IR and polarized raman spectra of a single crystal Rb3H(SeO4)2 in the ferroelastic phase

Pergamon

SpectrochimicaActa, Vol. 51A, No. 3, pp. 429-435, 1995 Copyright(~) 1995Elsevier Science Ltd Printed in Great Britain. All rights reserved 0584-8539(94)E0115-Q 0584-8539/95 $9.50+ 0.00

IR and polarized Raman spectra of a single crystal Rb3H(SeO4)2 in the ferroelastic phase G. SURESH,t R. RATHEESH,t V. U. NAYAR,tII M. ICHIKAWA:~ and G. KERESZTURY§ t Department of Physics, University of Kerala, Kariavattom, Trivandrum 695 581, India :~Department of Physics, Faculty of Science', Hokkaido University, Sapporo 060, Japan § Central Research Institute for Chemistry of the Hungarian Academy of Sciences, II Pusztaszeri UT 59-67H-1525 Budapest, Hungary (Received 7 December 1993; in final form and accepted 4 April 1994) Abstract--Infrared and polarized Raman spectra of a single crystal Rb3H(SeO4)2 and the polycrystalline spectra of its deuterated analogue are recorded and analysed. Bands indicate the existence of SeO42- ions and HSeO£ ions, in agreement with a non-centrosymmetric dimer in the crystal. The difference between v(Se-O) and v(Se-OH) bands is a measure of the hydrogen bond strength and it decreases on deuteration. The presence of an absorption continuum along with a transmission window shows that the proton motion is coupled with those of the oxygen atoms in the SeO]- ions. The ABC bands in the OH stretching region also support strong hydrogen bonding in the crystal. The lower wave numbers for the stretching OH mode show that the dimer formation is quite anharmonic, as judged from the isotopic shift. INTRODUCTION T r i r u b i d i u m h y d r o g e n diselenate belongs to a family of zero dimensional h y d r o g e n b o n d e d crystals with the general f o r m u l a M3H(XO4) 2 where M = Rb, K, Cs and X = Se, S. T h e various unique characteristics of these crystals are their isomorphism, low t e m p e r a t u r e phase transitions and geometrical isotope effect [1, 2]. As a part of the spectroscopic investigations on these i s o m o r p h o u s series of ferroelectrics, particular attention has been paid to Rb3H(SeO4)2, which has the longest crystallographically s y m m e t r i c h y d r o g e n b o n d (2.514 A,) with the p r o t o n s disordered within a dimer [3]. It is interesting to note that this c o m p o u n d does not show any low t e m p e r a t u r e phase transition, w h e r e a s it a p p e a r s on d e u t e r a t i o n as a s y m m e t r y invariant transition [4-6]. B o t h the p r o t o n a t e d and d e u t e r a t e d crystals are ferroelastic at r o o m t e m p e r a t u r e while the low t e m p e r a t u r e phase o f the d e u t e r a t e d crystal is antiferroelectric [7]. In this paper, the I R and polarized R a m a n spectra of a single crystal of Rb3H(SeO4)2, and the polycrystalline spectra of its d e u t e r a t e d analogue Rb3D(SeO4)2 are analysed to understand the nature of h y d r o g e n bonding, the isotope effect and the coupling b e t w e e n the internal vibrations in this zero dimensional h y d r o g e n b o n d system. EXPERIMENTAL Single crystals of Rb3H(SeO4)2 were prepared by the method reported by Gesi [5]. The deuterated compound was prepared by recrystallization from D20 solution containing a stoichiometric ratio of Rb2SeO4 and H2SeO4 [8]. A single domain was cut into paralleiopiped form with faces perpendicular to the axes of the index ellipsoid using a polarizing microscope. A Spex Ramalog 1401 double monochromator equipped with a Spectra Physics model 165 argon ion laser (2 = 488 nm) was used to record the Raman spectrum at room temperature (300+ 3 K), with a spectral resolution better than 3 c m - L Six different scattering geometries were utilized. The Raman spectrum of the deuterated compound was recorded in the powder form. IR spectra were recorded with a Perkin-Elmer 983 spectrophotometer using the KBr pellet technique. FACTOR GROUP ANALYSIS Rb3H(SeO4)2 and Rb3D(SeO4)2 crystallize in the monoclinic space g r o u p A 2 / a (C6h) with four f o r m u l a units in a primitive cell [3, 6]. T h e monoclinic cells of both the Author to whom correspondence should be addressed. 429

G. SURESH et al.

430

Table 1. Factor group analysis of Rb3H(SeO4) 2 Se04 Factor group species Czh

Ag Bg Au B.

Internal modes 1

2

3

4

2 2 2 2

4 4 4 4

6 6 6 6

6 6 6 6

External modes 6T, 6T, 6T, 6T,

6R 6R 6R 6R

4 x 3 Rb

4 x 1H

Optical modes

6T 6T 12T 12T

3T 3T 3T 3T

39 39 45 45

Acoustic modes

Spectral activity

-1 -2

R R IR IR

compounds are pseudo hexagonal. Two adjacent SeO4 ions are linked by a symmetric short hydrogen bond to form H(SeO4) 3- dimer with the proton either at the centre of symmetry or disordered between two equivalent sites in a double minimum potential. The atomic arrangement consists of these hydrogen bonded S e O 4 dimers and oxygenopolyhedra around Rb. The selenate groups occupy C~ site in the C2h symmetry. There are two kinds of Rb cations, Rb(1) occupying a Ci site and Rb(2) a general site. The Rb(1)-O bond lengths are shorter than the Rb(2)-O bond length by 0.029/~. The 0(2) • • • 0 ( 2 ' ) distances in the deuterated compound are larger by 0.018/~ than those in the protonated compound. The group theoretical analysis [9] predicts 165 (Table 1) fundamentals excluding the acoustic modes which are distributed as F = 39Ag + 39Bg + 44A. + 43B..

R E S U L T S AND DISCUSSION

SeO4 Vibrations A free selenate ion with tetrahedral symmetry possesses four internal vibrations: a symmetric stretching mode, v~(A l), a symmetric deformation mode, v2(E), an asymmetric stretching mode, v3(F2), and an asymmetric deformation mode, v4(F2). All the modes are active in the Raman while only the /72 species are active in the IR [10]. The wave numbers of the bands observed for the six orientations and the assignments are presented in Table 3. The most intense band around 866 cm-l in all the Raman scattering geometries is the totally symmetric stretching mode of the selenate ion (Fig. 1). If there is a substantial difference between the crystal fields at the sites of different types of ions the inequality between the force constants of the bonds will lead to the observation of separate bands corresponding to each type of ion. As the H atom in the H(SeO4) 3- dimer can exist at two disordered positions around the centre of symmetry, SeO 2- and HSeO4 ions are likely to coexist in the crystal with vibrational interaction between them [11]. Crystallographic data [3] reveal that the anion contains one distinctly longer Se-O bond (1.692 ,~) to which the proton is attached. Two of the other bonds have nearly the same length and the third one is slightly shorter. Therefore, the HOSeO3 ion can be approximated to the C3o symmetry group. The medium intense band around 776 cm- 1 in the Raman spectrum (Table 2) is assigned to the stretching mode of the Se-OH donor group [12]. The intensities of both 866 and 776 cm-~ bands are highest in the axx, ayy and azz polarizability tensor components, which belong to the Ag species of the C2h factor group. The appearance of this mode in the Bg species indicates the distortion of the anion. The frequency difference Vx_o- Vx_oH (X = Se, S) can be taken as a spectroscopic indication of the measure of hydrogen bonding [13] (Fig. 4). This separation is found to be less than that (112cm -1) observed in the isostructural (NH4)3H(SO4)2 [11]. It shows that the hydrogen bond is stronger in RbaH(SeO4)2 than those in the ammonium compound. The frequency difference decreases upon deuteration in agreement with the structural data.

IR and polarized Raman spectra in the ferroelastic phase

431

i,-

z la,i h,, z

1100

900

700

500

WAVENUMBER

300

100 0

(c~1)

Fig. 1. Raman spectra of the internal modes of Rb3H(SeO4)2 for different orientations.

Table 2. Correlation scheme showing the distribution of internal vibrations of SeO4 ion SeOa ion symmetry Td

HSeOa ion symmetry C3o

v~(SeO) (v,)Al (837)

v~(SeO3) A, (866)

6~(OSeO) (v2)E (345)

Site symmetry CI

Factor group symmetry C2h

A

Ag + Bs + A,, + B,,

R 866

6s(OSeO(H)) /

A

Ag + Bg + A. + B.

360

350

E (322)

A

Ag + Bg+ A~+ B.

326

332

A

Ag + Bg + A,, + B,,

776

692

Ag+B~+Au+B,,

900

902

A~+BI+A,,+B,,

910

A

A~+ Bg+ A. + B.

428

A

A~ + B~ + A,, + B,,

465

A~+Bg+A,,+Bu

408

~

./v~(Se-O(H)) v.s(SeO)~. At" (742) ~ A (v3)F2 (873) ~- a, t.qeO i 9 2 0 ~ ~"--3)~ 6s(SeO3)~ A t (432) 6a,(OSeO) ~ (va)F2 (413) c5.(Se03) A 3(94) E

A

IR 838

495

53w 70vw 83m 108vw 124vw

201vw

331s 370wbr

409m 431 m

778mbr

866vs

898m 909m 919w

49w 78m 98w 119vw

198vw

326s 359wbr

404m 428m

776mbr

866vs

892m 899sh 907w

890m 902sh 908w

866vs

403m 428s 465vw 774s

326m 356wbr

196vw

48w 52w 70w 78m 97m 118w

b(cc)a

896sh 901s 909vs

866vs

408m 430w 463wbr 772mbr

329s 366mbr

198vw

51w 61w 76sh 81m 98w

c(ba)c*

891m 900sh 908w

866vs

405m 429w 470vw 778wbr

328m 365wbr

198vw

52vw 70w 80m 95w 125vw

c(ac)b

902vs 910sh

866vs

410s 424s 465mbr 775mbr

331s 358vw

203wbr

52m 64vw 82w 100w

c(bc)a

v, very; s, strong; m, medium; w, weak; br, broad; sh, shoulder.

c(bb)a

c(aa)b

Raman

Rb3H(SeO4)2

890m 899s 904sh

866vs

405m 426m 465wbr 773sbr

326s 361mbr

197wbr

47w 58vw 71m 95w 111 vw 118w

Unpolarized

v2(SeOa) + v(O" "" O) v4(SeOD

v(Se-OH/OD) vl(SeO4)

338-351vw 391s

700mbr 836sh 902s

332s 370mbr 410m 433m 789wbr 868vs 908mbr

336-384s 387sbr 495m 640-740m 845sh 902vsbr

a(OH/OD) v(OH/OD) A, B, C bands

1105w

1225w 1720w

1010w 1230mbr 1590wbr 2258mbr 2370mbr

~'3(5eO4)

v(Rb-O)

Lattice modes

Assignments

229w 258w 275wbr

IR

189vw

51m 61w 82m 124vw

Raman

232w 253wbr 275wbr

IR

Rb3D(SeO4)z

Table 3. Observed wave n u m b e r s (cm i) and band assignments

-e

7~

IR and polarized Raman spectra in the ferroelastic phase

433

The free ion approach (with HSeO~- ion approximated to C3,, symmetry) predicts three bands each in the observed spectra for stretching ( 2 A I + E ) and bending (AI + 2 E ) modes in both Raman and IR for the SeO42- ion (Table 2). However, the site symmetry approach leads to four and five bands, respectively, for these modes. The observation of more bands than those predicted by the site symmetry approach [11] in the stretching and bending regions indicates the vibrational interaction between SeO 2- and HSeO~- ions. The H bond repulsive forces between the two co-ordinated S e O 4 ions in the dimer tend to prohibit a close O - H - . . O separation [14]. Therefore, the competition between the two equivalent anions for the shorter O-H bond may set up a double well potential for the equilibrium proton position. The observed higher frequency in the Raman spectrum for the symmetric v~ mode (of SeO42-) than in RbHSeO4 [11, 12] indicates a shortening of Se-O acceptor bond distance in the title compound. This implies a weakening of the hydrogen bond in Rb3H(SeO4)2 than in RbHSeO4. Thus, the positions for the location of proton become energetically inequivalent indicating the presence of a nonc e n t r o s y m m e t r i c ( S e O a H S e O 4 ) 3- dimer in this crystal. The asymmetric stretching mode is observed in the 890-910 cm-~ region with complete lifting of degeneracy. In the IR spectrum an intense broad band is observed at 902 cmfor this mode. ),(OH) bands occur usually in this region. The presence of this mode can cause a broadening of the v3 mode in the IR spectra. On deuteration this band becomes sharper due to the weakening of the ),(OH) band. The degeneracy of the v2(E) mode is completely lifted in the Raman spectra of both protonated and deuterated compounds. The triply degenerate v4 mode also appears with degeneracy completely lifted in all polarizations except in the a~x and ayy orientations of the Raman spectra of Rb3H(SeO4)2. In the deuterated compound the degeneracy of this mode is only partially lifted. The appearance of IR inactive modes in the IR spectra and the complete lifting of the degeneracy can be due to the lowering of the symmetry of the SeO42- ion. Isotopic substitution does not significantly shift the wave numbers of the fundamental modes of the SeO~- ions (Fig. 2). This is in agreement with the identical lattice constants

(a) 1100 1000 900

800

700

600

500

400

300

200

100

0

WAVENUMBER (crn1)

Fig. 2. Raman spectra of the internal modes of the polycrystalline(a) Rb3H(SeO4) 2 and (b) Rb3D(SeO4)z.

G. SURESH et al.

434

m,m O Z

(/) Z <

(b)

~r I-

2500

2000

1800

1600

11,00

1200

1000

800

600

t,00

200

WAVENUMBER (cr~ t ) Fig. 3. I n f r a r e d spectra o f (a) R b 3 H ( S e O 4 ) 2 and (b) R b 3 D ( S e O 4 ) 2.

in both protonated and deuterated crystals [8]. This is in contrast to other ferroelectrics where the wave numbers of the internal modes of the anion decrease on deuteration. The translational vibrations of Rb ÷ ions have the lowest frequencies owing to their high mass value.

Hydrogen bond vibrations According to the structural data [3, 6] the O - H . . - O distances in both the crystals are greater than the critical length 7c (2.47 A), indicating that protons are disordered in the double minimum potential. A very wide absorption with an Evan's hole around 320 cm- l in the IR spectrum as observed by Fukai et al. [7] shows that the motion of the hydrogen atom is coupled with that of the oxygen atoms in the selenate ions. This shows the presence of a short and strong hydrogen bond in the crystal.

6

150

3

"Io I

100

X

g I

X II 50

2.45

2.5

2.55

2.6

O.-- O d i s t a n c e

Fig. 4. Frequency difference v ( X - O ) - v ( X - O H ) versus 0 . . - O distance (1) Na3H(SO4) 2, (2) Rb3H(SeO4)2, (3) NH4HSeO4, (4) (NH4)3H(SO4)2, (5) RbHSeO4, (6) (NH4)3H(SeO4)2, (7) CsHSeO4.

IR and polarized Raman spectra in the ferroelastic phase

435

As in the case o f acid salts of carboxylic acids where the h y d r o g e n b o n d is very strong, the internal m o d e s v O H , 6 ( O H ) and ),(OH), t h o u g h allowed by selection rules, are not o b s e r v e d in the R a m a n spectra of this crystal. T h r e e b r o a d I R absorption bands are o b s e r v e d in the O H stretching region at 2370, 2258 and 1590 cm -~. T h e y give rise to an A B C triplet owing to Fermi resonance of the b r o a d O H stretching band with the o v e r t o n e s of the in-plane 6 ( O H ) and out-of-plane ) , ( O H ) m o d e s [14]. T h e presence of trio bands along with the I R absorption c o n t i n u u m supports the existence of strong h y d r o g e n b o n d s in the crystal. T h e b r o a d e n i n g of the v ( O H ) bands can be attributed to the result o f v ( O H ) vibrations o f the O . . . O system due to the correlated m o t i o n of the p r o t o n s in the dimer system [15]. Isotopic substitution causes a large change in the shapes and intensities of the B and C bands. T h e B b a n d b e c o m e s n a r r o w e r and appears at 1720 c m - i . T h e C b a n d b e c o m e s less intense and appears at 1225 cm -l. T h e in-plane b e n d i n g t~(OH) vibrations are o b s e r v e d at 1230 and 1010 cm-~ in the I R spectrum. O n d e u t e r a t i o n b ( O D ) bands fall in the region of absorption of the SeO42- ion. T h e b r o a d b a n d f r o m 335 to 380 cm -1 is due to the v ( O . . . O ) vibrations. T h e bending m o d e 6 ( ( O H ) - . - O ) a p p e a r s below 150 c m - l [16]. T h e isotopic f r e q u e n c y shift ratio is 1.3, which is considerably lower than the h a r m o n i c oscillator value (1.37). This lowering is a spectroscopic manifestation of a positive isotope effect of the a s y m m e t r i c h y d r o g e n b o n d [16, 17]. T h e lower wave n u m b e r s for v ( O H ) m o d e s show that d i m e r f o r m a t i o n is quite a n h a r m o n i c , as j u d g e d f r o m the isotopic shift [18].

REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9]

M. Ichikawa, T. Gustafsson, K. Motida, I. Olovson and K. Gesi, Ferroelectrics 108, 307 (1990). M. Kasahara, P. Kaung and T. Yagi, J. Phys. Soc. Jpn 61, 3432 (1992). I. P. Makarova, I. A. Verin and N. M. Shchagina, Soo. Phys. Crystallogr. 31, 105 (1986). M. Ichikawa, J. Phys. Soc. Jpn 47, 681 (1979). K. Gesi, J. Phys. Soc. Jpn 50, 3185 (1981). M. Ichikawa, T. Gustafsson and I. Olovsson, Acta Cryst. C48, 603 (1992). M. Fukai, T. Matsuo, H. Suga and M. Ichikawa, Solid State Comrnun. 84, 545 (1992). M. Ichikawa, K. Motida, T. Gustafsson and I. Olovsson, Solid State Cornrnun. 76, 547 (1990). W. G. Fately, F. R. Dollish, N. T. McDevitt and F. F. Bentley, Infrared and Raman Selection Rules for Molecular and Lattice Vibration--Correlation Method. Wiley-Interscience, New York (1972). [10] G. Herzberg, Molecular Spectra and Molecular Structure--Infrared and Raman Spectra of Polyatomic Molecules. Van Nostrand, New York (1945). [11] M. Damak, M. Kamoun, A. Daoud, F. Romain, A. Lautie and A. Novak, J. Molec. Struct. 130, 245 (1985). [12] J. Baran, Z. Czapla, M. M. Ilczyszyn and H. Ratajczak, Acta Physica Polonica 59A, 753 (1981). [13] B. Pasquier, N. Le Calve, A. Rozycki and A. Novak, J. Rarnan Spectrosc. 21,465 (1990). [14] J. B. Goodenough, Methods in Enzyrnology 127, 263 (1986). [15] C. Coulson and D. Hadzi, Hydrogen Bonding, p. 339. Pergamon Press, Oxford (1958). [16] B. Marchon and A. Novak, J. Chem. Phys. 78, 2105 (1983). [17] A. Novak, Structure and Bonding (Edited by J. D. Dunitz), Vol. 19, p. 176, Berlin, New York (1973). [18] L. W. Schroeder, T. H. Jordan and W. E. Brown, Spectrochirn. Acta 37A, 21 (1981).