IMF2017

Zone-boundary soft-modes have long been of interest in antiferrodistortive (AFD) systems such as SrTiO₃, and more recently the antiferromagnetically-ordering EuTiO₃. Their coupling to polar modes are also central in novel hybrid-improper ferroelectric compounds, or the archetypal improper ferroelectric, YMnO₃. This presentation will illustrate how inelastic neutron scattering (INS) measurements resolve anharmonic lattice dynamics across the Brillouin zone, providing important constraints for theory. In addition, we will discuss our anharmonic first-principles simulations (DFT), of the temperature-dependent lattice dynamics, including in the paraelectric phase, and direct comparison to INS measurements. In SrTiO₃ and EuTiO₃, alternative tilts of oxygen octahedra are the dominant lattice instability and result in AFD transition from cubic perovskite to a tetragonal phase at Tc=105 K and 280 K, respectively. This transition involves the softening of the R₂₅ phonon mode (R-point). The temperature dependence of this R₂₅ mode -as well as the zone-center TO ferroelectric mode- are clearly seen in INS data, and quantitatively reproduced in our anharmonic DFT simulations, including the renormalization of phonon frequencies. In EuTiO₃, the low-temperature magnetic ordering and magnetic excitations were also investigated. In the hexagonal manganite YMnO₃, the (1260 K) improper ferroelectric transition is driven by a K-point lattice instability, coupled to a polar mode. We have successfully mapped the unstable phonon branch and its temperature dependence with INS, and also achieved realistic simulations of the high-temperature lattice dynamics.