Resolving structural changes and symmetry lowering in spinel FeCr2S4

The cubic spinel FeCr2S4 has been receiving immense research interest because of its emergent phases and the interplay of spin, orbital and lattice degrees of freedom. Despite the intense research, several fundamental questions are yet to be answered, such as the refinement of the crystal structure in the different magnetic and orbital ordered phases. Here, using high-resolution synchrotron powder diffraction on stoichiometric crystals of FeCr2S4 we resolved the long sought-after cubic to tetragonal transition at ~65 K, reducing the lattice symmetry to I41/amd. With further lowering the temperature, at ~9 K, the crystal structure becomes polar, hence the compound becomes multiferroic. The elucidation of the lattice symmetry throughout different phases of FeCr2S4 provides a basis for the understanding this enigmatic system and also highlights the importance of structural deformation in correlated materials.