Temperature-dependent EXAFS-study of tungsten bronze Na2Sr4Nb10O30 and boracite Fe3B7O13Br ferroelectrics

Physica B 208&209 (1995) 627-628

ELSEVIER

Temperature-dependent EXAFS-study of tungsten bronze NazSrgNbloO3o and boracite Fe3BvO13Br ferroelectrics A.T. Shuvaev*, I.V. Pirog, I.A. Zarubin Institute of Physics, Rostov State University, Stachky Ave. 194, Rostov-on-Don 344104, Russian Federation

Abstract Local structures of Na2Sr4NbloO3o and FeaBTOlaBr have been studied by means of Nb and Fe K-edges EXAFS spectroscopy at temperatures from 20 ° C to 500 ° C. It has been found, that Nb atoms in NaESr4NbloO30 are shifted from the center of the oxygen octahedra (by about 0.085 A) and their displacements exist below and above the phase transition point. Thus it appears that the phase transition is of the order-disorder type. Temperature dependences of Debye-Waller factor (DWF) for F e - O and Fe Br bonds and interatomic distances (R) in FeaB7013Br have jumps in the vicinity of both phase transition points. This indicates the phase transition to be of the displacive type.

1. Introduction The structure of Na2Sr4NbloO3o ferroelectric is perovskite-like, so Nb atoms are located inside the oxygen octahedra. The structural phase transition at 270°C is attributed to the ferroelectric-dielectric type. FeBr-boracite undergoes two structural phase transitions: the first one at 132° C is of the ferroelectric-ferroelectric type and the second one - at 222°C - is of the ferroelectric-dielectric type. The nearest three shells of Fe atoms are formed by O, Br and B atoms. The purpose of this study is to investigate the nature of the ferroelectric phase transitions in Na2Sr4Nbl003o and FeBr-boracite using EXAFS technique.

2. Experimental details EXAFS measurement at the Nb and Fe K-edges have been performed in laboratory I-1]. The energy resolution is 9 eV in the region of the Nb K-edge and 2 eV near the * Corresponding author.

Fe K-edge. The analysis of the spectra has been performed by the routine procedure, i.e. Victorin polynomial pre-edge function and the 6th order Chebyshev polynomial for normalization/background removal. Fourier transformation over the range 3.5-12.8,A -1 and 2.5-11.5 ,~- 1 for Nb and Fe K-edge spectra, respectively. Curve fitting is performed in the reciprocal space on Fourier filtered spectra.

3. Results and discussion EXAFS data analysis of Na2Sr4Nba oO 30 indicate that Nb atoms shifted from the center of oxygen octahedra along axis of the third order so there were two oxygen shells around Nb with radii 19.5 and 2.12,g,. Temperature dependence of N b - O distances is shown in Fig. 1. These distances remain unchanged in phase transition point. So, the electric dipole moment of oxygen octahedra in the phase transition points is also unchanged. As it may be deduced from the results obtained the phase transition is of the "orde~disorder" type. Let us consider the EXAFS data for Fe Br boracite (Figs. 2 and 3). As one can see the temperature dependence

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A.T. Shuvaev et al. /Physica B 208&209 (1995) 627 628

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c h a n g e s significantly in the vicinity of the p h a s e t r a n s i t i o n points, a n d j u m p s of R a n d D W F values are noticed. In the paraelectric p h a s e t e m p e r a t u r e region all R a n d D W F values increase w i t h t e m p e r a t u r e . O n the o t h e r h a n d , in ferroelectric p h a s e s o m e i n t e r a t o m i c distances (Fe O) increase with t e m p e r a t u r e a n d the o t h e r s (Fe Br, F e - B ) decrease. F o r all a t o m i c pairs the D W F values increase in all phases. T h e s e peculiarities of R a n d D W F t e m p e r a t u r e d e p e n d e n c e s indicate the F e B r - b o r a c i t e to be of the displacive-type ferroelectric.

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Reference [1] A.T. Shuvaev and T.A. Lyubeznova, Pribori i Technica Experim. 3 (1988) 234.