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Issue		Eur. Phys. J. AP Volume 5, Number 1, January 1999
Page(s)		33 - 44
DOI		10.1051/epjap:1999109

DOI: 10.1051/epjap:1999109

Eur. Phys. J. AP 5, 33-44

Phase transitions in mixed disordered crystals Rb_1-xK_xCaF₃ (0<x<1) investigated by Raman spectroscopy

Ph. Daniel¹ - J. Toulouse² - M. Rousseau¹

¹ Laboratoire de Physique de l'État Condensé, Université du Maine, avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France
² Physics Department-Lehigh University 16, Memorial Drive, East PA 18015 Bethlehem, USA
daniel@lola.univ-lemans.fr

Received: 10 July 1999 / Revised: 24 September 1998 / Accepted: 9 October 1998

Abstract
Structural antiferrodistortive phase transitions have been studied in mixed disordered perovskites Rb_1-xK_xCaF₃ crystals over a wide concentration range (0<x<1) by Raman spectroscopy and a full group theory analysis has been carried out. Raman spectra have been indexed in all phases and a ``one mode" behavior is evidenced. The influence of the cationic substitution on the mechanism driving the transitions is characterized by: i) the disappearance of the strong first-order character of the low temperature transition in RbCaF₃; ii) the continuous evolution of the Raman spectra as a function of temperature, with no sharp indication of a transition except for the classical soft mode behavior; iii) the persistence of hard Raman modes even in the ideal cubic phase; iiii) the appearance of a broad central component associated with relaxation processes. Finally, the results of lattice dynamics calculations, when compared to inelastic neutron scattering measurements, show that the classical virtual crystal model fails. These results are examined in terms of a simple displacive transition for these fluoro-perovskites in which the interactions between F₆ octahedra are anisotropic and the importance of the substitutional disorder is highlighted.

PACS
61.72.-y Defects in crystals - 63.20.-e Phonons and vibrations in crystal lattices - 64.70.Hz Solid-solid transitions - 78.30.-j Infrared and Raman scattering

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