We report on the structural and spectroscopic characterization of the multiferroic ${\mathrm{Fe}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$. Synchrotron x-ray and neutron diffraction as well as thermal expansion measurements reveal a lattice anomaly at $T_{\mathrm{N}}\simeq 60\mathrm{K}$ but do not show any symmetry lowering in the magnetically ordered state. The lattice parameter $c$ 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 $100\text{–}8000{\mathrm{cm}}^{-1}$ reveal most of the expected phonon modes in comparison with the eigenfrequencies obtained by density-functional calculations. The $A_1$ phonons show an overall hardening upon cooling, whereas a nonmonotonic behavior is observed for some of the $E_1$ modes. These modes also show a strongly increased phonon lifetime below $T_{\mathrm{N}}$, 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 $d\text{−}d$ excitations of the tetrahedral ${\mathrm{Fe}}^{2+}$ site that become clearly visible below $T_{\mathrm{N}}$ only.

• Received 23 April 2020
• Revised 16 June 2020
• Accepted 5 August 2020

DOI:https://doi.org/10.1103/PhysRevB.102.094307