Phase transitions in K_1-xRb_xAlF₄: III - birefringence measurements

Abstract

Linear birefringence (LB) measurements versus temperature have been performed on K_1-xRb_xAlF₄ (x = 0.04; 0.06; 0.09). The corresponding curves exhibit a universal behaviour: the ab-LB step at the II-III_a transition (characteristic of a first order transition) as well as the LB's variations below T_{II-III a} do not depend upon the Rb concentration. The ac-LB behaves in the same way as the ab one's. This universal character is interpreted within the framework of a Landau-type free energy detailed in paper II.