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Phonon properties and structural phase transitions in ferroelectrics - high pressure Raman scattering studies

Mirosław Mączka 1,  Waldeci Paraguassu 2,  Antonio G. Souza Filho 3,  Paulo T. Freire 3,  Andrzej Majchrowski 5,  Josue Mendes Filho 3,  Jerzy Hanuza 4

1. Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.Ne 1410, Wrocław 50-950, Poland
2. Departamento de Fisica, Universidade Federal do Maranhão, São Luis 65085-580, Brazil
3. Departamento de Fisica, Universidade Federal do Ceara, Fortaleza 60455-970, Brazil
4. Department of Bioorganic Chemistry, University of Economics, Wrocław 53345, Poland
5. Institute of Applied Physics, Military University of Technology, Warsaw 00-908, Poland

Abstract

Raman spectroscopy is a very powerful tool in studies of ferroelectric materials since it gives information about the symmetry changes and the mechanisms by which phase changes proceed. In particular, there has been great interest in studies of soft modes since such studies give direct information on the nature of lattice instabilities. High pressure is a clean probe for investigating the delicate balance between long and short range forces which in turn should shed light on the lattice instabilities and ferroelectric order.

In this contribution, we present and discuss high pressure Raman scattering results for two ferroelectric materials, Bi2WO6 and K2MgWO2(PO4)2.

Bi2WO6 is a member of bismuth layered compounds (Aurivillius family). Its room temperature structure (space group symmetry Pca21) can be regarded as derived from a high symmetry body centered tetragonal structure (space group symmetry I4/mmm) [1]. We will show that Bi2WO6 exhibits two reversible second order phase transitions at 3.4 and 6.2 GPa. It will also be shown that the phase transition at 6.2 GPa is triggered by instability of the low frequency mode, which behaves as soft mode. The observed soft mode is weak and it may be easily overlooked in the temperature dependent studies. Our results demonstrate therefore that the Raman studies under high pressure are an alternative way to study soft mode dynamics in ferroelectrics with high Curie temperature.
K2MgWO2(PO4)2 is derived from KTiOPO4 (KTP) by replacement of two Ti4+ ions with Mg2+ and W6+ cations. This material exhibits five temperature-induced structural transitions from the P41212 to P1 structure [2]. We will show that this material exhibits a reversible first order phase transition near 2.0 GPa, which is driven by the soft mode corresponding to motions of the K+ ions.

References

[1] N. A. McDowell, K. S. Knight, and P. Lightfoot, Chem. Eur. J. 12, 1493 (2006).
[2] U. Peuchert, L. Bohaty, and J. Schreuer, J. Appl. Cryst. 31, 10 (1998).
 

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Related papers
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  2. Elastic properties of KGd(WO4)2 : Ho3+ single crystals studied by Brillouin spectroscopy

Presentation: Invited oral at E-MRS Fall Meeting 2008, Symposium A, by Mirosław Mączka
See On-line Journal of E-MRS Fall Meeting 2008

Submitted: 2008-04-01 14:42
Revised:   2008-06-13 16:22