Ground-state wave functions of Tb3+ ions in paramagnetic TbPO4: A neutron scattering study

The paramagnetic-excitation spectrum of TbPO4 has been investigated by means of inelastic neutron-scattering techniques, and the observed crystal-field transitions were analyzed using a Hamiltonian that included the atomic free-ion and crystal-field interactions for an f8 configuration. Five parameters for the crystal-field potential were obtained that provided a satisfactory description of the neutron data collected at temperatures between 4.2 and 150 K. The calculated crystal-field contribution to the specific heat in the 4--10 K temperature range and the paramagnetic susceptibility agree well with the experimental results. The large anisotropy of the magnetic ground-state doublet of the Tb ions with respect to the crystallographic c axis has important consequences for the spin structure in the antiferromagnetic phases at low temperatures. The effective internal magnetic field in antiferromagnetic TbPO4 was estimated based on the crystal-field-level scheme and a molecular-field approximation and was found to be in good agreement with the values reported from other measurements