Phonon renormalization at smallqvalues in the high-temperature phase of

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Abstract
The hexagonal perovskite exhibits a structural phase transition from a dynamically disordered high-temperature phase to an ordered low-temperature phase due to the cooperative Jahn - Teller effect. The lattice dynamics of the high-temperature phase has been studied by inelastic neutron scattering experiments. The investigations concentrated on small wave vectors q, where for the first time renormalized phonons at could be observed. The measurements confirm the predictions of a theoretical approach based on the coupling between dynamic reorientation processes and acoustic lattice waves (pseudo-spin phonon coupling).