Incommensurate structural phase transition in BaMnF4: Light scattering from phasons

We report an extensive series of optical investigations of BaMnF4 near its incommensurate structural phase transition at Ti254 K. We observe an inelastic central peak in the range (247±7) K. An anomaly occurs in the elastic scattered intensity at 241 K on heating, but the Raman peaks characteristic of the low-temperature phase persist up to 254 K. Measurements of the thermal diffusivity by a novel optical-electronic technique indicate that (1) the observed inelastic central peak is not due to entropy fluctuations, (2) the anomaly in the elastic central-peak intensity coincides with the peak in the pyroelectric coefficient, and (3) the value of pyroelectric coefficient changes sign within 1 K of Ti. The maximum in the inelastic central-peak intensity at 247 K may indicate another phase transition or may relate to the scattering mechanism. The anomaly in the elastic central peak is not a precise indicator of Ti, exhibits hysteresis, and may arise from static domain or defect scattering. The observed q→ dependence of the inelastic central-peak width is consistent with an interpretation based on phason scattering and provides evidence for a strongly anisotropic, overdamped phason response in the incommensurate phase. Finally, we present evidence from detailed line-shape analysis that the observed intensity may actually be the result of coupling between the phason and the LA phonon.