Phase transitions in K_1-xRb_xAlF₄: II - brillouin and ultrasonic studies

Abstract

The concentration dependence of the phase transitions features has been studied in K_1-xRb_xAlF₄ for x varying between 0 and 0.09 through Brillouin and ultrasonic measurements. For x larger than 0.02, a new phase (III_a) appears in the sequence phase II-phase IV of pure KAlF₄. The elastic constants show pretransitional and transitional anomalies around the II-III_a first order transition at T₀(x) which are supported by a Landau development of the free energy up to the 6th order using the appropriate order parameter (X₃ symmetry soft mode) adequately coupled to the strains, where only the second order term varies with x and is written as α(T-T_c(x)). This model also agrees with birefringence measurements (see paper III).