Suppression of phase transitions at low temperature by chromium substitution in vanadium spinel Fe(V_1−xCr_x)₂O₄

Abstract

Synchrotron powder diffraction, magnetization, and specific heat measurements have been used to investigate the structural and magnetic phase transitions in the spinel-type vanadate Fe(V1-x Cr x )2O4 (0.0 ⩽ x ⩽ 1.0). We observed five different transitions in our polycrystalline samples. The canted-type ferrimagnetic phase transition accompanied by the lattice distortion was suppressed by Cr substitution (x = 0.2). Additionally, high-resolution synchrotron powder diffraction revealed that the low-temperature orthorhombic phase appears in FeV2O4 below 30 K and disappears by subtle Cr doping. In contrast, for x ⩾ 0.7, we observed a different magnetic transition of possibly conical-type ferrimagnetic ordering that did not induce significant lattice distortion at the transition temperature. We performed structural refinements for low-temperature phases and suggest that the crystal system of Fe(V1-x Cr x )2O4 at 15K is an orthorhombic lattice for all x values. In the orthorhombic phase, a unique behaviour was observed, with the lattice constants a and b approaching each other as Cr doping increased. This behaviour can be explained by the change of orbitally ordered states of the Fe(2+) ions.

Even and odd crystal fields on Fe2+ ions, local lattice distortion parameters, electron-deformation interaction, and magnetoelectric coupling in FeCr2O4

“…The ground state of the iron ion is degenerate into orbital states. The structural phase transition from the cubic phase to the tetragonal phase (I41/amd) with an order of the ferrodistorsive type occurs at T = 135-150 K. As the temperature decreases, it exhibits the next transition at T ∼ 75 K, the orthorhombic lattice with the spaces symmetry group (Fddd) [7,8]. There are also successive magnetic phase transitions.…”