Elastic anomalies in HoNi2B2C single crystals
  We have measured temperature and magnetic field dependencies of the sound velocities and the sound attenuation in HoNi2B2C single crystals. The main result is a huge softening the velocity of C66 mode due to a cooperative Jahn-Teller effect, resulting in a tetragonal-orthorhombic structural phase transition. Anomalies in the behavior of the C66 mode through various magnetic phase transitions permit us to revise the low temperature H-T phase diagrams of this compound. 
V.D. Fil, A. Knigavko, A.N. Zholobenko, E.-M.Choi, S.-I. Lee