Incommensurate structural phase transition in BaMnF4: Light scattering from phasons

We report an extensive series of optical investigations of BaMn${\mathrm{F}}_4$ near its incommensurate structural phase transition at $T_i\approx 254$ 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 $T_i$. 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 $T_i$, exhibits hysteresis, and may arise from static domain or defect scattering. The observed $\overrightarrow{\mathrm{q}}$ 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.