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: Cac), consistent with the observed additional polarization below TN. 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.