We report on the structural and spectroscopic characterization of the multiferroic Fe2Mo3O8. Synchrotron x-ray and neutron diffraction, as well as thermal expansion measurements reveal a lattice anomaly at TN≃60K but do not show any symmetry lowering in the magnetically ordered state. The lattice parameter c exhibits a non-monotonic behavior with a pronounced minimum around 200K, 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 300K in the frequency range of 100−8000cm−1 reveal most of the expected phonon modes in comparison with the eigenfrequencies obtained by density-functional calculations. The A1 phonons show an overall hardening upon cooling, whereas a non-monotonic behavior is observed for some of the E1 modes. These modes also show a strongly increased phonon lifetime below TN, 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 d-d excitations of the tetrahedral Fe2+ site, which become clearly visible below TN only.
@article{arxiv.2004.14038,
title = {Structure, phonons, and orbital degrees of freedom in Fe$_2$Mo$_3$O$_8$},
author = {S. Reschke and A. A. Tsirlin and N. Khan and L. Prodan and V. Tsurkan and I. Kézsmárki and J. Deisenhofer},
journal= {arXiv preprint arXiv:2004.14038},
year = {2020}
}