Cooperative Jahn-Teller effect in the mixed crystal system (Tbx,Tm1−x)AsO4

Single crystals of the system (Tb_x,Tm_1?x)AsO₄ have been investigated by means of optical absorption spectroscopy, birefringence and specific heat. Over the entire concentration range we observe a cooperative Jahn-Teller distortion. The phase transition may be approximated in mean field theory, taking into account that due to local strains part of the Tb and Tm ions are rendered Jahn-Teller inactive, an effect which is much more pronounced on the Tm (x ? 0) than on the Tb (x ? 1) side.