Structural antiferrodistortive phase transitions have
been studied in mixed disordered perovskites
Rb1−xKxCaF3 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.