Molecular-Field Theory of Phase Transitions in TbPO4and in TmAsO4

The crystallographic and magnetic phase transitions observed in TbP${\mathrm{O}}_4$ and TmAs${\mathrm{O}}_4$ are described phenomenologically. A symmetry-adapted spin Hamiltonian, involving various dipolar and quadrupolar interactions, is constructed and treated in the molecular-field approximation. One or two phase transitions are found according to the values of the interaction parameters, and it is shown that the crystallographic distorsion in the paramagnetic distorted phase is monoclinic. The results are interpreted in terms of the Pytte and Stevens model for rare-earth vanadates.