Significant Structure Modulation in Polar Antiferromagnet FeCoMo₃O₈

The discovery of giant magnetoelectric effect in Fe₂Mo₃O₈ has sparked considerable interest in investigating the novel effects of M₂Mo₃O₈ (M = Zn, Mn, Fe, Co, Ni) family, in particular the emergent physics driven by the M-site ions. Herein, the structural evolution due to M-site mixing is studied in the compound FeCoMo₃O₈ whose unit cell volume and lattice constant are slightly tuned compared with those of Fe₂Mo₃O₈ and Co₂Mo₃O₈. However, significant structure distortion occurs on the octahedron and tetrahedron as a result of the relaxation of the unbalanced force, caused by the distinction of the M ion radius in the mixing case. The octahedron and tetrahedron are shifted toward each other along the c-axis which increases the coplanarity of the Fe and Co ions. The octahedron is compressed along the D_3d-symmetry axis and its bonds are elongated significantly. A complex tension-like distortion is induced for the tetrahedron. Its bond along the C_3v-symmetry axis is elongated and other three bonds are shortened. The study demonstrates that ions mixing at the M sites would introduce intrigue structure modulation in the M₂Mo₃O₈ family, which would tune the electronic orbit structure. This may shed new light on understanding the emergent behaviors of the material family.