Far infrared and near-millimetre spectroscopy of partially disordered crystals

Abstract

Disordered crystals show excess absorption in the far infrared and millimetre range in addition to one- and multi-phonon absorption processes. The effect is investigated in non–ferroelectric order-disorder systems (KSCN, NH₄Br:) and in weakly disordered and modulated improper and pseudoproper ferroelectrics ((NH₄)₂SO4 (NH₄)₂BeF₄, Rb₂ZnCl₄, Rb₂CoCl₄, Rb₂ZnBr₄). In displacive proper ferroelectrics the appearence of intrinsic central modes is reviewed and discussed on the example of TSCC where the central mode is assigned to critical cluster dynamics. In polarization glasses and relaxor ferroelectrics also central modes appear but their quantitative behaviour is not understood in details. In ferroelectric polymers the excess absorption is connected with microscopic piezoelectric resonances in lamellar crystallites.