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A significant spin-driven dielectric anomaly near the magnetic ordering temperature has been observed in single crystal Fe₄Nb₂O₉. By combining neutron and x-ray single crystal diffraction, we unambiguously determined its magnetic symmetry and studied the structural phase transition. The temperature-dependent static dielectric constant is strongly anisotropic, rendering two dielectric anomalies along the a axis in the hexagonal lattice with the first one coupled to the magnetic ordering around T_N = 97 K and the second one accompanying with a first-order structural transition around T_S = 70 K. Below T_N , we found that the anomalous dielectric constant is practically proportional to the square of the magnetic moment from neutron diffraction data, indicating that the exchange striction is likely responsible for the strong spin-lattice coupling.

L. Ding et al., Phy. Rev. Mater. 4, 084403, (2020).