A NOTE ON MIXED PHONON MODES

The relation between the temperature dependence of the spectroscopical ly-observable phonon-frequency square, omega(o)(2) of the optical mode or omega(A)(2) of the acoustic mode, and that of relevant thermodynam ic coefficients is considered. A special emphasis is laid on the piezo electric or sublattice-piezoelectric interaction between optical and a coustic modes in the lattice where not all the constituent atoms occup y the center of symmetry. The temperature dependence of the bilinearly -coupled-mode frequency is found to deviate from that of the renormali zed thermodynamic parameters observed experimentally, which results in some modification of the Lyddane-Sachs-Teller relation. The mixing of interacting phonons, their normal modes and possible strain-induced i onic-site displacements are also discussed.