Phenomenological theory of hard-mode frequency shift in ferroelectric phase transition

Abstract

Useful relations of hard-mode spectroscopy are rederived from Slater's simplified phenomenological theory of lattice dynamics. The phase transition anomaly in inverse dielectric susceptibility (1/chi), related to the hard-mode frequency shift (omega(Q)) as Delta(chi(-1)) = R omega(2)(Q), can be analyzed to study structural phase transitions associated with a nondegenerate hard mode. This hard-mode frequency shift represents a change in the symmetry-restricted chemical bonding due to the structural phase transition. The effect of phase transition on the respective chemical bonds can be probed to follow the order parameter change by observing frequency shifts of the corresponding vibrational hard mode. The proportionality constant R can be expressed in terms of mode parameters, which may be associated with the reorientational mobility of the dipole clusters in the phase transition system.