A Note on Mixed Phonon Modes

Abstract

The relation between the temperature dependence of thespectroscopically-observable phonon-frequency square, ω ²_Oof the optical mode or ω ²_A of the acoustic mode, and thatof relevant thermodynamic coefficients is considered. A specialemphasis is laid on the piezoelectric or sublattice-piezoelectricinteraction between optical and acoustic modes in the lattice wherenot all the constituent atoms occupy the center of symmetry. Thetemperature dependence of the bilinearly-coupled-mode frequency isfound to deviate from that of the renormalized thermodynamicparameters observed experimentally, which results in some modificationof the Lyddane-Sachs-Teller relation. The mixing of interactingphonons, their normal modes and possible strain-induced ionic-sitedisplacements are also discussed.