Origins of Polar Crystal Structure and Multiferroicity in the Antiferromagnet TbFeWO6

Cation-ordered polar (Pna2₁) materials RFeWO₆ (R = rare earth) have gained significant attention due to their multiferroic properties. These materials are derived from the parent aeschynite-type material CaTa₂O₆ possessing a centrosymmetric orthorhombic structure (Pnma). The ordering of M³⁺ and W⁶⁺ ions in the octahedral sites with distinct Wyckoff positions breaks the inversion symmetry of the parent structure. Our investigation of TbFeWO₆ using symmetry-adapted mode analysis, neutron powder diffraction, and density functional theory calculations (DFT) has revealed that the ordered polar (Pna2₁) structure can emerge from ordered centrosymmetric (Pnam or Pnan) structures through the relaxation of stress. Our low-temperature neutron diffraction experiments have provided evidence of a noncollinear commensurate magnetic structure exhibiting polar magnetic symmetry m (magnetic space group: C_ac), consistent with the observed additional polarization below T_N. Based on the spin structure, we propose that the mechanisms of exchange striction and/or inverse Dzyaloshinskii-Moriya may serve as origins of the multiferroic behavior exhibited by these materials.