Abstract
We report on the structural and spectroscopic characterization of the multiferroic . Synchrotron x-ray and neutron diffraction as well as thermal expansion measurements reveal a lattice anomaly at but do not show any symmetry lowering in the magnetically ordered state. The lattice parameter exhibits a nonmonotonic behavior with a pronounced minimum around 200 K, which is also reflected in an anomalous behavior of some of the observed infrared-active optical excitations and parallels the onset of short-range magnetic order. The infrared reflectivity spectra measured between 5 and 300 K in the frequency range of reveal most of the expected phonon modes in comparison with the eigenfrequencies obtained by density-functional calculations. The phonons show an overall hardening upon cooling, whereas a nonmonotonic behavior is observed for some of the modes. These modes also show a strongly increased phonon lifetime below , which we associate with the quenched direction of the orbital moment in the magnetically ordered state. A similar increase is observed in the lifetime of the higher-lying excitations of the tetrahedral site that become clearly visible below only.
7 More- Received 23 April 2020
- Revised 16 June 2020
- Accepted 5 August 2020
DOI:https://doi.org/10.1103/PhysRevB.102.094307
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