Lattice vibration spectra and thermal properties of SrWO4 single crystal

Chemical Physics Letters 426 (2006) 85–90 www.elsevier.com/locate/cplett

Lattice vibration spectra and thermal properties of SrWO4 single crystal Z.C. Ling

a,b,*

, H.R. Xia a,b,*, D.G. Ran a,b, F.Q. Liu a,b, S.Q. Sun H.J. Zhang b, J.Y. Wang b, L.L. Yu b

b

a,b

, J.D. Fan b,

a School of Physics and Microelectronics, Shandong University, Jinan 250100, PR China State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China

Received 18 April 2006; in final form 26 May 2006 Available online 3 June 2006

Abstract Large size strontium tungstate (SrWO4) single crystal was grown by Czochralski method. Raman scattering and infrared absorbance spectra measurements show that the characteristic lattice vibrational modes of SrWO4 arise mainly from the internal vibrations of the WO4 tetrahedra and partly by the external SrO8 polyhedra modes. Thermal properties including thermal expansion, specific heat, thermal diffusion and conductivities of the crystal were investigated to evaluate the thermal properties of SrWO4. The anisotropy of thermal properties was explained by crystal structure and its correlation with the lattice vibration spectra.  2006 Elsevier B.V. All rights reserved.

1. Introduction With the development of solid state lasers, there are several ways to extend the wavelength of lasers, one is doping crystals with different lanthanide ions, including Nd3+, Yb3+, Er3+, etc. [1–4]; another is to shift the laser wavelength by the application of stimulated Raman scattering (SRS) technique. Nowadays, a series of tungstate crystals including KGd(WO4)2, CaWO4, and BaWO4, etc., have attracting more and more attentions due to their large nonlinear optical susceptibility v3, making them to be efficient frequency converters by SRS [5–10]. More recently, highly efficient SRS experiment of strontium tungstate (SrWO4) was conducted by Shuanghong Ding et al., with a maxim total conversation efficiency 70% for the first and second stokes [11]. Spontaneous Raman can be a guide for SRS experiment to give constructive hints to optimize of the design of SRS cavity geometries. Hence we conducted the polarized Raman spectra and infrared absorbance measurements to

*

Corresponding authors. Fax: +86 53188565167. E-mail addresses: [email protected] (Z.C. Ling), hrxia@sdu. edu.cn (H.R. Xia). 0009-2614/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.05.093

investigate the phonon properties of lattice vibration in SrWO4 crystal. Thermal properties including specific heat, thermal expansion and thermal diffusion and conductivities of the crystal are important aspects to be considered in Raman crystals [13–15]. Thermal loading in the SRS operations often causes a temperature gradient in the crystal and leads to thermal expansion, which in turn causes thermal lensing and other thermal-optic effects. However, if the Raman crystal possesses larger specific heat and higher thermal conductivities, the heat can be more easily transferred to the environment, thereby decreasing the thermal loading effect [16]. We conducted the thermal properties experiments of SrWO4 to investigate the anisotropy of thermal properties from the viewpoint of crystal structure and its correlation with the lattice vibration spectra. 2. Experiment Large size SrWO4 single crystal up to 22-mm diameter · 40-mm length was grown by the Czochralski method. After grown, a polished sample is cut carefully along the X, Y, Z directions with precision of 1 in. and into a size of 7 · 6 · 4 mm3 for Raman measurement. There is a selection of the X//a, Y//b, and Z//c axis. The Raman spectra

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Z.C. Ling et al. / Chemical Physics Letters 426 (2006) 85–90

were accumulated using a Ventuno21 NRS-1000DT instrument at room temperature in frequencies ranging from 50 to 1200 cm1. The powder absorbance spectrum of Fourier-Transform infrared (FT-IR) was collected by the Nicolet-Nexus 670 FT-IR spectrometer from 50 to 1200 cm1. The thermal expansion of SrWO4 crystal was measured in the temperature range 323.15–1173.15 K at a constant rate of 5 K/min by using a Thermal mechanical Analysis (Perkin–Elmer Diamond TMA). Specific heat measurement was performed on the differential scanning calorimetry (NETZSCH STA 499C) with a sapphire calibration at a constant rate of 10 K/min. The thermal diffusion coefficients of SrWO4 crystal were measured on the laser flash apparatus (NETZSCH LFA447 Nanoflash) at an interval of 30 K in the temperature range from 298.15 to 563.15 K.

Among them, it is clearly that there are no more than 18 theoretically observable Raman peaks and 12 infrared absorption bands respectively. The concerned Raman scattering tensors are [19] 0 1 0 1 a 0 0 c d 0 B C B C Ag : @ 0 a 0 A Bg : @ d c 0 A

3. Space group theoretical consideration

Ag þ Bg : X ðYY ÞX ;

At room temperature, SrWO4 belongs to space group C64h ðI41 =aÞ, and contains two formulas in the primitive cell [17]. According to the crystal structure, the Wyckoff positions are 4a for W, 4b for Sr, 16e for O atoms. The reducible representations for the space group C64h at the C point of the first Brillouin zone are given in Table 1, in terms of The International Tables for Crystallography [18]. Using the result in Table 1, the irreducible representation of the normal modes can be obtained as follows: 16f : 3Ag þ 3Bg þ 3Eg þ 3Au þ 3Bu þ 3Eu 4b : Bg þ Eg þ Au þ Eu 4a :

Bg þ Eg þ Au þ Eu

Then the irreducible representation of the lattice vibration of the SrWO4 crystal in a unit cell can be obtained as follows: C ¼ 3Ag þ 5Bg þ 5Eg þ 5Au þ 3Bu þ 5Eu On the basis of the character table of the point groups C4h, the Raman (R) and infrared (IR) active optical modes at zero wave vector are

0 0 b 0 1 0 0 e B C Eg ð1Þ : @ 0 0 f A e f 0

0

0 0

0

0 B Eg ð2Þ : @ 0 f

0 0 e

1 f C e A 0

Therefore, the scattering geometrical configurations are determined as follows: X ðZZÞX ;

Ag :

Bg : ZðXY ÞZ; and

X ðYZÞX ; Y ðXZÞY

for Eg :

4. Results and discussion 4.1. Polarized Raman results and discussion Fig. 1 shows Raman spectra of the SrWO4 crystal corresponding to symmetry species Ag, Bg, Eg, with scattering geometry projects X ðZZÞX ; ZðXY ÞZ; X ðYY ÞX , and X ðYZÞX , respectively, recorded at room temperature in the frequency ranging from 50 to 1200 cm1. From the point of view of lattice dynamics, these stronger Raman peaks imply the strong interactions between the ions, which mainly arise from the stretching and bending of the shorter metal–oxygen bonds within the anionic groups. Therefore, the WO4 tetrahedra in the SrWO4 crystal, accordingly, should play an important role in the lattice vibration spectra. As a matter of experience for MO4 tetrahedra [20], they follow the relationship as: m3 > m1 for the stretching vibra(ZZ)

2

Cvib ¼ 3Ag ðRÞ þ 5Bg ðRÞ þ 5Eg ðRÞ þ 4Au ðIRÞ þ 4Eu ðIRÞ

1 0

Intensity (a.u)

2

Table 1 The reducible representations for C64h at C point Symmetry species of point group C4h

Seitz operator

E C4 C2 C34 I S34 rh S4

{1[0 0 0]|T(0, 0, 0)} {4[0 0 1]|T(0, 1/2, 1/4)} {2[0 0 1]|T(0, 0, 0)} {43[0 0 1]|T(1/2, 0, 3/4)} f1½0 0 0jT ð0; 1=2; 1=4Þg f4½0 0 1jT ð0; 0; 0Þg f2½0 0 1jT ð1=2; 0; 3=4Þg f43 ½0 0 1jT ð0; 0; 0Þg

Character of Wyckoff position 16f

4b

4a

24 0 0 0 0 0 0 0

6 0 2 0 0 2 0 2

6 0 2 0 0 2 0 2

(XY)

1 0 (YY)

1

* (YZ)

1 *

0 200

400

600

800

1000

1200

Raman shift cm-1 Fig. 1. Raman spectra of SrWO4 recorded from 50 to 1200 cm1 at room temperature with scattering geometry project X ðZZÞX ; ZðXY ÞZ; X ðYY ÞX , and X ðYZÞX , respectively. w denotes the leakage of peaks from other configurations.

Z.C. Ling et al. / Chemical Physics Letters 426 (2006) 85–90

cm1 show obvious different scattering intensity yet other peaks remain the same intensity, that might be an indication for the not fully polarization of the two peaks. That is also found in the NaY(WO4)2 by Macalik et al. [23]. The authors ascribe that by the local disorder of the cationic layers, which forms the domain-like structure and leads the not fully polarized modes to be seen in other configurations. However, except for the not fully polarized peaks 923 (m1) and 339 (m2) cm1, the remains 803, 384, 240, 137, and 103 cm1 should be Eg modes. The Raman shift of the observed vibration modes of the SrWO4 crystal are listed in Table 2. Spontaneous Raman scattering data could be used to evaluate the feasibility of SrWO4 as a stimulated Raman scattering (SRS) converter. In that case, the peak at 923 cm1(m1) is dominant for E perpendicular to the c-axis, and the mode at 841 cm1(m3) is dominant for E perpendicular to the b-axis, which can be meaningful hints to optimize the design of SRS cavity geometry.

tions and m4 > m2 for the bending vibrations. Compared to the Raman data of Na2WO4, KYb(WO4)2, CaWO4, NaY(WO4)2 crystals, etc. [12,21–23], Raman peak 928 cm1 could be assigned as m1 (W–O symmetric stretching), peak 338 cm1 as m2 (O–W–O symmetric bending), 841 cm1 as m3 (W–O anti-symmetric stretching), and 375 as m2 (O–W–O anti-symmetric bending), respectively. In X ðZZÞX spectrum, we observe three Ag modes locating at 923, 338, and 193 cm1, which agrees well with our prediction. In the ZðXY ÞZ projection corresponding to the Bg modes, five peaks can be found in the spectrum. What’ interesting is that, the peak position of ZðXY ÞZ for Bg modes is the same as that in X ðYY ÞX projection, which should record Ag and Bg modes simultaneously. However, Ag modes are often observed not fully polarized and due to the cationic disorder. So it is often observed in the Bg and Eg configurations [23]. Considering a 75 cm1 peak observed by Scott et al. [12], we could assign 841, 375, 339, 192 cm1 as Bg modes. From X ðYZÞX spectrum of Eg modes we observe seven Raman peaks. Comparing the two projections X ðYZÞX and Y ðXZÞY , as shown in Fig. 2, 923(m1) and 339(m2)

803

1.4

4.2. Infrared analysis The infrared spectrum of SrWO4 is shown in Fig. 3. Compared to the IR data of Na2WO4, CaWO4, NaY(WO4)2 crystals, etc. [12,21,23], it is easy to find that m3 and m4 locate at 818 and 413 cm1, respectively. The broad band around 300 cm1 could be ascribed as m2, and it splits into 312 and 274 cm1, which might also be ascribed by the cationic disorder mentioned above. Other modes below 200 cm1, such as 73, 135 and 193 cm1 could be attributed as external modes between the tetrahedra and Sr atoms. It is worth noting that 135 and 193 cm1 modes exhibit strong intensity, especially the 193 cm1 peak split into 193 and 162 cm1, which might implies the striking coupling of the Sr atoms and the around O atoms. These modes could be may be treated as Sr–O stretching and O–Sr–O bending vibrations being active in the 150–250 and 100–140 cm1 spectral regions, respectively. As can be seen in the infrared spectrum, two strong absorption bands below 350 cm1 indicate striking coupling of the phonon energy between the WO4 and Sr atoms, which

(YZ) (XZ)

Intensity (a.u)

1.2 1.0 0.8 0.6

923

384 0.4

137

103

339 240 *

0.2 200

400

600

800

1000

87

1200

Raman shift cm-1

Fig. 2. Comparison of modes of SrWO4 recorded in the projections X ðYZÞX and Y ðXZÞY corresponding to Eg. w denotes the leakage of peaks from other configurations.

Table 2 Raman and IR phonon frequency (cm1) measured in the SrWO4 crystal X ðZZÞX

ZðXY ÞZ

X ðYY ÞX

X ðYZÞX

Ag

Bg

Ag + Bg

Eg

923 (m1)

923 841 (m3) 375 (m4)

923 (m1) 841 (m3) 375 (m4)

923 803 (m3) 384 (m4)

339 (m2)

339 (m2)

339

192

192

240

338 (m2) 193

137 103

IR

818 552 471 413 312 274 193 162 135 73

Approximate assignments

WO4 symmetric stretching WO4 anti-symmetric stretching WO4 anti-symmetric bending

WO4 symmetric bending Sr–O stretching Sr–O bending WO4 libration

Z.C. Ling et al. / Chemical Physics Letters 426 (2006) 85–90

0.6

0.4

0.2

0.0 200

400

600

800

1000

1200

wavenumbers cm-1 Fig. 3. IR absorbance spectra of SrWO4 from 50 to 1200 cm1 at room temperature.

might be the sign for good thermal diffusion and conductivity in SrWO4 crystal. 4.3. Thermal properties 4.3.1. Thermal expansion and density of SrWO4 The thermal expansion coefficient [aij] of crystal is a symmetrical second tensor [24]. The solid lines in Fig. 4 are the thermal–expansion ratio curves along the three crystallographic axes. The thermal–expansion ratio is almost linear over the entire measured temperature range from 323.15 to 1173.15 K. The average linear thermal–expansion coefficients in the temperature range from 373.15 to 1173.15 K are aa = 5.0979 · 106/K, ab = 5.3900 · 106/K, and ac = 17.2353 · 106/K. These results indicate that the crystal possesses a large anisotropic thermal expansion, which may due to the anisotropy of the crystal structure. Along the c-axis exist 0.0200 6.46 6.44

0.0150 0.0125

Density (gcm -3)

Expansion ratio dL/dL0

0.0175

6.42 6.40 6.38 6.36 6.34 6.32

0.0100

6.30 6.28 200

400

600

800

1000

1200

Temperature (K)

0.0075 0.0050

a b c

0.0025 0.0000 200

400

600

800

1000

1200

1400

Tem perature (K) Fig. 4. Thermal–expansion ratio curves of SrWO4 along different directions. The inset shows the calculated density variation of SrWO4 at different temperatures.

4.3.2. Specific heat of SrWO4 The specific heat of SrWO4 versus temperature is measured by the method of differential scanning calorimetry and shown in the inset of Fig. 5. It is easy to find the value of specific heat of our crystal remains almost constant with a value of 0.30–0.34 J g1 K1 in the temperature range from 333.15 to 1063.15 K, which is in compliance with the Dulong–Petit law as have been shown in the BaWO4 crystal. Hence we could get a prediction that the Debye temperature of the SrWO4 is apparently not higher than 333.15 K. 4.3.3. Thermal diffusion and thermal conductivity of SrWO4 The thermal conductivity and thermal diffusion coefficients are also symmetrical second-rank tensors [24], which are diagonal like the thermal expansion coefficient tensor in the principal coordinate system. The thermal conductivity tensor components were then calculated by using the following formula: 1.8

3.6

0.6 0.5

1.7 1.6 1.5

3.4

0.4 0.3

3.2

0.2

3.0

0.1

1.4

0.0 200

400

600

800

1000

Temperature K

1200

2.8

1.3 2.6

1.2

[100] direction [001] direction

1.1

2.4

1.0 300

350

400

450

500

550

Thermal conductivity(Wm-1 K-1 )

Absorbance(a.u)

0.8

the two Sr atom layers while only one layer of Sr separate the tetrahedra along a- or b-axis. Comparing to small distortion of WO4 tetrahedra, as is shown by neutron diffraction studies on the SrWO4 crystal in Ref. [17], SrO8 polyhedra is much more easily distorted with the increasing of temperature. However, the value of thermal expansion of SrWO4 crystal is smaller than the BaWO4 [25], indicating better thermal tolerance and less likely to crack under the same cooling rate. The density of SrWO4 was calculated by the same method in reference [25], using the thermal expansion data mentioned above. The density of SrWO4 had been measured to be 6.439 g cm3 at 295.15 K by using the buoyancy method. The density versus temperature of the crystal in the temperature range from to is shown in the inset of Fig. 4. It is found to be linearly decreasing as the temperature increasing and the density is calculated to be 6.298 g cm3 at 1173.15 K.

Specific heat Jg-1 K-1

1.0

Thermal diffusion coefficient ( mm 2/s)

88

2.2 600

Temperature (K) Fig. 5. (Solid lines) Thermal diffusion coefficients of SrWO4. (Dotted lines) The calculated thermal conductivity of SrWO4 crystal. The inset shows the specific heat curve versus temperature of SrWO4 crystal.

Z.C. Ling et al. / Chemical Physics Letters 426 (2006) 85–90

k ¼ kqC p where k, k, q, Cp denote the principal thermal conductivity, thermal diffusion coefficient, density, and specific heat of the crystal, respectively. The solid lines in Fig. 5 show the thermal diffusion coefficients of SrWO4 crystal at temperature range from 303.15 to 563.15 K at 20 K intervals. The dotted lines show the calculated thermal conductivity of SrWO4 crystal. From the Fig. 5, the thermal diffusion and thermal conductivity of SrWO4 exhibit relatively anisotropy and decrease with the increasing of temperature. This could be ascribed by the decrease of phonon free path with the enhancement of molecular thermal dynamics. Thermal diffusion and conductivity along the [1 0 0] direction are better than that along [0 0 1], with the k[1 0 0]/k[0 0 1] = 1.06. From the point view of crystal structure, along [0 0 1] more Sr atoms (two according to the unit cell) layers interrupt the lattice vibration of WO4 tetrahedra. However, only one layer of Sr atoms exists along [1 0 0] direction between the two WO4 tetrahedra, which might indicate less interruption of thermal diffusion and phonons energy transfer. That is also testified by the polarized Raman spectra of SrWO4, which show that phonons propagate along [0 0 1] direction (with projection ZðXY ÞZ) possess lower Raman scattering intensity than that along [1 0 0] direction (with projection X ðYZÞX ) of external modes of SrO8 polyhedra below 300 cm1. Hence the thermal diffusion and conductivity along [1 0 0] are better than those along [0 0 1]. It should be noted that the thermal conductivity of SrWO4 are 3.165 and 2.987 W m1 K1 along [1 0 0] and [0 0 1] directions at 303.15 K, better than that of BaWO4 crystal with 2.324 and 2.256 W m1 K1 [25]. That might due to the fact that Sr is lighter in mass and smaller in ionic radius than Ba atom, which leads to higher phonon energy in the O–Sr–O vibration than that in O–Ba–O link. For Raman crystals, the heat generated in the SRS process is deposited within a region and would be conducted away from this region leading to a temperature gradient in the crystal and hence a refractive index profile, which acts as a lens, just as in a solid-state laser crystal. In most laser crystals, the contribution by thermo-optic coefficient (dn/dT) is assumed to be the dominant one for the thermal lens. For most of the Raman crystals, thermo-optic coefficients (dn/dT) are negative, for instance, the isostructural CaWO4 possesses the values as 7.1 · 106 (o) and 10.2 · 106 (e) [26]. Higher thermal conductivities in SrWO4 crystal imply potential for better performance against the thermal focus and other negative thermal-optic effects in the laser operations. 5. Conclusion Large size SrWO4 single crystal was grown by Czochralski method. Raman scattering and infrared absorbance spectra measurement was performed to assign the phonons

89

properties of the crystal combined with the Space group analysis. The experiments show that the characteristic lattice vibrational modes of SrWO4 arise mainly from the internal vibrations of the WO4 tetrahedra and partly by the external SrO8 polyhedra modes. Two not fully polarized Raman peaks were observed in other configurations, which might due to the cationic disorder in SrWO4 crystal. Thermal properties including thermal expansion, specific heat, thermal diffusion and conductivities of the crystal were investigated to evaluate the thermal properties of the crystal thoroughly. The anisotropy of thermal properties were explained from the viewpoint of crystal structure and its correlation with lattice vibration spectra. The external modes derived from Sr atoms in the crystal play an important role in the thermal expansion and especially the thermal diffusion and conductivity properties. From the point views of lattice vibration and thermal properties, SrWO4 crystal is a promising candidate for Raman lasers. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No. 10274043) and The State Key Laboratory of Crystal Materials of Shandong University, China also supports it. References [1] J.E. Bernard, A.J. Alcock, Opt. Lett. 18 (1993) 968. [2] A.A. Kaminskii, Phys. Status Solidi A 148 (1995) 9. [3] V. Petit, J.L. Doualan, P. Camy, V. Menard, R. Moncorge, Appl. Phys. B 78 (2004) 681. [4] H.R. Xia, H.D. Jiang, M. Guo, J.Y. Wang, J.Q. Wei, X.B. Hu, Y.G. Liu, Opt. Commun. 188 (2001) 233. [5] H.M. Pask, Prog. Quantum Electron. 27 (2003) 3. [6] H.M. Pask, J.A. Piper, Opt. Commun. 148 (1998) 285. [7] P. Cerny, H. Jelinkova, P.G. Zverev, T.T. Basiev, Prog. Quantum Electron. 28 (2004) 113. [8] Y. Chen, L. Major, V. Kushawaha, Appl. Opt. 35 (1996) 3203. [9] I.V. Mochalov, Opt. Eng. 366 (1997) 1660. [10] O. Musset, J.P. Boquillon, Appl. Phys. B: Lasers Opt. 65 (1997) 13. [11] Shuanghong Ding, Xingyu Zhang, Qingpu Wang, Fufang Su, Shutao Li, Shuzhen Fan, Zhaojun Liu, Jun Chang, Sasa Zhang, Shumei Wang, Yuru Liu, IEEE J. Quantum Electron. 42 (1) (2006) 73. [12] S.P.S. Porto, J.F. Scott, Phys. Rev. 157 (1967) 716. [13] D. Elwell, H.J. Scheel, Crystal Growth from High-temperature Solutions, Academic Press, London, 1975, p. 138. [14] K.A. Jackson, Liquid Metals and Solidification, ASM, Cleveland, 1958, p. 174. [15] K.A. Jackson, Prog. Solid State Chem. 4 (1967) 53. [16] P. Cerny, H. Jelinkova, M. Miyagi, T.T. Basiev, P.G. Zverev, Proc. SPIE 4630 (2002) 108. [17] Erdogan Gurmen, Eugene Daniels, J.S. King, J. Chem. Phys. 55 (1971) 1903. [18] T. Hahn, The International Tables for Crystallography, USA, Boston, 1983, p. 469. [19] R. Loudon, Adv. Phys. 13 (1964) 423; R. Loudon, Errata 14 (1965) 629. [20] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compound, fourth edn., Wiley, New York, 1986, p. 134.

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