Phonon anomaly in Sm₂BaNiO₅

We present low-temperature far-infrared study of Sm₂BaNiO₅. The lowest-frequency phonon of Sm₂BaNiO₅ generated by the motion of the Sm³⁺ ion demonstrates anomalous behaviour at temperatures lower than the Néel temperature, T<T _N=55 K. This phonon hardens at the magnetic ordering although its frequency shift does not follow the magnetic order parameter. The observed phenomenon is responsible for an unusual dielectric response of Sm₂BaNiO₅ reported in the literature. A correlation between the temperature behaviour of crystal-field levels and the phonon anomaly is discussed. (paper)