Research Papers in Physics and Astronomy

 

Date of this Version

2-1-1983

Comments

Published in Physical Review B Volume 27, Number 3, 1 February 1983. Copyright © 1983 The American Physical Society. Used by permission.

Abstract

Raman scattering from K2SeO4 crystals has been studied in the (20-800)-K temperature range. Three portions of the spectrum are discussed: defect-induced scattering, primarily below 100 cm-1, the external mode spectrum below 200 cm-1, and the internal mode spectra in two regions, 300-500 and 800-950 cm-1. The temperature dependence of the low-frequency, defect-induced scattering has been correlated (in previous studies) with the temperature dependence of certain nonzero-wave-vector phonons that have been observed by others using inelastic neutron scattering. Close to the incommensurate transition temperature, Ti= 129 K, a large enhancement and line narrowing develops below 10 cm-1; no satisfactory interpretation of this effect has yet emerged. Above 129 K, in the paraelectric phase, the multiplicity of the external modes agrees with the predictions of the Raman-scattering selection rules. The internal mode spectra, however, consistently contain more lines than prescribed by the selection rules. These observed internal mode spectra would have the correct multiplicity if the crystal did not possess the center of symmetry of the presumed Pnam space group, thus suggesting a distortion of the selenate sublattice. Small frequency shifts above 375 K in the external mode spectra are similar to those observed in the incommensurate phase and, also, differential thermal analyses of powders and crystals show small, diffuse, reproducible peaks in the (373 -473)-K range. Both effects may be associated with glasslike phase changes in the selenate sublattice. A qualitative model of orientational disorder in the selenate sublattice is offered to account for all these observations.

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