂ with high quality and determined its structural character to be a cyclic geometry via single crystal structural analysis and IR and Raman spectroscopy. The spectral characteristics of the Nd:KGdP₄O₁₂ crystal were studied through its absorption spectrum, emission and excitation fluorescent spectra, and fluorescence decay curve. Finally, we measured its thermal stability, specific heat and thermal conductivity.

Materials and Methods

1. Crystal growth

The crystal growth used a single-zone vertical tubular electric furnace with a Kantal AF heater. The temperature of the furnace was measured near the heater with a B-type thermocouple and was regulated with a SHIMADEN FP21 type temperature controller.

We grew Nd:KGdP₄O₁₂ crystals with the top-seeded solution growth and slow cooling (TSSG−SC) technique from a self-flux. The starting material was prepared by mixing Nd₂O₃ (4N), Gd₂O₃ (4N), K₂CO₃ (99.0%) and NH₄H₂PO₄ (99.0%) at a molar ratio of 0.05∶0.95∶3∶12 in an agate mortar. This was then transferred into a Φ60×60 mm² Pt crucible to melt in batches. During the melting, a stoichiometric Nd₂O₃, Gd₂O₃, K₂CO₃ and NH₄H₂PO₄ generated the title compound KGd_0.95Nd_0.05P₄O₁₂. An excess of K₂CO₃ and NH₄H₂PO₄ reacted to produce potassium metaphosphate compound, which served as a self-flux to avoid foreign ions during the crystal growth. The typical chemical reactions are shown below.

There was marked bubbling of NH₃, CO₂ and H₂O, and the mixture was heated slowly to a melt. The melt was homogenized at 900°C with a stirrer until the melt was clear and there were no bubbles. Meanwhile, the crucible was rotated alternately clockwise and counterclockwise at 15 revolutions per minute for complete homogenization. The oscillating crucible rotation was also necessary during the crystal growth for effective mass transport. The coldest position in the solution was 0.5 cm below the melt surface and the temperature gradient in the axial direction was 1.3°C·cm⁻¹. After several attempts, the melt temperature decreased to 812°C. Meanwhile, a thick wire of platinum attached to a corundum rod was introduced into the furnace and was placed above the melt. About 10 min later, the platinum wire was immersed below the melt surface as a seed. While maintaining the oscillating crucible rotation, the melt temperature was reduced at 0.4°C·d⁻¹ until it reached 794°C. During this process, the solute first crystallized on the platinum wire, which served as the seed for larger crystals.

After the growth was completed, the crystals were removed from the solution. The furnace was cooled to room temperature slowly and then some single crystals clustering around the platinum wire were taken out of the furnace. Figure 1(a) shows that the biggest one of these had good transparency with no cracks or inclusions. The colour of the crystal shows that the neodymium ion had already entered the crystal lattice. The stoichiometry of the crystal was analyzed by inductively coupled plasma−atomic emission spectrometer (ICP−AES) (Thermo IRIS Advantage). The density of the Nd:KGdP₄O₁₂ crystal at room temperature (23°C) was measured using a density unit of Sartorius balance (YDK01−C) by the buoyancy method.

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Figure 1. Photograph (a) of the as-grown single crystal with a size of 20 mm×10 mm×5 mm and the theoretical morphology (b) of the Nd:KGdP₄O₁₂ crystal.

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2. X-ray diffraction analysis

The X-ray diffraction (XRD) technique was used to analyze several aspects of the as-grown Nd:KGdP₄O₁₂ single crystal such as determining its structure, identifying the crystalline phase, studying the crystal morphology as well as crystallinity.

A single crystal of Nd:KGdP₄O₁₂ cut from the as-grown crystal was used for indexing and intensity data collections on a Rigaku Saturn CCD area detector diffractometer using Mo Kα graphite monochromated radiation (λ = 0.71073 Å). The structure was resolved with the SHELXS-97 computer utility, and refined by a full-matrix least-squares method with SHELXL-97 software.

Polycrystalline powder of Nd:KGdP₄O₁₂ was prepared from the as-grown single crystal. The powder X-ray diffraction (PXRD) pattern of Nd:KGdP₄O₁₂ was recorded at room temperature with a Fangyuan DX2700 (Dandong, China) powder diffractometer using graphite monochromatized Cu Kα radiation in the 2θ range 10°−70° with a step of 0.02°.

An XRD goniometer was used to identify and orientate the crystalline forms that comprised the morphological habit for the as-grown single crystal had well-developed facets. XRD was also used to check the quality of the as-grown Nd:KGdP₄O₁₂ crystal. The crystal in Figure 1(a) was polished and used as a sample. The rocking curve of the (004) diffraction crystal face was obtained on an XRD diffractometer (PANalytical X’Pert PRO) equipped with a four-crystal Ge (022) monochromator. The setting of the X-ray tube (PW3373/10) was 40 kV and 10 mA. The step time and step size were 0.1 s and 0.001°, respectively.

3. IR and Raman spectroscopy

The molecular spectroscopy and lattice vibrations of Nd:KGdP₄O₁₂ were studied with Fourier transform infrared (FTIR) and Raman spectroscopy. The IR spectrum of a KBr pressed pellet of the powered sample was recorded from 4000 to 400 cm⁻¹ on a Nicolet Magna-IR 560 ESP FTIR spectrometer. The as-grown crystal shown in Figure 1(a) was also studied on a Renishaw inVia Raman microscope. The output wavelength of the excited CW argon laser was 514 nm.

4. Thermal analysis

The thermal stability of Nd:KGdP₄O₁₂ was measured with both thermogravimetry and differential scanning calorimetry (TG−DSC) using a Netzsch STA 449C from 40 to 1000°C at 10°C·min⁻¹. The specific heat of the Nd:KGdP₄O₁₂ crystal was also measured by DSC using a simultaneous thermal analyzer (Netzsch DSC 200 F3) in an atmosphere of N₂.

The thermal diffusivity was measured by the laser flash method using a laser flash apparatus (Netzsch LFA 457 Nanoflash) in the 25–300°C range at intervals of 50°C. A 0.95 mm×4 mm×4 mm sample was used to carry out the measurements whose {001} faces were polished and coated with graphite on both sides.

5. Spectroscopic characterization

A 4.30-mm thick sample was polished and used to measure the spectroscopic properties. The polished facet was {001}. The unpolarized absorption spectrum was measured on a Hitachi UV−Vis−NIR spectrophotometer (U4100) from 175 to 2000 nm. The fluorescence spectra were recorded using an Edinburgh Instruments FLS920 spectrophotometer with a xenon lamp as the excitation source. The incident light and fluorescence were dispersed with two M300 monochromators using ruling gratings from Bentham Instruments. A cooled Hamamatsu R5509-72 photomultiplier was used for detection. The decay curve was measured using a pulsed xenon lamp as the pump source. The pulse duration was 10 ns, and the excitation wavelength was 808 nm. All the measurements were performed at room temperature (300 K). The splitting of the Nd³⁺ emission band was also studied at 10 K using the same FLS920 spectrophotometer equipped with a close cycle helium cryostat (Advanced Research Systems DE202).

Results and Discussion

1. Crystal growth and chemical composition

As mentioned above, KGdP₄O₁₂ is a polymorphous compound. In addition to the C2/c phase, it has two isomers, i.e. KGd(PO₃)₄ in P2₁ and P2₁/n. Both KGdP₄O₁₂ in C2/c and KGd(PO₃)₄ in P2₁/n can be prepared by crystallizing KH₂PO₄ and Gd₂O₃ from phosphoric acid below 550°C [27], [28]. Because of the lower crystallization temperature and the resulting higher viscosity, it is difficult to grow bulk crystals of Nd:KGdP₄O₁₂. Parreu grew a KGd(PO₃)₄ crystal in P2₁ from a melt made from NH₄H₂PO₄, K₂CO₃ and Gd₂O₃ at a higher temperature [20]. In studying the crystallization region of KGd(PO₃)₄ (P2₁) in the ternary system Gd₂O₃−K₂O−P₂O₅, six neighbouring phases were found, and KGdP₄O₁₂ in C2/c was one of them. By choosing the ratio in the melt composition, we successfully grew bulk crystals of Nd:KGdP₄O₁₂ (Figure 1(a)). The composition of a melt influences the formation of phase. A similar situation occurred to a BaTeMo₂O₉ polymorphous compound recently reported–the α and β crystal phases can be grown from a melt with different compositions [29].

The chemical composition of the as-grown crystal was analyzed by ICP−AES (Table 1). The chemical formula of the crystal corresponds to KGd_0.95Nd_0.05P₄O₁₂, which is consistent with results of single crystal diffraction. The distribution coefficient of the neodymium ion at the structural sites of gadolinium ion is defined as K_Nd = ([Nd]/([Nd]+[Gd]))_crystal/([Nd]/([Nd]+[Gd]))_solution. This can be calculated from the data in Table 1 and the stoichiometry of the starting materials. The value (0.984) is very close to 1 because the ionic radius of Nd³⁺ is only slightly bigger than that of Gd³⁺. This means that the active Nd³⁺ ion can be easily doped in the KGdP₄O₁₂ host crystal, and that the composition inside a single crystal would be homogeneous in general.

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Table 1. The result of chemical analysis for the as-grown crystal of 5 at% Nd³⁺-doped KGdP₄O₁₂.

https://doi.org/10.1371/journal.pone.0100922.t001

2. Crystal structure

Nd:KGdP₄O₁₂ crystallizes in the monoclinic space group C2/c with cell parameters a = 7.812(2) Å, b = 12.307(3) Å, c = 10.474(2) Å, β = 110.84(3)° and Z = 4. Information regarding crystal data, data collection and refinement is given in Table S1. The atomic coordinate information is given in Table S2, and the anisotropic displacement parameter is given in Table S3. The lengths of P−O, Gd(Nd)−O and K−O bonds are listed in Table S4, and the bond valences have been calculated according to the Brown and Altermatt parameters [30] for the structure of Nd:KGdP₄O₁₂. The bond valence sums are reasonable for both cations and anions. The crystal structure of Nd:KGdP₄O₁₂ we show here is consistent with that of undoped KGdP₄O₁₂ reported by Ettis et al [27]. The Nd³⁺-active ion occupies the lattice site of Gd³⁺. The density of a single crystal of Nd:KGdP₄O₁₂ at 23°C is 3.54 g·cm⁻³. This agrees well with the calculated value from the crystallographic data.

The basic structural unit of Nd:KGdP₄O₁₂ is a centrosymmetric cyclotetraphosphate ring anion of [P₄O₁₂]⁴⁻ that is composed of four PO₄ tetrahedra. Each PO₄ tetrahedron shares its two corners (i.e. bridging oxygen) with the others. The four phosphorus atoms of each [P₄O₁₂]⁴⁻ ring lie in the same plane due to symmetry, and all the oxygen atoms are either above or below the P₄ plane. As shown in projection into the ac plane (Figure 2), the [P₄O₁₂]⁴⁻ rings form layers parallel to (001) at z = 0 and 1/2. A previous description [27] suggesting that these layers were perpendicular to the [001] direction is erroneous because the normal of the (001) faces is different from the [001] direction. We note that the [P₄O₁₂]⁴⁻ rings stack in the structure similar to fish scales. The P₄ plane, i.e. (0.2908, −0.0415, 0.9559), is not parallel to (001), and the interfacial angle between them is 17.08(2)°.

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Figure 2. Structure projection of Nd:KGdP₄O₁₂ along the [010] direction. (Here, all the oxygen atoms have been omitted.).

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The Gd(Nd)O₈ polyhedra and KO₁₀ polyhedra link the [P₄O₁₂]⁴⁻ rings to form a three-dimensional framework. The eight vertexes of the Gd(Nd)O₈ polyhedra come from terminal oxygens of the PO₄ polyhedra, and they belong to six [P₄O₁₂]⁴⁻ rings that are equal in the two adjacent layers. The Gd(Nd)O₈ polyhedra do not share any oxygen atoms and are isolated by the PO₄ and KO₁₀ polyhedra. Ettis et al [27] erroneously reported the shortest distance between the two rare earth ions to be 5.269 Å. Thus, they thought that it was small compared to the other alkali metal and rare earth tetraphosphates. Actually, the shortest Gd(Nd)−Gd(Nd) distance in the Nd:KGdP₄O₁₂ crystal is 6.0059 Å, which is actually relatively long for tetraphosphates.

Figure S1 shows the experimental XRD pattern of the pulverized Nd:KGdP₄O₁₂ crystal as well as our simulated pattern. The peak positions and diffraction intensities are consistent between the experimental and simulated XRD patterns. This confirms that our proposed structure is accurate.

For a laser crystal, the mechanical strength is due to the host material. Cleavage behaviour of the host crystal is related not only to the bond strength but also the geometry. There are three kinds of chemical bonds in potassium gadolinium phosphates: P−O, Gd−O and K−O. According to their covalent character, P−O bonds are much stronger than Gd−O and K−O bonds. The K−O bonds are the weakest of all. This is confirmed by the calculated results of bond valences shown in Table S5. The KGd(PO₃)₄ structures (both P2₁ and P2₁/n) are formed by intrachain P−O bonds and interchain Gd−O and K−O bonds. The P−O bonds run along the [100] direction for KGd(PO₃)₄ in P2₁ and [101] for KGd(PO₃)₄ in P2₁/n to generate long chains, respectively. Therefore, the KGd(PO₃)₄ crystals have the strongest strength along the above [100] or [101] directions and cleavage very easily occurs between the chains. In KGdP₄O₁₂ however, the GdO₈ polyhedra and KO₁₀ polyhedra link [P₄O₁₂]⁴⁻ rings to form a three-dimensional framework, and the strengths along the different directions are quite similar. Therefore, the Nd:KGdP₄O₁₂ crystal does not have a prominent cleavage behaviour. During the processing of Nd:KGdP₄O₁₂ crystals, no crack and cleavage as seen in KGd(PO₃)₄ crystals was found.

3. Crystal morphology and rocking curve

The as-grown crystal has some well-developed facets (Figure 1(a)) oriented using an XRD goniometer. The habit contains crystalline forms {001}, {010}, {110} and {021}. The measured interfacial angles among the facets were in good agreement with the calculated angles.

The morphology of a crystal reflects its structure. Taking into account only the point group and the crystal cell parameters, we established the theoretical morphological scheme of the Nd:KGdP₄O₁₂ crystal via the WinXMorph software [31] using the Bravais-Friedel and Donnay-Harker (BFDH) law. This morphological scheme is shown in Figure 1(b). Table 2 lists the crystalline forms {hkl}, equivalent faces and corresponding d_hkl for Nd:KGdP₄O₁₂.

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Table 2. The crystalline forms {hkl} observed in the Nd:KGdP₄O₁₂ crystal and the corresponding d_hkl arranged by decreasing sense.

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While the as-grown crystal did not show a completely ideal morphology due to the restriction of the crystal growth method, all the predicted crystalline forms can be found on the as-grown single crystal. It should be noted that the facet was smaller and limited by the growth conditions. Thus, it was quite pristine and we did not identify for the as-grown crystal. The narrow tetragonal facet (021) on the as-grown crystal shows that {021} has a faster growth rate, which is consistent with the predicted scheme. The ideal morphology of KGdP₄O₁₂ is like a drum and hexagonal {001} facets reveal much more than other facets similar to drumheads. This relates to the structural feature of KGdP₄O₁₂ in which [P₄O₁₂]⁴⁻ rings regularly array to form layers parallel to (001). The distance between the two [P₄O₁₂]⁴⁻ rings adjacent to the two layers is larger than that from the same layer.

The rocking curve of the (004) diffraction plane of the as-grown Nd:KGdP₄O₁₂ crystal is presented in Figure S2. The diffraction peak is intense and with good symmetry without splitting, and its FWHM value is 0.006°. These results demonstrate that the as-grown Nd:KGdP₄O₁₂ crystal has a nearly perfect lattice structure.

4. IR and Raman spectra

The IR and Raman spectra of Nd:KGdP₄O₁₂ at room temperature are shown in Figure 3. The vibration frequencies above 600 cm⁻¹ and their corresponding assignment are listed in Table S5 and are based on published findings [27], [32]. In the low-frequency region below 600 cm⁻¹, it is very difficult to distinguish the symmetric and antisymmetric bending modes of the O−P−O and P−O−P groups.

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Figure 3. The IR and Raman spectra of Nd:KGdP₄O₁₂ at room temperature.

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Compared to other cyclotetraphosphate compounds, we notice an absence of bands in the 750−1000 cm⁻¹ region of the IR spectrum. This result is in agreement with the cyclic structure of Nd:KGdP₄O₁₂. The non-coincidence of the observed bands in the IR and Raman spectra indicates that the [P₄O₁₂]⁴⁻ anion is centrosymmetric, which is in agreement with the above structural results.

A distinguishing characteristic also exits in the Raman spectrum. The symmetric stretching vibration of the P−O−P linkage–ν_s(P−O−P)–has a single peak at 684 cm⁻¹. This is the strongest of all the Raman vibration peaks. However, for ν_s(P−O−P) of KGd(PO₃)₄ (P2₁), two peaks are observed in the range of 660–730 cm⁻¹ and have the same intensity [24]. These are less than one half of the intensity of the ν_s(O−P−O). That is because of the symmetrical difference in the crystal structures and the different positions of the lanthanide and alkali ions. The obvious differences between the IR and Raman spectra of KGdP₄O₁₂ and KGd(PO₃)₄ highlight how IR and Raman spectroscopy can identify the structure of alkali-metal lanthanide metaphosphates.

5. Thermal properties

Generally, double phosphate compounds are not stable at high temperatures. To measure the decomposition, a TG−DSC analysis for the Nd:KGdP₄O₁₂ crystal was performed. The TG−DSC curves of the Nd:KGdP₄O₁₂ crystal are given in Figure S3. A single sharp endothermic peak is observed at 920°C, which exhibits the characteristics of a first-order phase transition. The sample weight of Nd:KGdP₄O₁₂ does not show representative variation in the measurement temperature up to 1000°C. Apart from a crystalline phase, an amorphous phase was formed in the sample cell after the DSC measurement. The M^ILn^IIIP₄O₁₂ compounds often decompose irreversibly into lanthanide tri-metaphosphates and alkali metal metaphosphates [33]. We thus conclude that the exothermic peak at 920°C may be related to a decomposition of Nd:KGdP₄O₁₂ in accordance with the reaction:The decomposition temperature of the Nd:KGdP₄O₁₂ crystal is much higher than that of KGd(PO₃)₄ (P2₁) (878°C) [20]. The difference in stability between these two potassium gadolinium metaphosphates can be explained by comparing their crystal data. The lengths of P−O bonds vary from 1.4752 to 1.6009 Å for KGdP₄O₁₂. However, the P−O bond lengths vary from 1.4414 to 1.6886 Å for KGd(PO₃)₄. That is, the PO₄ tetrahedra in KGd(PO₃)₄ are heavily distorted as are the GdO₈ polyhedra. The Gd(Nd)−O distances in the Nd:KGdP₄O₁₂ are from 2.3665 to 2.4189 Å with an average of 2.3933 Å. The Gd−O distances in KGd(PO₃)₄ vary widely from 2.2903 to 2.4767 Å with an average of 2.4059 Å. In addition, the volume per formula of KGdP₄O₁₂ (235.3 Å³) is smaller than that of KGd(PO₃)₄ (240.4 Å³). Thus, the thermal stability of the cyclic C2/c space group should be better than that of the chain structure in the P2₁ space group.

For laser crystal materials, the damage threshold and possible laser applications can be greatly influenced by the specific heat. The specific heat is also an important value used to calculate thermal conductivity. Figure S4 shows that the constant pressure specific heat of the Nd:KGdP₄O₁₂ crystal varies as a function of the temperature. The specific heat of the Nd:KGdP₄O₁₂ crystal at room temperature (25°C) is 0.521 J·g⁻¹·K⁻¹. This is comparable to Nd:YAG (0.59 J·g⁻¹·K⁻¹) and Nd:YVO₄ (0.51 J·g⁻¹·K⁻¹) [34]. Because Nd:YAG and Nd:YVO₄ both have a high optical damage threshold, it follows that Nd:KGdP₄O₁₂ should also have a high damage threshold [5]. The specific heat increases almost linearly from 0.485 to 0.799 J·g⁻¹·K⁻¹ with temperature increases from −10 to 510°C. This suggests that the Nd:KGdP₄O₁₂ crystal can tolerate even more thermal energy at a high temperature.

Figure 4 shows the thermal diffusivity and thermal conductivity of the Nd:KGdP₄O₁₂ crystal. The thermal conductivity (κ) was calculated using the measured thermal diffusivity and specific heat according to κ = λρC_p, where λ, ρ and C_p denote the thermal diffusivity, density and specific heat of the crystal at the same temperature, respectively. The thermal diffusivity of the crystal is 0.906 mm²⋅s⁻¹ along the direction perpendicular to (001), i.e. the c* direction, at 27.8°C. The calculated thermal conductivity of the crystal is 1.66 W⋅m⁻¹⋅K⁻¹ along the c* direction correspondingly. This value is similar to that of Nd:SrLaGa₃O₇ [4] and larger than those of some laser crystals with broad absorption such as NaLn(WO₄)₂ [2], [3] and Ca₃La₂(BO₃)₄ [5]. In laser designs, the thermal loading causes a temperature gradient in the crystal and leads to thermal expansion that results in thermal lensing and other thermo-optic effects. All these effects would cause a decline in the quality of the laser beams and even crack the active crystal. The deposited heat would be easily transferred to the environment if the laser crystal possesses high thermal conductivity–this would minimize the thermal loading effects.

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Figure 4. Thermal diffusivities and thermal conductivities of the Nd:KGdP₄O₁₂ crystal along the c* direction.

https://doi.org/10.1371/journal.pone.0100922.g004

It is clearly shown in Figure 4 that the thermal diffusivity and thermal conductivity component of the Nd:KGdP₄O₁₂ crystal decreases with increasing temperature. Thermal conductivity is dominated primarily by its phonon thermal conductivity for the dielectric. This has two important determinants: the heat capacity and the phonon mean free path. Though the heat capacity of the Nd:KGdP₄O₁₂ crystal increases with temperature, the phonon-phonon scattering becomes much stronger and the phonon mean free path markedly decreases. Therefore, the final result is that the thermal conductivity decreases with temperature increases.

6. Absorption spectrum

Figure 5 shows the unpolarized absorption spectrum of the Nd:KGdP₄O₁₂ crystal at room temperature. The UV cut off is below 200 nm. This means that a large band gap would increase the damage resistivity of the crystal. All absorption lines are due to the 4f³–4f³ transition of the Nd³⁺ ions. Very strong absorption lines occur near 581, 747 and 798 nm, corresponding to the transitions of ⁴I_9/2→²G_7/2+⁴G_5/2, ⁴I_9/2→⁴S_3/2+⁴F_7/2 and ⁴I_9/2→²H_9/2+⁴F_5/2, respectively. The interesting feature of this spectrum relative to the potential application in the diode-pumped laser, is the strong absorption band with a FWHM of 14.8 nm near 798 nm. This is close to the laser output of the AlGaAs laser diode. It is well known that temperature control of the laser diode is crucial, because the emission wavelength increases with temperature. Large line-width in the Nd:KGdP₄O₁₂ crystal is very suitable for laser diode pumping because it is not temperature dependent.

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Figure 5. Absorption spectrum of the Nd:KGdP₄O₁₂ crystal at room temperature.

https://doi.org/10.1371/journal.pone.0100922.g005

The Judd-Ofelt theory was used to calculate the optical parameters of the electric dipole transition within the 4f electronic configuration of the Nd³⁺ ion. Important parameters included the oscillator intensity parameters (Ω_λ), spontaneous emission probability (A), fluorescence branching ratio (β) and radiative lifetime (τ_r) [35], [36]. A detailed calculation procedure was used similar to literature [37]. Nine absorption bands were used to obtain the intensity parameters (Ω_2,4,6), which are Ω₂ = 1.79×10^–20cm², Ω₄ = 3.48×10^–20cm² and Ω₆ = 6.05×10^–20cm², respectively. The values of the experimental (f_exp) and calculated (f_cal) oscillator strengths are listed with the absorption cross sections (σ) in Table 3. The root mean square error (δ) between f_exp and f_cal is 6.97×10⁻⁸. The properties of ⁴F_3/2→⁴I_J_’ (J’ = 9/2, 11/2, 13/2, 15/2) transitions are well known. These are the main channels of a Nd³⁺ laser and depend only on the values of Ω₄ and Ω₆ because the reduced matrix elements |<||U⁽²⁾||>| for these transitions are zero. The spectroscopic quality parameter (X = Ω₄/Ω₆) is 0.57, which is less than 1 and indicates that the emission to the ⁴I_11/2 manifold is more feasible than that to ⁴I_9/2 manifold. This result can be confirmed by the fluorescence emission spectrum (see below). In this regard, it is similar to the most common laser materials based on the Nd³⁺ ion including Nd:YVO₄ and Nd:YAG. Table 4 presents the spontaneous emission probability (A), radiative branching ratio (β) and radiative lifetime (τ_r) of the multiplet ⁴F_3/2. It can be seen that the calculated branching ratios are in good agreement with the experimental ones obtained from the measured fluorescence spectrum later.

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Table 3. The barycenter wavelength, experimental and calculated oscillator strengths, and absorption cross section of the Nd:KGdP₄O₁₂ crystal at room temperature.

https://doi.org/10.1371/journal.pone.0100922.t003

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Table 4. The calculated spontaneous emission probability, radiative branching ratios, radiative lifetime and experimental branching ratios of ⁴F_3/2 multiplet of Nd³⁺ in the Nd:KGdP₄O₁₂ crystal.

https://doi.org/10.1371/journal.pone.0100922.t004

7. Fluorescence spectra analysis

The room temperature fluorescence spectrum of the Nd:KGdP₄O₁₂ crystal at 808 nm excitation was recorded from 830 to 1600 nm (Figure 6). Three emission bands corresponding to the ⁴F_3/2 →⁴I_9/2, ⁴I_11/2 and ⁴I_13/2 transitions are observed at 860–920 nm, 1040–1080 nm and 1300–1380 nm, respectively. The room temperature emission FWHM of the transition ⁴F_3/2→⁴I_11/2 is 14 nm. This is evidence of the dominant contribution of inhomogeneous broadening to the spectra line-width in the low symmetry structure. The emission cross sections for the laser channel from ⁴F_3/2 to ⁴I_9/2, ⁴I_11/2 and ⁴I_13/2 were calculated through the Füchtbauer-Ladenburg (FL) formula [12], [18]. These values are 1.11×10⁻²⁰, 6.25×10⁻²⁰ and 2.13×10⁻²⁰cm², respectively.

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Figure 6. Emission fluorescence spectrum of the Nd:KGdP₄O₁₂ crystal at room temperature.

https://doi.org/10.1371/journal.pone.0100922.g006

The emission spectra of the Nd:KGdP₄O₁₂ at low temperature were studied to determine the Stark sublevels of the ⁴I_13/2 and ⁴I_11/2 excited multiplets and the ground state multiplet ⁴I_9/2. Figure S5 shows the emission spectra of Nd:KGdP₄O₁₂ at 10K and 300K. These correspond to the transitions from ⁴F_3/2 to ⁴I_9/2, ⁴I_11/2 and ⁴I_13/2 levels, respectively. Figure 7 shows a schematic diagram of the Stark sublevels of Nd³⁺ in KGdP₄O₁₂ crystals obtained from the low temperature emission spectra. The ⁴F_3/2 term is split into two components with Δ = 70 cm⁻¹. The low value of the crystal field splitting indicates that the crystal field of the KGdP₄O₁₂ host is weak. This is due to weak interactions between rare earth ions and oxygen atoms in contrast with the strong covalent bonds between P and O. In this regard, the Nd:KGdP₄O₁₂ crystal is similar to other double phosphates of alkali and lanthanide ions [38], [39].

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Figure 7. Stark structure of the energy levels of the Nd³⁺ ion in the KGdP₄O₁₂ crystal.

https://doi.org/10.1371/journal.pone.0100922.g007

The fluorescence lifetime (τ_f) of the ⁴F_3/2 energy level of Nd³⁺ in the Nd:KGdP₄O₁₂ crystal was determined to be 300 µs by fitting the decay curve exponentially (Figure S6). The long fluorescence lifetime would be beneficial to high energy storage during laser operation. Though the interatomic distance (6.0059 Å) between the nearest rare earth ions for KGdP₄O₁₂ is smaller than that for KGd(PO₃)₄ (P2₁) (6.5865 Å), it does not obviously affect the fluorescence lifetime. This is a function of the isolated GdO₈ polyhedra in KGdP₄O₁₂. The τ_f of the Nd:KGd(PO₃)₄ crystal (P2₁) is 246 µs with a very low doping concentration (0.6 at%) [38]. The values of Nd:LaP₅O₁₄ and Nd:GdP₅O₁₄ crystals are around 310 µs when doped with ∼1 at% Nd³⁺ [40].

It is well known that the τ_f of Nd³⁺ ions generally decreases with increasing Nd³⁺ concentrations due to the interactions between the Nd³⁺ ions, i.e. the concentration quenching effect. However, the Nd:KGdP₄O₁₂ crystal still has a long fluorescence lifetime of 300 µs under a higher doping concentration of 5 at%. Versus the above phosphates, the KGdP₄O₁₂ crystal is a distinct laser host material with a very low concentration quenching for Nd³⁺ because the Nd:KGdP₄O₁₂ crystal ensures a long fluorescence lifetime even under a high active ion concentration.

The calculated τ_r of the ⁴F_3/2 energy level is 312 µs (Table 4). Thus, the fluorescent quantum efficiency (η = τ_f/τ_r) of the ⁴F_3/2 level is 96%. For solid-state laser materials, low crystal field strength leads to weak electron-phonon interactions and further leads to high quantum efficiency. The previous data describing weak Stark splitting of the Nd³⁺ energy levels further supports the calculated result showing that Nd:KGdP₄O₁₂ has a high quantum efficiency.

Some spectroscopic properties of the Nd:KGdP₄O₁₂ crystal are listed in Table 5 as well as those of other Nd³⁺-doped crystals including YAG and YVO₄ crystals with ordered structures and SrLaGa₃O₇ and KBaGd(MoO₄)₃ crystals with disordered structures. The FWHM value of Nd:KGdP₄O₁₂ at λ_a is not only much larger than those of Nd:YAG and Nd:YVO₄ crystals, but also larger than those of Nd:SrLaGa₃O₇ and Nd:KBaGd(MoO₄)₃ crystals. This suggests that the Nd:KGdP₄O₁₂ crystal can be pumped more effectively by AlGaAs laser diode, but not be restricted to the temperature stability of the output wavelength in the laser diode. The quality factor (M) of a laser material is proportional to its doping concentration (N) and fluorescence lifetime (τ_f). A high value of M generally means a low oscillation threshold in subsequent laser operations. Therefore, the high doping concentration and long fluorescence lifetime of Nd:KGdP₄O₁₂ makes it possible to achieve a continuous wave laser action. In addition, Nd:KGdP₄O₁₂ can be used as a tunable laser material with a very broad emission band near 1060 nm.

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Table 5. Comparison of spectroscopic properties of the Nd:KGdP₄O₁₂ crystal with other Nd³⁺-doped crystals.

https://doi.org/10.1371/journal.pone.0100922.t005

Conclusions

We successfully grew a macro-defect-free single crystal of 5 at% Nd³⁺-doped KGdP₄O₁₂ with very good crystallinity using TSSG−SC from self-flux. To the best of our knowledge, both the growth of the bulk crystals and doping with neodymium are shown here for the first time in KGdP₄O₁₂. The Nd:KGdP₄O₁₂ crystallizes in space group C2/c, and the phosphoric anions have a cyclic geometry of [P₄O₁₂]⁴⁻. The crystal has good chemical stability and does not show cleavage like its isomer KGd(PO₃)₄. The distribution coefficient of neodymium ion is very close to 1, so gadolinium ion can be evenly substituted. The absorption and fluorescence spectra of the Nd:KGdP₄O₁₂ crystal were investigated at room temperature. The peak absorption cross section at 798 nm is 2.59×10⁻²⁰cm² with a FWHM of 14.8 nm. Such a broad FWHM in the absorption band is suitable for InGaAs laser diode pumping. The radiation and fluorescence lifetimes of the excited state ⁴F_3/2 are 312 and 300 μs, respectively. These result in a high luminescent quantum efficiency of 96%. The emission from the ⁴F_3/2 energy level to the ⁴I_11/2 manifold is more feasible than that of the ⁴I_9/2 manifold. The peak emission cross section at 1057 nm (corresponding to ⁴F_3/2→⁴I_11/2) is 6.25×10⁻²⁰cm². In addition, the KGdP₄O₁₂ crystal presents positive thermal characteristics. It has a good thermal stability with decomposition at 920°C. At room temperature, the specific heat and the thermal conductivity along the c* direction are 0.521 J·g⁻¹·K⁻¹ and 1.66 W⋅m⁻¹⋅K⁻¹, respectively. In summary, the Nd:KGdP₄O₁₂ crystal may be regarded as a potential solid-state laser material for laser diode pumping.

Supporting Information

Figure S1.

The experimental XRD pattern of Nd:KGdP₄O₁₂ from polycrystalline powder and the simulated pattern from single crystal data.

https://doi.org/10.1371/journal.pone.0100922.s001

(DOCX)

Figure S2.

XRD rocking curve of the (004) diffraction plane of the as-grown Nd:KGdP₄O₁₂ single crystal.

https://doi.org/10.1371/journal.pone.0100922.s002

(DOCX)

Figure S3.

TG and DSC curves of the Nd:KGdP₄O₁₂ crystal.

https://doi.org/10.1371/journal.pone.0100922.s003

(DOCX)

Figure S4.

Specific heat versus temperature curve of the Nd:KGdP₄O₁₂ crystal.

https://doi.org/10.1371/journal.pone.0100922.s004

(DOCX)

Figure S5.

Emission fluorescence spectra of the Nd:KGdP₄O₁₂ crystal at 10K and 300K.

https://doi.org/10.1371/journal.pone.0100922.s005

(DOCX)

Figure S6.

Fluorescence decay curve of the ⁴F_3/2 manifold of the Nd:KGdP₄O₁₂ crystal at room temperature.

https://doi.org/10.1371/journal.pone.0100922.s006

(DOCX)

Table S1.

Crystal data, data collection and refinement of Nd:KGdP₄O₁₂.

https://doi.org/10.1371/journal.pone.0100922.s007

(DOCX)

Table S2.

Atomic coordinates and equivalent isotropic displacement parameters of Nd:KGdP₄O_12.

https://doi.org/10.1371/journal.pone.0100922.s008

(DOCX)

Table S3.

Anisotropic displacement parameters (Å²) of Nd:KGdP₄O₁₂.

https://doi.org/10.1371/journal.pone.0100922.s009

(DOCX)

Table S4.

Bond lengths and bond valences in the Nd:KGdP₄O₁₂ crystal.

https://doi.org/10.1371/journal.pone.0100922.s010

(DOCX)

Table S5.

Frequencies (cm⁻¹) and assignments of IR absorption and Raman scattering for Nd:KGdP₄O₁₂.

https://doi.org/10.1371/journal.pone.0100922.s011

(DOCX)

Acknowledgments

The authors thank Dr. Shifeng Jin (Institute of Physics, Chinese Academy of Science) for his help in the rocking curve measurements.

Author Contributions

Conceived and designed the experiments: TS YK JX. Performed the experiments: TS YZ. Analyzed the data: TS YZ PS ZZ. Contributed reagents/materials/analysis tools: TS SC. Wrote the paper: TS.

References

1. Kaminskii AA (2007) Laser crystals and ceramics: recent advances. Laser & Photon Rev 1: 93–177.
- View Article
- Google Scholar
2. Faure N, Borel C, Couchaud M, Basset G, Templier R, et al. (1996) Optical properties and laser performance of neodymium doped scheelites CaWO₄ and NaGd(WO₄)₂. Appl Phys B 63: 593–598.
- View Article
- Google Scholar
3. Cheng Z, Zhang S, Fu K, Cheng H (2001) Growth, thermal and laser properties of neodymium doped sodium yttrium double tungstate crystal. Jpn J Appl Phys 40: 4038–4040.
- View Article
- Google Scholar
4. Zhang YY, Zhang HJ, Yu HH, Sun SQ, Wang JY, et al. (2011) Characterization of disordered melilite Nd:SrLaGa₃O₇ crystal. IEEE J Quantum Elect 47: 1506–1513.
- View Article
- Google Scholar
5. Pan ZB, Cong HJ, Yu HH, Tian L, Yuan H, et al. (2013) Growth, thermal properties and laser operation of Nd:Ca₃La₂(BO₃)₄: a new disordered laser crystal. Opt Express 21: 6091–6100.
- View Article
- Google Scholar
6. Li H, Zhang LZ, Wang GF (2009) Growth, structure and spectroscopic characterization of a new laser crystals Nd³⁺:Li₃Ba₂Gd₃(WO₄)₈. J Alloy Compd 478: 484–488.
- View Article
- Google Scholar
7. Li H, Wang GJ, Zhang LZ, Huang YS, Wang GF (2010) Growth and structure of Nd³⁺-doped Li₃Ba₂Y₃(WO₄)₈ crystal with a disorder structure. CrystEngComm 12: 1307–1310.
- View Article
- Google Scholar
8. Xiao B, Zhang LZ, Lin ZB, Huang YS, Wang GF (2012) Growth and spectral properties of Nd³⁺:Li₃Ba₂La₃(WO₄)₈ crystal. Optoelectron Adv Mat 6: 404–410.
- View Article
- Google Scholar
9. Song MJ, Wang GJ, Lin ZB, Zhang LZ, Wang GF (2007) Growth and spectral properties of Nd³⁺-doped Li₃Ba₂Y₃(MoO₄)₈ crystal. J Cryst Growth 308: 208–212.
- View Article
- Google Scholar
10. Song MJ, Zhang LZ, Wang GF (2009) Growth and spectral properties of Nd³⁺-doped Li₃Ba₂Ln₃(MoO₄)₈ (Ln = La, Gd) crystals. J Alloy Compd 480: 839–842.
- View Article
- Google Scholar
11. Song MJ, Zhao W, Wang GF, Zhao ML, Wang LT (2011) Growth, thermal and polarized spectral properties of Nd³⁺-doped Li₃Ba₂Gd₃(MoO₄)₈ crystal. J Alloy Compd 509: 2164–2169.
- View Article
- Google Scholar
12. Meng XM, Lin ZB, Zhang LZ, Huang YS, Wang GF (2011) Structure and spectral properties of Nd³⁺-doped KBaGd(MoO₄)₃ crystal with a disorder structure. CrystEngComm 13: 4069–4073.
- View Article
- Google Scholar
13. Xiao B, Huang YS, Zhang LZ, Lin ZB, Wang GF (2012) Growth, structure and spectroscopic characterization Nd³⁺-doped KBaGd(WO₄)₃ crystal with a disordered structure. PLoS ONE 7: e40229.
- View Article
- Google Scholar
14. Cheng Y, Xu XD, Xin Z, Yang XB, Xiao XD, et al. (2009) Crystal growth, optical properties, and continuous-wave laser operation of Nd³⁺-doped CaNb₂O₆ crystal. Laser Phys Lett 6: 740–745.
- View Article
- Google Scholar
15. Xu XD, Cheng SS, Meng JQ, Li DZ, Zhou DH, et al. (2010) Spectral characterization and laser performance of a mixed crystal Nd:(Lu_xY_1–x)₃Al₅O₁₂. Opt Express 18: 21370–21375.
- View Article
- Google Scholar
16. Li DZ, Xu XD, Zhou DH, Zhuang SD, Wang ZP, et al. (2010) Crystal growth, spectral properties, and laser demonstration of laser crystal Nd:LYSO. Laser Phys Lett 7: 798–804.
- View Article
- Google Scholar
17. Li DZ, Xu XD, Cong ZH, Zhang J, Tang DY, et al. (2011) Growth, spectral properties, and laser demonstration of Nd:GYSO crystal. Appl Phys B 104: 53–58.
- View Article
- Google Scholar
18. Li DZ, Xu XD, Zhang J, Cong ZH, Tang DY, et al. (2011) Growth, spectral properties, and laser demonstration of Nd:(Lu_0.5Gd_0.5)₂SiO₅ crystal. Laser Phys Lett 8: 647–652.
- View Article
- Google Scholar
19. Boulon G (2003) Yb³⁺-doped oxide crystals for diode-pumped solid state lasers: crystal growth, optical spectroscopy, new criteria of evaluation and combinatorial approach. Opt Mat 22: 85–87.
- View Article
- Google Scholar
20. Parreu I, Solé R, Gavaldà J, Massons J, Díaz F, et al. (2005) Crystal growth, structural characterization, and linear thermal evolution of KGd(PO₃)₄. Chem Mater 17: 822–828.
- View Article
- Google Scholar
21. Parreu I, Carvajal JJ, Solans X, Díaz F, Aguiló M (2006) Crystal structure and optical characterization of pure and Nd-substituted type III KGd(PO₃)₄. Chem Mater 18: 221–228.
- View Article
- Google Scholar
22. Parreu I, Solé R, Gavaldà J, Massons J, Díaz F, et al. (2003) Crystallization region, crystal growth, and phase transitions of KNd(PO₃)₄. Chem Mater 15: 5059–5064.
- View Article
- Google Scholar
23. Parreu I, Solé R, Massons J, Díaz F, Aguiló M (2007) Crystal growth, crystal morphology and surface micromorphology of type III KGd(PO₃)₄ and KNd(PO₃)₄. Cryst Growth Des 7: 557–563.
- View Article
- Google Scholar
24. Parreu I, Solé R, Massons J, Díaz F, Aguiló M (2007) Crystal growth and characterization of type III ytterbium-doped KGd(PO₃)₄: a new nonlinear laser host. Chem Mater 19: 2868–2876.
- View Article
- Google Scholar
25. Parreu I, Pujol MC, Aguiló M, Díaz F, Mateos X, et al. (2007) Growth, spectroscopy and laser operation of Yb:KGd(PO₃)₄ single crystals. Opt Express 15: 2360–2368.
- View Article
- Google Scholar
26. Solé R, Ruiz X, Pujol MC, Mateos X, Carvajal JJ, et al. (2009) Physical properties of self-flux and WO₃-containing solutions useful for growing type III KGd(PO₃)₄ single crystals. J Cryst Growth 311: 3656–3660.
- View Article
- Google Scholar
27. Ettis H, Naïli H, Mhiri T (2003) Synthesis and crystal structure of a new potassium-gadolinium cyclotetraphosphate, KGdP₄O₁₂. Cryst Growth Des 3: 599–602.
- View Article
- Google Scholar
28. Rekik W, Naïli H, Mhiri T (2004) Potassium gadolinium polyphosphate, KGd(PO₃)₄. Acta Cryst C60: i50–i52.
- View Article
- Google Scholar
29. Zhang JJ, Zhang ZH, Zhang WG, Zheng QX, Sun YX, et al. (2011) Polymorphism of BaTeMo₂O₉: a new polar polymorph and the phase transformation. Chem Mater 23: 3752–3761.
- View Article
- Google Scholar
30. Brown ID, Altermatt D (1985) Bond-valence parameters obtained from a systematic analysis of the Inorganic Crystal Structure Database. Acta Cryst B41: 244–247.
- View Article
- Google Scholar
31. Kaminsky W (2007) From CIF to virtual morphology using the WinXMorph program. J Appl Crystallogr 40: 382–385.
- View Article
- Google Scholar
32. Horchani K, Ferid M, Gacon JC, Lecocq S, Trabelsi-Ayedi M, et al. (2002) Structure refinement, infrared and Raman spectra of KDyP₄O₁₂. Mater Res Bull 37: 1259–1267.
- View Article
- Google Scholar
33. Jungowska W, Znamierowska T (1990) Studies on thermal stability of mixed metaphosphates KLa(PO₃)₄ and K₂La(PO₃)₅. J Therm Anal Calorim 36: 2095–2098.
- View Article
- Google Scholar
34. Koechner W (2006) Solid-State Laser Engineering (6th). New York: Springer Press. 54–73.
35. Judd BR (1962) Optical absorption intensities of rare-earth ions. Phys Rev 127: 750–761.
- View Article
- Google Scholar
36. Ofelt GS (1962) Intensities of crystal spectra of rare-earth ions. J Chem Phys 37: 511–520.
- View Article
- Google Scholar
37. Wang GF, Chen WZ, Lin ZB, Hu ZS (1999) Optical transition probability of Nd³⁺ ions in a alpha-Nd³⁺:LaSc₃(BO₃)₄ crystal. Phys Rev B 60: 15469–15471.
- View Article
- Google Scholar
38. Parreu I (2006) Crystal growth and characterization of ytterbium or neodymium doped type III-KGd(PO3)4-a new bifunctional nonlinear and laser crystal. Tarragona: Rovira i Virgili University. 114.
39. Maksimova SI, Palkina KK, Loshchenov VB (1981) Structure and luminescence properties of α’-CsNd(PO₃)₄. Izv Akad Nauk SSSR Nerog Mater 17: 116–120.
- View Article
- Google Scholar
40. Weber HP (1975) Nd pentaphosphate lasers. Opt Quant Electron 7: 431–442.
- View Article
- Google Scholar
41. Sato Y, Taira T (2005) Comparative study on the spectroscopic properties of Nd:GdVO₄ and Nd:YVO₄ with hybrid process. IEEE J Sel Top Quantum Electron 11: 613–620.
- View Article
- Google Scholar

[ref1] 1. Kaminskii AA (2007) Laser crystals and ceramics: recent advances. Laser & Photon Rev 1: 93–177.
View Article
Google Scholar

[2] View Article

[3] Google Scholar

[ref2] 2. Faure N, Borel C, Couchaud M, Basset G, Templier R, et al. (1996) Optical properties and laser performance of neodymium doped scheelites CaWO₄ and NaGd(WO₄)₂. Appl Phys B 63: 593–598.
View Article
Google Scholar

[5] View Article

[6] Google Scholar

[ref3] 3. Cheng Z, Zhang S, Fu K, Cheng H (2001) Growth, thermal and laser properties of neodymium doped sodium yttrium double tungstate crystal. Jpn J Appl Phys 40: 4038–4040.
View Article
Google Scholar

[8] View Article

[9] Google Scholar

[ref4] 4. Zhang YY, Zhang HJ, Yu HH, Sun SQ, Wang JY, et al. (2011) Characterization of disordered melilite Nd:SrLaGa₃O₇ crystal. IEEE J Quantum Elect 47: 1506–1513.
View Article
Google Scholar

[11] View Article

[12] Google Scholar

[ref5] 5. Pan ZB, Cong HJ, Yu HH, Tian L, Yuan H, et al. (2013) Growth, thermal properties and laser operation of Nd:Ca₃La₂(BO₃)₄: a new disordered laser crystal. Opt Express 21: 6091–6100.
View Article
Google Scholar

[14] View Article

[15] Google Scholar

[ref6] 6. Li H, Zhang LZ, Wang GF (2009) Growth, structure and spectroscopic characterization of a new laser crystals Nd³⁺:Li₃Ba₂Gd₃(WO₄)₈. J Alloy Compd 478: 484–488.
View Article
Google Scholar

[17] View Article

[18] Google Scholar

[ref7] 7. Li H, Wang GJ, Zhang LZ, Huang YS, Wang GF (2010) Growth and structure of Nd³⁺-doped Li₃Ba₂Y₃(WO₄)₈ crystal with a disorder structure. CrystEngComm 12: 1307–1310.
View Article
Google Scholar

[20] View Article

[21] Google Scholar

[ref8] 8. Xiao B, Zhang LZ, Lin ZB, Huang YS, Wang GF (2012) Growth and spectral properties of Nd³⁺:Li₃Ba₂La₃(WO₄)₈ crystal. Optoelectron Adv Mat 6: 404–410.
View Article
Google Scholar

[23] View Article

[24] Google Scholar

[ref9] 9. Song MJ, Wang GJ, Lin ZB, Zhang LZ, Wang GF (2007) Growth and spectral properties of Nd³⁺-doped Li₃Ba₂Y₃(MoO₄)₈ crystal. J Cryst Growth 308: 208–212.
View Article
Google Scholar

[26] View Article

[27] Google Scholar

[ref10] 10. Song MJ, Zhang LZ, Wang GF (2009) Growth and spectral properties of Nd³⁺-doped Li₃Ba₂Ln₃(MoO₄)₈ (Ln = La, Gd) crystals. J Alloy Compd 480: 839–842.
View Article
Google Scholar

[29] View Article

[30] Google Scholar

[ref11] 11. Song MJ, Zhao W, Wang GF, Zhao ML, Wang LT (2011) Growth, thermal and polarized spectral properties of Nd³⁺-doped Li₃Ba₂Gd₃(MoO₄)₈ crystal. J Alloy Compd 509: 2164–2169.
View Article
Google Scholar

[32] View Article

[33] Google Scholar

[ref12] 12. Meng XM, Lin ZB, Zhang LZ, Huang YS, Wang GF (2011) Structure and spectral properties of Nd³⁺-doped KBaGd(MoO₄)₃ crystal with a disorder structure. CrystEngComm 13: 4069–4073.
View Article
Google Scholar

[35] View Article

[36] Google Scholar

[ref13] 13. Xiao B, Huang YS, Zhang LZ, Lin ZB, Wang GF (2012) Growth, structure and spectroscopic characterization Nd³⁺-doped KBaGd(WO₄)₃ crystal with a disordered structure. PLoS ONE 7: e40229.
View Article
Google Scholar

[38] View Article

[39] Google Scholar

[ref14] 14. Cheng Y, Xu XD, Xin Z, Yang XB, Xiao XD, et al. (2009) Crystal growth, optical properties, and continuous-wave laser operation of Nd³⁺-doped CaNb₂O₆ crystal. Laser Phys Lett 6: 740–745.
View Article
Google Scholar

[41] View Article

[42] Google Scholar

[ref15] 15. Xu XD, Cheng SS, Meng JQ, Li DZ, Zhou DH, et al. (2010) Spectral characterization and laser performance of a mixed crystal Nd:(Lu_xY_1–x)₃Al₅O₁₂. Opt Express 18: 21370–21375.
View Article
Google Scholar

[44] View Article

[45] Google Scholar

[ref16] 16. Li DZ, Xu XD, Zhou DH, Zhuang SD, Wang ZP, et al. (2010) Crystal growth, spectral properties, and laser demonstration of laser crystal Nd:LYSO. Laser Phys Lett 7: 798–804.
View Article
Google Scholar

[47] View Article

[48] Google Scholar

[ref17] 17. Li DZ, Xu XD, Cong ZH, Zhang J, Tang DY, et al. (2011) Growth, spectral properties, and laser demonstration of Nd:GYSO crystal. Appl Phys B 104: 53–58.
View Article
Google Scholar

[50] View Article

[51] Google Scholar

[ref18] 18. Li DZ, Xu XD, Zhang J, Cong ZH, Tang DY, et al. (2011) Growth, spectral properties, and laser demonstration of Nd:(Lu_0.5Gd_0.5)₂SiO₅ crystal. Laser Phys Lett 8: 647–652.
View Article
Google Scholar

[53] View Article

[54] Google Scholar

[ref19] 19. Boulon G (2003) Yb³⁺-doped oxide crystals for diode-pumped solid state lasers: crystal growth, optical spectroscopy, new criteria of evaluation and combinatorial approach. Opt Mat 22: 85–87.
View Article
Google Scholar

[56] View Article

[57] Google Scholar

[ref20] 20. Parreu I, Solé R, Gavaldà J, Massons J, Díaz F, et al. (2005) Crystal growth, structural characterization, and linear thermal evolution of KGd(PO₃)₄. Chem Mater 17: 822–828.
View Article
Google Scholar

[59] View Article

[60] Google Scholar

[ref21] 21. Parreu I, Carvajal JJ, Solans X, Díaz F, Aguiló M (2006) Crystal structure and optical characterization of pure and Nd-substituted type III KGd(PO₃)₄. Chem Mater 18: 221–228.
View Article
Google Scholar

[62] View Article

[63] Google Scholar

[ref22] 22. Parreu I, Solé R, Gavaldà J, Massons J, Díaz F, et al. (2003) Crystallization region, crystal growth, and phase transitions of KNd(PO₃)₄. Chem Mater 15: 5059–5064.
View Article
Google Scholar

[65] View Article

[66] Google Scholar

[ref23] 23. Parreu I, Solé R, Massons J, Díaz F, Aguiló M (2007) Crystal growth, crystal morphology and surface micromorphology of type III KGd(PO₃)₄ and KNd(PO₃)₄. Cryst Growth Des 7: 557–563.
View Article
Google Scholar

[68] View Article

[69] Google Scholar

[ref24] 24. Parreu I, Solé R, Massons J, Díaz F, Aguiló M (2007) Crystal growth and characterization of type III ytterbium-doped KGd(PO₃)₄: a new nonlinear laser host. Chem Mater 19: 2868–2876.
View Article
Google Scholar

[71] View Article

[72] Google Scholar

[ref25] 25. Parreu I, Pujol MC, Aguiló M, Díaz F, Mateos X, et al. (2007) Growth, spectroscopy and laser operation of Yb:KGd(PO₃)₄ single crystals. Opt Express 15: 2360–2368.
View Article
Google Scholar

[74] View Article

[75] Google Scholar

[ref26] 26. Solé R, Ruiz X, Pujol MC, Mateos X, Carvajal JJ, et al. (2009) Physical properties of self-flux and WO₃-containing solutions useful for growing type III KGd(PO₃)₄ single crystals. J Cryst Growth 311: 3656–3660.
View Article
Google Scholar

[77] View Article

[78] Google Scholar

[ref27] 27. Ettis H, Naïli H, Mhiri T (2003) Synthesis and crystal structure of a new potassium-gadolinium cyclotetraphosphate, KGdP₄O₁₂. Cryst Growth Des 3: 599–602.
View Article
Google Scholar

[80] View Article

[81] Google Scholar

[ref28] 28. Rekik W, Naïli H, Mhiri T (2004) Potassium gadolinium polyphosphate, KGd(PO₃)₄. Acta Cryst C60: i50–i52.
View Article
Google Scholar

[83] View Article

[84] Google Scholar

[ref29] 29. Zhang JJ, Zhang ZH, Zhang WG, Zheng QX, Sun YX, et al. (2011) Polymorphism of BaTeMo₂O₉: a new polar polymorph and the phase transformation. Chem Mater 23: 3752–3761.
View Article
Google Scholar

[86] View Article

[87] Google Scholar

[ref30] 30. Brown ID, Altermatt D (1985) Bond-valence parameters obtained from a systematic analysis of the Inorganic Crystal Structure Database. Acta Cryst B41: 244–247.
View Article
Google Scholar

[89] View Article

[90] Google Scholar

[ref31] 31. Kaminsky W (2007) From CIF to virtual morphology using the WinXMorph program. J Appl Crystallogr 40: 382–385.
View Article
Google Scholar

[92] View Article

[93] Google Scholar

[ref32] 32. Horchani K, Ferid M, Gacon JC, Lecocq S, Trabelsi-Ayedi M, et al. (2002) Structure refinement, infrared and Raman spectra of KDyP₄O₁₂. Mater Res Bull 37: 1259–1267.
View Article
Google Scholar

[95] View Article

[96] Google Scholar

[ref33] 33. Jungowska W, Znamierowska T (1990) Studies on thermal stability of mixed metaphosphates KLa(PO₃)₄ and K₂La(PO₃)₅. J Therm Anal Calorim 36: 2095–2098.
View Article
Google Scholar

[98] View Article

[99] Google Scholar

[ref34] 34. Koechner W (2006) Solid-State Laser Engineering (6th). New York: Springer Press. 54–73.

[ref35] 35. Judd BR (1962) Optical absorption intensities of rare-earth ions. Phys Rev 127: 750–761.
View Article
Google Scholar

[102] View Article

[103] Google Scholar

[ref36] 36. Ofelt GS (1962) Intensities of crystal spectra of rare-earth ions. J Chem Phys 37: 511–520.
View Article
Google Scholar

[105] View Article

[106] Google Scholar

[ref37] 37. Wang GF, Chen WZ, Lin ZB, Hu ZS (1999) Optical transition probability of Nd³⁺ ions in a alpha-Nd³⁺:LaSc₃(BO₃)₄ crystal. Phys Rev B 60: 15469–15471.
View Article
Google Scholar

[108] View Article

[109] Google Scholar

[ref38] 38. Parreu I (2006) Crystal growth and characterization of ytterbium or neodymium doped type III-KGd(PO3)4-a new bifunctional nonlinear and laser crystal. Tarragona: Rovira i Virgili University. 114.

[ref39] 39. Maksimova SI, Palkina KK, Loshchenov VB (1981) Structure and luminescence properties of α’-CsNd(PO₃)₄. Izv Akad Nauk SSSR Nerog Mater 17: 116–120.
View Article
Google Scholar

[112] View Article

[113] Google Scholar

[ref40] 40. Weber HP (1975) Nd pentaphosphate lasers. Opt Quant Electron 7: 431–442.
View Article
Google Scholar

[115] View Article

[116] Google Scholar

[ref41] 41. Sato Y, Taira T (2005) Comparative study on the spectroscopic properties of Nd:GdVO₄ and Nd:YVO₄ with hybrid process. IEEE J Sel Top Quantum Electron 11: 613–620.
View Article
Google Scholar

[118] View Article

[119] Google Scholar

Figures

Abstract

Introduction

Materials and Methods

1. Crystal growth

2. X-ray diffraction analysis

3. IR and Raman spectroscopy

4. Thermal analysis

5. Spectroscopic characterization

Results and Discussion

1. Crystal growth and chemical composition

2. Crystal structure

3. Crystal morphology and rocking curve

4. IR and Raman spectra

5. Thermal properties

6. Absorption spectrum

7. Fluorescence spectra analysis

Conclusions

Supporting Information

Acknowledgments

Author Contributions

References