  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.
