Ordering process and ferroelectricity in a spinel derived from FeV ₂O ₄

The spinel FeV ₂O ₄ is known to exhibit peculiar physical properties, which is generally ascribed to the unusual presence of two cations showing a pronounced interplay between spin, orbital, and lattice degrees of freedom (Fe2 ⁺ and V3 ⁺ on the tetrahedral and octahedral sites, respectively). The present work reports on an experimental reinvestigation of this material based on a broad combination of techniques, including x-ray diffraction, energy dispersive, and Mössbauer spectroscopies, as well as magnetization, heat capacity, dielectric, and polarization measurements. Special attention was first paid to establish the exact cationic composition of the investigated samples, which was found to be Fe _1.18V _1.82O ₄. All the physical properties were found to point out a complex ordering process with a structural transition at T _S = 138 K, followed by two successive magnetostructural transitions at T _N1 = 111 K and T _N2 = 56 K. This latter transition marked the appearance of electric polarization, and magnetization data were analyzed in detail to discuss the nature of the magnetic state at T<T _N2. An overall interpretation of the sequence of transitions was proposed, taking into account two spin couplings, as well as the Jahn-Teller effects and the mechanism of spin-orbit stabilization. Finally, the origin of ferroelectricity in Fe _1.18V _1.82O ₄ is discussed on the basis of recent models.