Phonon instabilities in TmVO4

The lattice instability of TmVO4 near its cooperative Jahn-Teller phase transition is shown to be a competitive process involving distortions of B1g and B2g symmetry. In order to understand the soft-mode behavior, we must consider the spin-lattice coupling of the entire J=6 ground multiplet of the Tm3+ ion. Axial magnetic fields are found to affect dramatically the soft acoustic-mode behavior. © 1973 The American Physical Society.