Tunneling model of phase changes in tetragonal rare-earth crystals

The crystal-field theory for a rare-earth ion in tetragonal symmetry is studied in a semiclassical description. The low-lying quantum states are interpreted as tunneling states between classical minima. At low temperatures, spontaneous lattice distortions occur to lower some of the minima relative to others. The model is applied to DyVO4 and TbVO4 and is shown to lead to a good understanding of why they show phase changes and unusual paramagnetic properties. © 1971 The American Physical Society.