Cooperative Jahn-Teller effects in the mixed crystals TbpGd1-pVO4and DypY1-pVO4

The structural phase change arising from the cooperative Jahn-Teller effect has been studied by Raman scattering and optical birefringence methods in the mixed crystals Tb p Gd 1-p VO 4 and Dy p Y 1-p VO 4 with p varying from one to zero. The introduction of Gd and Y dilutes the active components causing the phase change, resulting in a reduction of the transition temperature T D (p). Varying p varies the splitting of E g phonons below T p (p) and also the energy of low-lying collective electronic excitations. The Tb system shows unique behaviour with p=0.365; a transition from tetragonal to orthorhombic symmetry occurs at 8.5K but the crystal reverts to tetragonal symmetry below 2.3K. The effect in the Tb system can be explained by mean-field theory. The Dy compounds are explained using series expansion and modified mean-field methods.