Structural and Thermal Dependence of Normal Mode Condensations in K sub 2 TeBr sub 6.

The structural thermal dependence in K sub 2 TeBr sub 6 resulting from the condensation of normal modes at the monoclinic-to- tetragonal and tetragonal-to-cubic phase transitions has been investigated over the temperature range 20 to 430K. Opposed TeBr sub 6 sup (2-) octahedral rotations and A sub (2g) mode symmetry lead to the phase transition at 434K from Fm3m to P4/mnc with order parameter obeying the classical Landau relationship Q sub A proportional to (T sub c -T) sup (1/2). The E sub g mode leads to octahedral tilting from the tetragonal axis and the K sup + ion becoming displaced as the symmetry transforms at 400K to P2 sub 1 /n, with order parameter Q sub E sup Br proportional to (e sub 22 -e sub (11)) sup (1/2) and Q sub E sup K proportional to e sub 13, where e sub (ij) are monoclinic strain tensors.