JAHN-TELLER PHASE-TRANSITION IN J=1 TETRAGONAL PARAMAGNETIC INSULATOR

The equations of motion method is used to study elastic and magnetic p roperties of the J=1 paramagnetic insulator to fourth order in the mag netoelastic coupling. The temperature dependence of the sound velocity as well as the phonon damping and dispersive shift of the energy caus ed by the magnetoelastic interaction in the presence of an external ma gnetic held are obtained. It is shown that for the system with excited state close to a doublet ground state the fourth order contribution c annot be neglected in the description of the Jahn-Teller phase transit ion. The braodenings and shifts of the crystal-field levels are also c alculated.