Growth, structure and spectroscopic characterization of Nd3+-doped KBaGd(WO4)3 crystal with a disordered structure.

The undoped and the Nd(3+):KBaGd(WO(4))(3) crystals were grown by the top seeded solution growth (TSSG) method from a flux of K(2)W(2)O(7). The structure of the pure crystal was determined by the single-crystal X-ray diffraction method. It crystallizes in the monoclinic symmetry with space group C2/c. In the structure, K(+) and Ba(2+) ions share the same 8f site with occupancy of 0.464 and 0.536, respectively. The investigation of spectral properties of Nd(3+):KBaGd(WO(4))(3) crystal indicates that it exhibits broad absorption and emission bands, which are attributed to locally disordered environments around the Nd(3+) centers. The broad absorption band is suitable for diode laser pumping.

Introduction

The diode-laser pumped solid-state lasers are useful in a wide variety of applications in the fields of military [1], [2], industry [3] and medical treatments [4] due to their advantages, such as outstanding stability, high efficiency, compact size and long lifetime. A lot of well-known Nd3+-doped laser crystals (Nd:YAG, for example [5]) are commercially available, however, they can be hardly pumped with diode-lasers. Because they have very narrow absorption bands near their pumped wavelength compared to the emission bandwidths of common diode-lasers (several nanometers), they cannot absorb the pump energy of diode-lasers efficiently. Besides, the temperature stability of the emission wavelength of diode-lasers needs to be crucially controlled for laser crystals with narrow absorption bands, since the emission wavelength of the diode-lasers changes at 0.2–0.3 nm/K with the operating temperature of the laser device. [6] As a consequence, it is necessary to explore novel laser crystals with large absorption bandwidths for diode pumping.

The broadening of the spectral features can be expected for solid-state materials with multisites, defects, or local disorder [7]. In the past few years, there have been a lot of studies on the family of scheelite-related disordered molybdate and tungstate crystals AT(XO4)2 (A = monovalent Li, Na; T = trivalent Bi, Y, La-Lu; X = Mo, W) [8]–[10]. These disordered crystals have been demonstrated as promising materials in the domains of tunable and ultrashort femto-second laser pulse due to the obvious advantage of large spectral broadening. Recently, we synthesized a new triple tungstate KBaGd(WO4)3 compound also with disordered structure. Because of the mixed K(Ba) occupancy of the same crystallographic site, 8f, the structure of KBaGd(WO4)3 presents some local disorder around Gd3+ sites, and thus can lead to the spectral broadening when rare-earth luminescent ions (such as Nd3+) replace Gd3+. Therefore, this paper reports on the growth, structure and spectral properties of Nd3+-doped KBaGd(WO4)3 crystal.

Materials and Methods

1.Synthesis and Crystal Growth

Polycrystalline samples of undoped KBaGd(WO4)3 were synthesized by means of the solid-state reaction method. The synthesis process followed that of ref. [11]. To evaluate melting point of the synthesized KBaGd(WO4)3 compound, the thermal analysis was carried out by differential scanning calorimetry (DSC) using a NETZCH STA 499C Simultaneous Thermal Analyzer, which was performed up to 1563 K at a heating and cooling rate of 15 K min−1.

The undoped and Nd3+:KBaGd(WO4)3 crystals were grown by the top seeded solution growth method. The crystal growth was carried out in a vertical tubular furnace, as shown in Fig. 1. An AL-708 controller with controlling accuracy of ±0.1 K was used to control the furnace temperature and the cooling rate.

Figure 1

Schematic diagram of crystal growth apparatuses: (1) seed holder; (2) Al2O3 tube; (3) melt; (4) thermocouple; (5) crucible; (6) thermal insulation material; (7) seed.

2.Crystal Structure Analysis

A single crystal of undoped KBaGd(WO4)3 with dimensions of 0.10×0.10×0.10 mm3 was selected for X-ray diffraction determinations. The diffraction data were collected on a Mercury70 CCD diffractometer equipped with graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation at 293 K. A total of 1214 independent reflections were collected in the range of 3.35° < θ <27.45°, of which 1102 with I ≥2σ(I) were independent. The absorption correction based on the empirical PSI-scan technique was applied. The structure was solved by direct methods and refined by the full-matrix least squares technique with the SHELXL97 program package [12]. The final unweighted residual factor R which is used to judge the structure model is 0.0229. The weighted residual factor wR is 0.0514 with the weighting factor w:in the above equations, σ(F 2) is the estimated standard uncertainty of the observed reflections, F 2 and F 2 means the observed and calculated squared structure factors, respectively. After the refinement, the residual electron density (Δρ)max = 1.542, and (Δρ)min  = −2.045 e/Å3, (Δ/σ)max = 0.000. The value of goodness of fit S which is used to adjust the weighting scheme is 1.050. The details of the X-ray structure analysis are listed in (Table S1). The X-ray power diffraction (XRD) pattern of undoped KBaGd(WO4)3 was determined using a D-max-rA type diffractometer with Cu Kα radiation (λ = 1.54056 Å) at room temperature. The power electron diffraction pattern of undoped KBaGd(WO4)3 crystal which was carried out by a JEM-2010 transmission electron microscope.

3.Spectral Characterization

Since Nd3+:KBaGd(WO4)3 crystal with the monoclinic system is optically biaxial, three optical indicatrix axes (X, Y, Z) should be determined before the measurement of polarized spectra. One of the optical indicatrix axes (Y) is parallel to the unique two-fold crystallographic axis (b-axis, which is also the C 2 symmetry axis), and the other two lie in a perpendicular plane to this axis. The orientation of the crystal was performed by means of crossed polarized microscope, and the orientating procedure follows that of ref. [13]. The c-axis is at 16° anticlockwise with respect to the indicatrix Z-axis, and the angular relationship between the optical indicatrix axes and the crystallographic axes (a, b, c) for Nd3+:KBaGd(WO4)3 crystal is shown in Fig. 2. After orientation, a sample with dimensions of 5.5×3.8×2.6 mm3 and high optical quality was cut from the as-grown crystal, as shown in Fig.3 (c).

Figure 2

Relative orientation between the optical indicatrix axes (X, Y, Z) and the crystallographic axes (a, b, c) of the Nd3+:KBaGd(WO4)3 crystal.

The plot means view from the positive b-axis direction.

Figure 3

(a) Nd3+:KBaGd(WO4)3 crystal grown by TSSG method. (b) Stimulated facets marked by Miller indices (hkl). (c) An orientated rectangle sample cut from the as-grown crystal. Each face is perpendicular to one of the optical indicatrix axes.

The polarized absorption spectra were measured with a Perkin-Elmer UV-VIS-NIR spectrophotometer (Lambda 900). Excited by a continuous Xe-lamp at 805 nm, the polarized emission spectra were recorded using an Edinburgh Instruments FLS920 spectrophotometer. All the spectral experiments were carried out at room temperature. In spectral experiment, the electric field of the light is parallel to each one of the three optical indicatrix axes, E||X, E||Y, and E||Z.

Results and Discussion

1.DSC Analysis and Crystal Growth

The DSC result reveals that the undoped KBaGd(WO4)3 crystal melts incongruently (Fig. 4). In the heating cycle, an endothermic peak corresponding to a solid-liquid transformation is observed, which means the KBaGd(WO4)3 is decomposed at 1372 K, and then melts completely to form the liquid phase at 1477 K. The cooling curve shows two exothermic peaks with maxima at 1463 K and 1355 K, respectively, with the second one corresponding to resolidification of KBaGd(WO4)3.

Figure 4

DSC curves of undoped KBaGd(WO4)3 compound.

Since the KBaGd(WO4)3 crystal exists a phase transition at temperatures lower than the melting point, hence the best technique to grow it is the top seeded solution growth method (TSSG) [14]. The Nd3+:KBaGd(WO4)3 crystal was grown in a flux of K2W2O7 by the this method, where the molar ratio of KBaGd(WO4)3 to K2W2O7 is 1∶1. The chemical used were K2CO3, BaCO3, WO3 with analytical-grade as well as Gd2O3 and Nd2O3 with 99.99% purity. According to their stoichiometric composition, 3.0 at % Nd3+-doped KBaGd(WO4)3 and K2W2O7 were weighed. The weighed materials were mixed and put in a platinum crucible with a volume of 100 mm3. The mixture was slowly heated up to 1273 K in air atmosphere, and then maintained at that temperature for 48 hours to achieve a homogeneous melt. The saturation temperature of the solution was determined by repeated seeding. The starting growth temperature was about 1203 K. The crystals were grown at a cooling rate of 1K/day and a rotating rate of 45 rpm. When the growth ended, the grown crystals were carefully withdrawn from the solution and cooled to room temperature at 15 K h−1. The Nd3+:KBaGd(WO4)3 crystal with few inclusions and dimensions of 21×24×12 mm3 was obtained, as shown in Fig. 3(a). The morphology of the crystal is shown in Fig. 3(b), which was simulated with Bravais-Friedel and Donnay-Harker based WinXMorph software. [15].

The composition of the grown crystal was measured in an inductively coupled plasma by atomic emission spectrometry (ICP-AES) technique. For this purpose, the Nd3+:KBaGd(WO4)3 sample was heated at 363 K and dissolved in 37% HCl, then the signal of ICP intensity was quantified from the comparison of the standard elements and corrected by the calibration curve. The result of ICP is listed in Table 1. The measured molar ratio K: Ba: (Gd+Nd): W is close to 1∶1∶1:3, which agrees with the composition of Nd3+:KBaGd(WO4)3 compounds.

Table 1

Summary of the elemental analysis results of 3.08 at% Nd3+-doped KBaGd(WO4)3 crystal by the ICP-AES technique.

				(Gd+Nd)a
Elements	W	K	Ba	Gd	Nd
Required (Wt%)	51.22	3.63	12.75	14.02	0.54
Measured (wt%)	50.96±1.02	3.27±0.07	12.43±0.25	13.84±0.28	0.43±0.009
Calculated molar number (10−2)	27.72±0.55	8.36±0.18	9.05±0.18	8.80±0.18	0.30±0.006
Molar ratio	3.06±0.12	0.92±0.038	1±0.04	0.97±0.039	0.0309±0.0013
				(Gd+Nd) ≈ 1

the Gd3+ ions were generally substituted by Nd3+ ions in the Nd3+-doped KBaGd(WO4)3 crystal because the Gd3+ and Nd3+ ions have the same valence.

2.Crystal Structure

Fig. 5(a) shows the XRD pattern of undoped KBaGd(WO4)3 crystal that can be indexed on the basis of lattice parameters. The indexed result of power electron diffraction pattern of undoped KBaGd(WO4)3 crystal is in agreement with that of the powder X-ray diffraction pattern, as shown in Fig. 5(b), which confirmed the XRD result. The powder XRD data of undoped KBaGd(WO4)3 crystal are listed in Table S2.

Figure 5

(a) Powder XRD patterns of undoped KBaGd(WO4)3 crystal. (b) Powder electron diffraction pattern of the undoped KBaGd(WO4)3 crystal.

The crystallographic analysis of the undoped KBaGd(WO4)3 crystal reveals that it crystallizes in the monoclinic system with space group C2/c. The lattice parameters of KBaGd(WO4)3 crystal are: a = 17.544(4) Å, b = 12.1742(16) Å, c = 5.3202(9) Å, β = 105.498(11)°, and V = 1095.0(3) Å3 (also see CIF file: CIF S1). The atomic coordinates and thermal parameters are given in Table S3. In the refined structure, the Gd and W(2) fully occupy 4e sites, and W(1) as well as the eight types of O are found in different 8f sites. After least-squares refinement, the valence charge equilibrium and the magnitude of temperature factors demonstrate the existence of a statistic distribution of K and Ba, in single 8f site with occupancy factors fixed to 0.464∶0.536, which shows that KBaGd(WO4)3 crystal has a high disordered structure.

The crystalline structure is constituted by K/BaO8 distorted square antiprisms with C 1 symmetry, distorted GdO8 polyhedra which form chains lying along the c-axis (see Fig. 6 (a)) and two kinds of distorted WO4 tetrahedra. It can be described as layers stacked along the a-axis formed by corrugated six-membered rings of disordered K/BaO8 polyhedra, which share edges (Fig. 6 (b)). Adjacent layers are connected through distorted WO4 polyhedra and GdO8 chains (Fig. 7). The distinctive layered arrangement further increases the locally variable crystal field around Gd sites. The Gd3+ ions could provide appropriate sites for Nd3+ replacements because the Gd3+ and Nd3+ ions have the same valence, and additionally offer multiple distributions of cationic environments around these sites due to the coexistence of K+, Ba2+ and Gd3+ ions. Therefore, when Nd3+ replaces Gd3+ ions, the high disordered environment around the Nd3+ dopant ions leads to inhomogeneous broadening of the absorption and emission bands [16], [17].

Figure 6

View of the ab plane of the KBaGd(WO4)3 crystal.

(a) Chain formed by GdO8 disordered polyhedra. (b) Six-membered rings of square antiprisms K/BaO8.

Figure 7

View along the ac plane shows the layers stacked along the a-axis.

3.Spectral Properties

Fig. 8 shows the polarized absorption spectra of the Nd3+-doped KBaGd(WO4)3 at room temperature. These absorption lines are due to transition from the ground state 4 I 9/2 to the various excited states of Nd3+ ions. In the absorption spectra, the most interesting is the broad absorption bands in the range of 780–840 nm, which is close to the output wavelength of commercially diode laser devices. The absorption spectra are strong polarization dependent because of the anisotropy of the monoclinic crystal. The absorption spectrum for E||Y polarization is strongest among the three absorption spectra and it has a full-width at half-maximum (FWHM) of 14 nm at 803 nm, which is larger than that of the ordered crystal, like Nd3+:BaGd2(MoO4)4 crystal (4 nm, E||Z) or LaB3O6 (5 nm, E||X) [18], [19]. Such large FWHM caused by the highly disordered structure around Nd3+ centers is suitable for diode-laser pumping. This demonstrates the Nd3+-doped KBaGd(WO4)3 crystal can be pumped effectively and not restricted to the temperature stability of the output wavelength of diode-laser.

Figure 8

Polarized absorption spectra of Nd3+-doped KBaGd(WO4)3 crystal at 300 K.

The inset shows the spectra in the range of 780 to 840 nm.

The absorption cross-section σ was calculated using σα/N c formula, where α is the absorption coefficient and N c is the concentration of Nd3+. The concentration of Nd3+ ions in Nd3+:KBaGd(WO4)3 crystal was measured to be 3.08 at% (i.e. 1.12 ×10−20 cm−3), and the results of the σ are listed in Table 2.

Table 2

Comparison of spectral parameters of Nd3+:KBaGd(WO4)3 with other Nd3+-doped laser crystals.

Crystal	Polarization	Absorption(∼805nm)		Emission (∼1060 nm)		Disordered	Ref.
		FWHM	σa	FWHM	σe
		(nm)	(10−20cm2)	(nm)	(10−20 cm2)
KBaGd(WO4)3	E||X	11	4.5	19	4.5
	E||Y	14	6.9	24	6.5	Yes
	E||Z	19	4.1	19	4.1
LiLa(WO4)2	π-polarization	20	2.2	–	8.9	Yes	[24]
	σ-polarization	20	2.8	–	7.8
NaY(WO4)2	π-polarization	17	–	≈ 13	–	Yes	[25]
	σ-polarization	17	–	≈ 13	–
KBaGd(MoO4)3	unpolarized	9	11.1	24	5.17	Yes	[16]
BaGd2(MoO4)4	E||X	–	4.3	4	8.6
	E||Y	–	3.3	4	7.2	No	[18]
	E||Z	4	17.5	3	19.2
LaB3O6	E||X	3	5.0	4	10.2
	E||Y	3	4.5	3	8.3	No	[19]
	E||Z	4	5.5	4	7.2

The emission cross-sections were calculated using the Füchtbauer-Ladenbrug (F-L) equation, [20].

where I(λ) is the emission intensity at wavelength λ with q polarization, A is the emission probability which can be calculated from the Judd-Ofelt theory [21], [22], the refractive index n is calculated to be 1.83 using the method mentioned in Ref. [23].

Fig. 9 shows the wavelength dependences of the stimulated emission cross-sections. Three emission bands observed at 850–950, 1020–1150 and 1300–1450 nm are due to the transitions of 4 F 3/2→4 I 9/2, 4 F 3/2→4 I 11/2 and 4 F 3/2→4 I 13/2, respectively. The most important transition of 4 F 3/2→4 I 11/2 is centered at about 1060 nm. The FWHM for E||Y polarization is 24 nm, which is much larger than that of ordered structure crystals (see Table 2). The peak stimulated emission cross-sections for the transition of 4 F 3/2→4 I 11/2 are 4.5, 6.5 and 4.1×10−20 cm2 for E||X, E||Y and E||Z, respectively.

Figure 9

Polarized emission spectra of Nd3+-doped KBaGd(WO4)3 crystal excited with 805 nm radiation at 300 K.

4.Conclusion

The 3.08 at% Nd3+-doped KBaGd(WO4)3 crystal was successfully grown by the TSSG method. The thermal analysis shows it melts incongruently at 1372 K. The structure analysis indicates that the undoped KBaGd(WO4)3 crystal has a statistic distribution of K(0.464) and Ba(0.536) atoms. Each Ba(K) is coordinated by eight oxygen atoms to form a distorted polyhedron. This fact and its layered arrangement structure lead to locally disordered environments around the Gd3+ ions. When Gd3+ ions are substituted by Nd3+ dopant ions, the disordered structure results the broad FWHM of absorption and emission bands. Such broad FWHM of absorption band is suitable for diode-laser pumping. The Nd3+:KBaGd(WO4)3 crystal with broad FWHM of emission band is promising as a candidate for tunable and short pulse solid-state laser operation. The spectroscopic properties of the Nd3+:KBaGd(WO4)3 crystal are anisotropic with the largest absorption and emission cross sections for E||Y polarization. The Nd3+:KBaGd(WO4)3 crystal has large absorption and emission cross-sections, which are 6.9×10−20 cm2 at 805 nm and 6.5×10−20 cm2 at 1060 nm respectively.

Crystal data and structure refinement details for undoped KBaGd(WO

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Powder XRD data of undoped KBaGd(WO

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Atomic coordinates and thermal parameters for undoped KBaGd(WO

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The CIF file of the undoped KBaGd(WO

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