Precursor order and Raman scattering near displacive phase transitions

Reports detailed experimental studies of the onset of hard-mode Raman activity near the cubic-tetragonal structural phase transitions in KMnF 3 and RbCaF3. The results are discussed in the context of a theory taking into account the influence not only of the long-range order, reflected in the macroscopic order parameter, but also that of the precursor order reflected in the central peak and phonon sideband of the soft-mode spectral function. The precursor order, dominant near the phase transition, is responsible for the persistence of quasi-first-order hard-mode features above the transition temperature. The extent of this persistence is used as a basis for specific estimates of the degree of precursor order in KMnF3 and RbCaF3.