Jahn-Teller effects in the mixed vanadate/phosphate crystals TbV1-xPxO4 (0≤x≤0.32) -: art. no. 014423

X-ray-diffraction measurements were performed on mixed vanadate/phosphate single crystals of the type: TbV1-xPxO4 (where x=0, 0.10, 0.18, 0.24, and 0.32) in order to investigate the effects of P doping on the Jahn-Teller phase transition. The transition temperature, below which the tetragonal symmetry of the lattice is lowered to orthorhombic, decreases with increasing P concentration. In order to explain the order parameters of the transition and the striking reduction in the transition temperature with increasing P content, a modified mean-field model was developed for the mixed system. A simple calculation assuming only a distribution in the value of the quadrupolar crystal-field parameter B-2(0) is unsatisfactory. Since the local tetragonal symmetry is broken due to the random distribution of phosphorous, a term involving the operator P-xy=1/2 (J(x)J(y)+J(y)J(x)) was introduced in the Hamiltonian that includes both the crystal-field and magnetoelastic effects. This term, in conjunction with the distribution in the magnitude of the corresponding crystal-field parameter, was essential in order to reproduce the observed temperature dependence of the order parameter (orthorhombic strain). The results of the diffuse scattering measurements performed on the x=0.32 crystal confirmed the existence of local strains that should give rise to variation in B-2(0). The diffuse intensity due to strains having the symmetry associated with P-xy could not be resolved, however, because of the overlapping thermal diffuse scattering.