In situ high-pressure infrared spectra of α-NaVO3

~

Solid State Communications, Vol. 99, No. 12, pp. 869-871, 1996 Copyright © 1996 Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-1098/96 $12.00 + .00

t Pergamon

PII: S0038-1098(96)00343-2

IN SITU HIGH-PRESSURE INFRARED SPECTRA OF a-NaVO3

Andrzej Grzechnik and Paul F. McMillan Materials Research Group for High Pressure Synthesis, Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604, U.S.A. (Received 4 April 1996; in revised form 3 June 1996 by S.G. Louie) In-situ infrared absorbance spectra of a-NaVO3 are obtained in the range of 450-1200 cm-t up to 300 kbar. Two distinct phase transformations are observed at 50 and 70 kbar, respectively. The phase transition at 50 kbar is indicated by a change in slope of the pressure dependence of the band frequencies. The crystalline-to-disordered phase transformation takes place at 70 kbar. Copyright © 1996 Published by Elsevier Science Ltd

Keywords: C. crystal structure and symmetry, D. phase transitions.

1. INTRODUCTION The structure of a-NaVO3 is related to the monoclinic C2/c silicate pyroxenes [1,2], where corner-shared chains of the tetrahedral VO43- units along [00 1] are bound in a crystal network by the Na external cations. However, Ramani et al. [3] and Shaikh [4] deduced from Wilson's statistics that the space group for NaVO 3 at room temperature is either C2/c or Cc. They described a ferroelectric-paraelectric phase transition at 380°C (Cc ~ C2/c), that occurs through the reorientation of thermal vibration ellipsoids. In 3-NaVO3 (Pnma space group), the V atom, in the metavanadate chains parallel to [0 10], is coordinated to five oxygen atoms forming a distorted trigonal bipyramid [5]. Vibrational features of NaVO 3 metavanadates were reported by Chisler and Lazarev [6], Lazarev et al. [7], de Waal [8], Seetharaman et al. [9], Shen et al. [10, 11], Kuok et al. [12], Tang et aL [13], and Kristallov and Perelyaeva [14]. Seetharaman et al. [9] described a ferroelectric-paraelectric phase transition (/3 ~ transformation). Chisler and Lazarev [6] measured polarized IR and Raman spectra of ~-NaVO3 and found that the TO-LO splitting for the IR mode corresponding to the antisymmetric V - O - V stretching oscillation (Bu) is equal to 253cm -l. Results of the optical mode calculations (C2/c space group) revealed that a microscopic electrostatic field softens the effective elasticity of the in-phase atom displacements along the metavanadate chain [7]. Shen et al. [10, 11] and Tang et al. [13] investigated Raman features at high

pressures. They reported a first-order phase transition at 43 kbar and pressure-induced amorphization at 60 kbar. The goal of this study is to investigate highpressure behavior of a-NaVO3 with in situ infrared spectroscopy. 2. EXPERIMENTAL Transparent crystals of a-NaVO3 were prepared from a 1 : 1 mixture of NaECO3 and V205 using the Bridgman method. The phase purity of the sample was checked with X-ray diffraction. The IR absorbance data were collected on a BioRad Digilab FTS-40 FTIR instrument (Michelson interferometer) equipped with a KBr beamsplitter, MCT detector and a ceramic globar source in the range 450-1200cm -1. The sample (thin platelets of single crystals) and CsI in an approximate 1 : 10 weight proportion were loaded into a Mao-Bell-type cell with type IIa diamonds, brilliant cut with 600#m culets, and a sample chamber diameter of 250 #m. Pressures were determined from the shift of the R~ ruby fluorescence line using the scale by Mao et al. [15]. The pressure variation in CsI across the sample chamber was q-5% of the total pressure. In order to remove the interference fringes from the spectra due to the multiple IR beam reflection between the culets of the diamonds, the collected interferograms were deconvoluted into two interferograms (the interferograms from a Michelson interferometer and multiple

869

INFRARED SPECTRA OF a-NaVO3

870

Table 1. Infrared vibrational modes in a-NaVO3 in the 450-1100 cm -l region This study

[8]

962sh 950s 916sh 828s 651s 482m

961 940 911 836

Assignment [7] vs(V_O) v~(V-O) vas(V_O_V) vs(V_O_V)

480

sh, shoulder; s, strong; m, medium. reflection) and then only the interferogram from a Michelson interferometer, modulated by sample and diamond absorption, was Fourier transformed. 3. RESULTS AND DISCUSSIONS The results of a vibrational analysis (C2/c space group) and comparison between the observed midinfrared spectra collected in this study and the one presented by de Waal [8] are displayed in Table 1. The spectrum measured from single crystals of a-NaVO 3 is better resolved than the one collected from a poly-

Vol. 99, No. 12

crystalline sample [8]. The strong resolved band at 651 cm -s corresponds to the TO frequency of the B, mode (va~V-O-V oscillation) [6]. This vibration, shifting all the similarly charged atoms in the same direction, is strongly softened by the electrostatic field [7]. Figure 1 shows the absorbance IR spectra at various pressures up to 300 kbar for the frequency range of 450-1100 cm -z. There is no major structural transformation up to 70kbar. The change in the pressure dependence of the band frequencies at about 50kbar can be attributed to the first-order phase transition, reported from the Raman experiments, at 43kbar [13]. There is only one additional band observed in the mid-infrared re#on (469 cm -l) that is associated with this transformation. We assign this band to the mixed V - O - V bending and stretching oscillation. This observation suggests that this transition involves chain librational and bending modes of the VO43- metavanadate chains and does not cause significant changes in bond lengths or the V atom coordination number. There are also no features in the spectra that can be related to the pressure-induced amorphization at 60 kbar, reported earlier by Shen et al. [10, 11], and Tang et al. [13]. The onset of a second phase transition is observed 1000

000

A

e"

5 kbar

0oo

>(D J:

G)

35 kbar

C3

=E g

C

700

>

0 t4~ JD m

50 kbar .

78 kbar

.

-

-

~

600 90 kbar 158 kbar 500 217 kbar 286 kbar

,

4 0 0

J

,

I

,

500

,

,

i

J

i

600

i

i

i

I

. . . .

700

I

,

800

,

,

,

I

,

,

,

900

,

I

i

1000

i

i

i

I

,

,~, kp,,

1100

wavenumber (cm "1) Fig. 1. Mid-infrared spectra upon pressurizing.

,

.

I

,

,

,

I

,

,

,

I

,

,

,

I

,

,

,

I

,

,

,

F

,

,

,

I

,

,

,

/

i

1200

0

40

80

120

160

200

240

280

320

pressure (kbar)

Fig. 2. Pressure dependence of the mid-infrared frequencies. Lines are guides for the eyes.

INFRARED SPECTRA OF t~-NaVO3

Vol. 99, No. 12

871

previously interpreted to be due to a pressure-induced amorphization [10, 11, 13], are in fact due to a crystalline-to-disordered phase transition where the V atoms possess positional disorder [16]. The spectra taken with decreasing pressure show hysteresis in transformations (Fig. 3). After complete release of pressure, features of both phases are present in the IR spectrum of the decompressed sample. Our multi-anvil synthesis (the LiVO3-LiNbO3 intermediate compounds) and in situ X-ray diffraction experiments (LiVO3 and t~-NaVO3) indicate that the highpressure phases of LiVO3 and a-NaVO3 have the LiNbO3-type structure [19].

c 0 m

g @

8 quench

28 kbar

0

III

47 k:bar 6'7 kbar

Acknowledgements--A.G. acknowledges a Junior Fulbright Fellowship through US-Polish Educational Exchanges. This work was supported under the NSFMRG grant, DMR 91-21570.

96 k:bm" 172 k:bw

2O3 kbar 253 kbar 282 kbar . . . .

400

I

. . . .

500

i

,

600

,

,

,

I

. . . .

700

I

,

600

wavenumber

,

,

t

i

,

,

900

,

,

I

. . . .

1000

i

. . . .

1100

i

1200

(cm "~)

Fig. 3. Mid-infrared spectra upon decompressing. at 70 kbar, where additional bands appear (Fig. 2). The new bands in the region 600-820cm -1 can be correlated to the change in the coordination number of the V cation. The band at 812 cm -1 corresponds to the strong Raman band at about 800 cm -t, occuring above 60 kbar [10, 11, 13]. These bands are characteristic of the stretching modes with the six-fold coordination of the cation. Both phases, the pyroxene and highpressure phases, are observed up to the pressure of 160 kbar (Fig. 2). This phase transition is sluggish, with an additional structural rearrangement at 90 kbar, and may involve breakdown of the chain pyroxene structure through a displacive first-order phase transition, as was proposed for isostructural LiVO3 [16]. The increase in width of the bands can be attributed to the significant positional disorder of the V cation in the lattice, as was documented previously by Adams et al. [17, 18] for in situ infrared spectra of KVO 3 and NH4VO3, respectively. Also, the mid-infrared spectrum collected from the polycrystalline sample of a-NaVO3 shows broad bands [8]. The varying intensity of the IR absorption bands is related, through the Beer-Lambert law, to changes in the absorption coefficient, e.g. changes in the density of the material and its changes due to structural transformations. We suggest that the features observed in the Raman spectra of a-NaVO3,

REFERENCES 1.

Hawthorne, F.C. and Calvo, C., J. Solid State Chem. 22, 1977, 157. 2. Marumo, F., Isobe, M. and Iwai, S., Acta Cryst. B30, 1974, 1628. 3. Ramani, K., Shaikh, A.M., Swaminatha Reddy, B. and Visvamitra, M.A., Ferroelectrics 9, 1975, 49. 4. Shaikh, A.M., Ferroelectrics 107, 1990, 219. 5. Kato, K. and Takayama, E., Acta Cryst., 1340, 1984, 102. 6. Chisler, A.E. and Lazarev, A.N., Solid State Physics (in Russian) 30, 1988, 1683. 7. Lazarev, A.N., Chisler, A.E. and Smirnov, M.B., Opt. Spektrosc. (in Russian) 71, 1991, 294. 8. de Waal, D., Mar. Res. Bull. 26, 1991, 893. 9. Seetharaman, S., Bhat, H.L. and Narayanan, P.S.J. Raman Spektrosc. 14, 1983, 401. 10. Shen, Z.X., Ong, C.W., Tang, S.H. and Kuok, M.H., Phys. Rev. B49, 1994, 1433. 11. Shen, Z.X., Ong, C.W., Tang, S.H. and Kuok, M.H., J. Phys. Chem. Solids 55, 1994, 665. 12. Kuok, M.H., Tang, S.H., Shen, Z.X. and Ong, C.W., J. Raman Spektrosc. 26, 1995, 301. 13. Tang, S.H., Kuok, M.H., Shen, Z.X. and Ong, C.W., J. Phys.: Condens. Matter 6, 1994, 6565. 14. Kristallov, L.V. and Perelyaeva, L.A., Russian J. Inorg. Chem. 36, 1991, 879. 15. Mao, H.K., Bell, P.M., Shaner, J.W. and Steinberg, D.J., J. Appl. Phys. 49, 1978, 3276. 16. Grzechnik, A. and McMillan, P.F., J. Phys. Chem. Solids 59, 1995, 159. 17. Adams, D.M., Christy, A.G., Haines, J. and Leonard, S., J. Phys.: Condens. Matter 3, 1991, 6135. 18. Adams, D.M., Haines, J. and Leonard, S. J. Phys.: Condens Matter 3, 1991, 2859. 19. Grzechnik, A. and McMillan, P.F. In preparation.