Raman and infrared reflectance spectra study of CsLiB6O10

Spectrochimica Acta Part A 55 (1999) 2565 – 2571 www.elsevier.nl/locate/saa

Raman and infrared reflectance spectra study of CsLiB6O10 Wang Yufang *, Liu Jianjun, Hu Shifen, Lan Guoxiang Department of Physics, Nankai Uni6ersity, Tianjin 300071, People’s Republic of China Accepted 18 February 1999

Abstract Raman spectra and infrared reflectance spectra of a cesium lithium borate (CLBO: CsLiB6O10) single crystal have been recorded in various geometrical configurations. The vibrational modes are classified by the factor group analysis method and the spectral bands are assigned to vibrational frequencies of (B3O6)3 − ring and BO4 tetrahedra. A comparison of the vibrational frequencies of various borate crystals suggests a correlation between the highest vibrational frequency of borate crystals and their nonlinear optical efficiency. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Nonlinear optical crystal; Vibrational spectrum

1. Introduction Borate series crystals, including LiB3O5 (LBO) [1,2], CsB3O5 (CBO) [3], and b-BaB2O4 (BBO) [4], have received much attention because of their excellent properties for ultra-violet (uv) nonlinear optics. Recently, another new uv nonlinear optical crystal cesium lithium borate (CLBO: CsLiB5O10) is developed [5,6]. CLBO has a larger nonlinear optical coefficient, high laser damage threshold and transparent into far uv. Because CLBO has such characteristics, it would be useful to understand its fundamental structure and lattice vibrational properties. Vibrational spectroscopy can provide useful information. The Raman spectra and infrared * Corresponding author. E-mail address: [email protected] (W. Yufang)

reflectance spectra of CLBO have not been reported. Since the resolution of the Raman spectra at low temperature is better than that at room temperature, we expected that more complete data may be obtained at low temperature. In this paper, we recorded the Raman spectra at low temperature and at room temperature. The infrared reflectance spectra are also measured at room temperature. A comparison with the vibrational spectra of CBO and LBO was made.

2. Crystal structure and group analysis Sasaki et al. [7] had determined the crystal structure. CLBO has tetragonal structure with a space group of I42( d and two chemical formulae (36 atoms in all) in the primitive unit cell. The structure comprises of isolated Cs cations and a

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network of chains formed from (B3O7)5 − groups and Li+ cations. The basic structure unit (B3O7)5 − , which is shown in Fig. 1, is a six-membered ring in which two of the B atoms are three-fold coordinated and the third B atom is four-fold coordinated by O atoms. We use the factor group analysis method to classify the vibrational modes of CLBO crystal. The factor group of G point in Brillouin zone of CLBO crystal is isomorphic to the point group D2d. The characters x (i) q (R) of the irreducible representation of the D2d point group, the representation operations (R, tR) and the number of atoms U(R, tR) in the primitive cell which remain invariant under the operation (R, tR) are listed in the Table 1. The symmetry of the vibrational modes of CLBO crystal is classified by using the characterreducing equation:

prepared and mechanically polished, one sample was cut along directions perpendicular to x, y, and z, and another sample to x+y, x − y, and z. All Raman spectra were recorded on a Spex1403 double monochromator excited by 514.5 nm radiation from an Ar+ ion laser. For the low temperature measurements, the sample was mounted inside the vacuum chamber of a refrigerator with liquid nitrogen. The infrared reflectance spectra were recorded on a Nicolet-170SX Fourier transform infrared spectrometer with resolution 4 cm − 1 covering the range 160–4000 cm − 1. The IR spectra of E and B2 vibration modes were obtained with the polarization of incident light parallel to x- (or y-) and z-axis, respectively.

1 (i) n (i) q = % x q (R) × U(R, t R) ×( 9 1 + 2 cos uR) g

The infrared reflectance spectra of B2 and E modes of CLBO crystal are shown in Fig. 2. By using the Kromers–Kronig inversion technique, we obtained the dispersion curves of the refractive index, n(v), extinction coefficient, k(v), the real and imaginary parts of the complex dielectric coefficient, or (v) and oi (v), and the imaginary parts of the reciprocal of dielectric coefficient, (1/o)i. From the dispersion curves of oi and (1/o)i, we determined the transverse and longitudinal frequencies of the polar modes, vTO and vLO,

Where g is the order of point group. The calculated results indicate that there are 11A1 + 13A2 +13B1 +15B2 +28E vibrational modes, including three acoustic modes B2 +E. Among the optical modes 11A1 +13A2 +13B1 + 14B2 + 27E, the B2 and E modes are polar phonons and are both infrared and Raman active. A1 and B1 are nonpolar phonons and are only Raman active.

4. Results and discussion

3. Experimental CLBO crystals used here are grown by means of the top-seed Kyropoulos method [5]. In order to distinguish the vibrational symmetries of the Raman active modes, the Raman spectra were recorded in the various geometrical configurations, which are listed in Table 2. The x-, y-, and z-axis are selected paralleling to crystallographic a-, b-, and c-axis, respectively. The definition of the crystallographic axes is the same as that in the IRE standards on piezoelectric crystals [8]. In order to record Raman spectra in the various configurations listed in Table 2 and infrared reflectance spectra, two cuboid samples have been

Fig. 1. Structure of triborate group in the free state. , O atom; , B atom.

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Table 1 Characteristics of the reducible and irreducible representations of D2d point group D2d {R tR}

E {E (0,0,0)}

2S4 {S4 (0,0,0)}

C2 {C2 (0,0,0)}

2C%2 {C%2 (O,12,14)}

2sd {sd (0,12,14)}

A1 A2 Bl B2 E U{R t R} 91+2 cos uR x(R)

1 1 1 1 2 36 3 108

1 1 1 −1 0 4 −1 −4

1 1 1 1 −2 4 −1 −4

1 −1 1 −1 0 4 −1 −4

1 −1 −1 1 0 0 1 0

respectively, and the results are given in Table 3. The ordinary and extraordinary refractive indices (no and ne) of CLBO at near infrared region were determined from the n(v) dispersion curves, and the results give no =1.552 and ne =1.476. The value of ne is consistent with the result of [6], but the value of no is larger than the result of [6]. The refractive index difference between ordinary and extraordinary components is much larger which results in easy leakage of modes in Raman spectrum (discussed below). The Raman spectra of the CLBO crystal at room temperature (293 K) and at liquid nitrogen temperature (78 K) are similar, which shows that no phase transition occurs in this temperature range. Since the resolution is better at the low temperature than that at room temperature, only the Raman spectra of CLBO at 78 K are given here. The Raman spectra of the CLBO crystal in the various configurations at 78 K are shown in Figs. 3 and 4. The wavenumbers of the observed Raman bands are given in Table 3. In these Raman spectra, the assignment of vibrational modes symmetry of the observed Raman bands are usually straightforward except for z(yx)z¯ spectrum, which correspond to B2(LO) modes. From Fig. 3, it can be seen that more than twenty Raman bands are found in the B2(LO) spectrum. Apparently, this number is not consistent with the results of group theory analysis, some of them belonging to A1 or B1 vibrational modes. The leakages of vibrational modes are often observed in uniaxial crystals when either the incident or scattered light propagates in a direc-

x 2+y 2, z 2 x 2−y 2 Tz, xy Tx, Ty, xz, yz

tion close to the optic axis. The same case was found by Porto et al.[9] in Calcite. The convergent incident or scattered light directed along the optic axis can be strongly depolarized due to interference of its ordinary and extraordinary components. Under these circumstances, there would be an apparent leakage of some vibrational modes. For a ray diverging as little as 1° from the optic axis, the depolarization can be complete after a 5-mm path in calcite. In our experiments, both the divergence angles of incident light and collecting angles of scattered light greatly exceed 1°. In addition, the birefringence of CLBO is larger as mentioned above. These give rise to an apparent leakage of modes in Raman spectrum. The leakage Raman bands of A1 and B1 modes in z(yx)z¯ spectrum are easily subtracted by comparison with x(zz)y spectrum of A1 modes and x+y(x− y, x+ y)x− y spectrum of B1 modes. The symmetry assignments of Raman bands are given in Table 3, the observed Raman vibrational bands are 12A1 + 12B1 + 14B2 + 25E, which agree roughly with that predicted by group theory. As was previously mentioned, the structure of CLBO crystal consists of triborate groups (B3O7)5 − , which form a three-dimensional network. Therefore, some Raman and infrared reflectance bands of CLBO can be assigned to characteristic vibrations of the triborate groups. But the (B3O7)5 − groups in CLBO crystal are not isolated, so this assignment only is an approximate treatment. In addition, no detailed vibrational spectral data of (B3O7)5 − group can be available. Thus, it is nearly impossible to assign

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Table 2 The configuration of measuring Raman spectrum of single crystal CLBOa Symmetry of the vibration

Al

B1

B2(TO)

B2(LO)

E(TO)

E(LO)

Measurement configuration

x(zz)y

x+y(x−y, x+y)x−y

x(yx)y

z(yx)z¯

x(zx)z

x(yz)x¯

a

The label of the configuration follows standard porto notation.

the observed bands to specific vibrational modes of (B3O7)5 − group. According to [10], the nonplanar triborate group (B3O7)5 − can be equivalent to a planar six-membered ring (B3O6)3 − , and the vibrational bands of the crystal with this kind of ring can be attributed to the vibrational modes of the (B3O6)3 − ring and BO4 tetrahedra. On the basis of these reasons, we simply associate some vibrational bands of CLBO crystal with the vibrational modes of (B3O6)3 − ring and BO4 tetrahedra. The vibrational frequencies of the (B3O6)3 − ring have been calculated in our work for bBaB2O4[11]. The vibrational spectral data of BO4 tetrahedra can be found in [12]. The vibrational bands at about 1500 cm − 1 can be attributed to 6BO(A1% ) mode of (B3O6)3 − ring (calculated frequency 1519 cm − 1), this mode is mainly a stretching vibration of the extra-ring BO% bonds. The band at 1418 cm − 1 in A1 Raman spectrum and the corresponding infrared reflectance bands should be attributed to 6BO(E%) (calculated frequency 1427 cm − 1), which is also a kind of stretching vibration of extra-ring BO% bonds. The band at 1254 cm − 1 of B2 Raman spectrum should belong to 6BO(E%) (calculated frequency 1239 cm − 1), which is mainly the stretching vibration of intra-ring BO bonds. The Raman bands at about 1000 cm − 1 could be assigned to another 6BO%(E%) mode of (B3O6)3 − ring with calculated frequency 967 cm − 1. It is generally accepted that the ring-breathing vibrational mode 6ring(A1% ) of the six-membered ring with tetrahedra gives rise to a Raman band at the 760– 780 cm − 1 region [12]. The Raman bands in range of 782 – 785 cm − 1 in the spectra of CLBO crystal should be assigned to 6ring(A%1), its calculated frequency is 780 cm − 1. It is worth attention that the Raman band of CLBO crystal is not highly polarized and its wavenumber is larger than that of LBO [10] and CBO [13] crystals with same triborate group.

The Raman bands at about 700 cm − 1 are very weak and there exists a counterpart in IR spectra, these bands are assigned to rw(A%2%) (calculated frequency 682 cm − 1), which belongs to out-ofplane wagging vibration of (B3O6)3 − ring. The bands at about 620 cm − 1 are caused by the bending vibration of intra-ring BO bonds dring(A%1) (calculated frequency 634 cm − 1). The bands in range of 447–467 cm − 1 are attributed to another dBO(E%), involved mainly with the vibration of the intra-ring BO bonds. The Raman bands at about 370 cm − 1 are assigned to d(E%) of (B3O6)3 − ring, which is the bending vibration.

Fig. 2. Infrared reflectance spectra of B2(z) and E(x,y) modes of CLBO crystal at room temperature, obtained with the polarization of incident light parallel to x- and z-axis, respectively. The size of crystal used is x*y*z =7.5*5.1*3.9 mm3.

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Table 3 Frequencies (cm−1) of the vibrational modes and their assignments of the CLBO crystala A1 modes

B1 modes

B2 modes

Raman

Raman

Raman TO

E modes

IR LO

TO

Raman LO

TO

LO

TO

1468 vw

1487 w

1462 1379 1341 1286 1096 1030

1418 w 1254 vw 1102 vw

978 m 953 vw

782 s

620 vs 500 vs

954 vw 841 785 772 741

w vw vw vw

624 vw 543 vw 502 vw

378 vw 303 w 254 w

188 s

189 vw 141 w

53 vw 47 vw a b

1412 1273 1133

984 s

1030

918 s

947

1023 vw

370 vw 315 vw 246 w

98 w

1360 m 1251 w 1099 m

100 vw 76 w 52 vs

968 vw 916 vw

953 w

783 vw

783 m

704 vw 609 vw 540 w

740 715 621 544

vw vw m w

702 vw

716

540 vw

547

494 vw

500 404 366

361 vw

377 vw

395 vw 356 w

263 w

270 w

262 w

272

200 w

221

152 vw 97 vw 74 vs 62 vw

1108 vw 1046 vw 1014 vw 993 vw 959 vw

1294 1132 1051 1014

vw vw vw vw

976 vw

830 vw 783 vw 769 vw

836 w 784 vw 770 vw

705 614 555 510

706 615 566 512

vw vw w vw

Assignmentsb

IR

vw vw vw vw

467 vw 447 w 404 vw

467 vw 450 w 405 vw

245 vw 224 vw 188 vw

247 vw 224 vw 188 vw

125 w 98 vs 80 vw 63 vw 53 s 46 m

126 vw 99 s

LO w s s w m w

1510 1459 1370 1302 1131 1048

" " "

989 m 957 m 827 w

1008 978 837

765 vw

773

611 vw 552 w 504 vw

614 565 510

(B3O6)3− (B3O6)3− (B3O6)3−

6B−O%(A%1) 6B−O%(E%) 6B−O%(E%)

(B3O6)3−

6B−O%(E%)

BO4 6as(F2)

(B3O6)3−

456 vw

" 467 "

392 vw 347 vw

405 360

202 vw

229

6ring(A1% )

(B3O6)3− rw(A2¦) (B3O6)3− dring(A%1) BO4 6s(A1)

(B3O6)3− (B3O6)3−

dB−O(E%) d(E%)

153 w

86 s 65 s

75 vw 61 w 48 m

Relative intensity: vs, very strong; s, strong; m, medium, w, weak; vw, very weak. Assignment: 6, stretching vibration; d, bending vibration; r, out-of-plane vibration; s, symmetric; w, wagging; t, gorsional.

The reflectance bands at about 920 cm − 1 are most intense in all IR spectra, and the wavenumbers of their counterparts in Raman spectra are at 910–960 cm − 1 region. The band could be assigned to the asymmetric stretching vibration of BO4 tetrahedra 6as(F2). The Raman bands at 500– 555 cm − 1 region are strong polarized and the corresponding infrared reflectance band is very

weak, which led us to assign it to the symmetric stretching vibration 6s(A1) of BO4 tetrahedra. A similar assignments for 6as(F2) and 6s(A1) were suggested by Kamitsos [12]. The assignment of the characteristic vibrations mentioned above are presented in Table 3. The vibrational spectra and mode frequencies observed in CLBO crystal are not quite similar to

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that found in LBO and CBO crystals with same triborate groups, this may be due to the substitution of Li atoms for B atoms in the borate network of CLBO crystal. For the borate crystals only with three-coordinated or four-coordinated boronoxygen group, the maximum frequency observed in vibrational spectra is in the range of 1000 – 1300 cm − 1. Such crystals do not exhibit good nonlinear optical properties. In BBO, LBO, and CBO crystals, large nonlinear optical activity have been observed, the basic structural units of which are the plane sixmembered ring (B3O6)3 − or the six-membered ring with BO4 tetrahedra (B3O7)5 − with highest vibrational frequencies exceeding 1500 cm − 1.

Fig. 4. Raman spectra of CLBO crystal in various configurations at 78 K, corresponding to E(TO)[x(zx)z] and E(LO)[x(yz)x]. The size of crystal used is x*y*z = 7.5*5.1*3.9 mm3.

Fig. 3. Raman spectra of CLBO crystal in various configurations at 78 K, corresponding to A1[x(zz)y], B1[x +y(x −y, x + y)x − y], B2(TO)[x(yx)y] and B2(LO) [z(yx)z¯ ] modes. The sizes of crystals used are x*y*z = 7.5*5.1*3.9 mm3 and (x + y)*(x− y)*z= 3.6*3.4*3.9 mm3.

Thus, it can be seen that the highest vibrational frequency is an indication of the nonlinear optical activity [14]. Our vibrational spectra data for CLBO crystal also verifies this conclusion. From Table 3 we find that the largest value of LO-TO splittings in infrared reflectance spectra is 80 cm − 1. For BBO, LBO, and CBO crystals, this value goes to 116, 94, and 83 cm − 1, respectively [10,13,15]. The second harmonic generation (SHG) coefficients of BBO, LBO. CBO, and CLBO crystals can be found in literature’s [1– 3,6]. The largest SHG coefficient is observed in BBO crystal, the smallest is in CLBO crystal. LBO and CBO crystals possess medium SHG coefficients. Therefore, there may exist a correlation between the non-linear optical coefficients and the LO-TO splittings of vibrational modes of above borate crystals. Namely, the crystals with larger LO-TO splittings of polar vibrational modes have stronger non-linear optical coefficients.

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Acknowledgements We would like to thank Sasaki laboratory, Osaka Japan, for providing the CLBO crystal. This research was supported by the Doctoral Research Fund c97005521 of Chinese University.

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