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

¹ Laboratoire de Physique de l'État Condensé (UPRES A CNRS 6087), 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

Received: 10 July 1999
Revised: 24 September 1998
Accepted: 9 October 1998
Published online: 15 January 1999

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.