A study of the cooperative Jahn-Teller effect in rare-earth vanadates by linear birefringence. I. TbVO₄ and Tb_cGd _1-cVO₄

Measurements of optical linear birefringence have been used to study the cooperative Jahn-Teller phase transition in the mixed crystals Tb _CGd_1-cVO₄ for 0.34<or=c<or=1. For two concentrations (c=0.34 and c=0.36) a double phase transition was observed as a function of temperature, in which an orthorhombic phase exists between two tetragonal phases. This is in agreement with earlier Raman studies. The origin of the birefringence and its relationship to the order parameter is discussed. A mean-field theory of the variation of the order parameter (i.e. the orthorhombic strain) with temperature is found to give a satisfactory explanation of the observed birefringence. Measurements of the critical behaviour of the order parameter are discussed.