Soft-phonon response function: Inelastic neutron scattering from LaA1O3

High-resolution inelastic-neutron-scattering measurements have been performed at the cubic-rhombohedral (Pm3m→R3̄c) phase transitions in a low-mosaic flux-grown single crystal of LaA1O3. The transition occurred at (489 ± 2)°C, somewhat lower than reported for Verneuil-grown samples. The scattering from the soft R25 mode at the corner of the Brillouin zone in the cubic phase exhibits the central component up to 610 °C. In addition, phonon sidebands centered around ±ω are observed which are overdamped below 550 °C. The observed intensities are well described by a model which relates the central component to the phonon sidebands through an anharmonic coupling parameter δ(T), as recently reported by Shapiro et al. for SrTiO3. ω2(T) is found to deviate from the mean-field behavior near the transition temperature and δ2(T) is found to be nearly constant. A comparison is made between LaA1O3, SrTiO3, and KMnF3 with respect to these parameters as well as the anisotropy of the dispersion surfaces near the R point. © 1973 The American Physical Society.