Structural and magnetic properties of spinel compound Fe_1+xCo_xV₂O₄

Vanadium spinel oxides AV₂O₄ have attracted much attention for recent years because they show the peculiar physical properties which are caused by competition and cooperation of spin, orbital and lattice degrees of freedom. Among such compounds, FeV₂O₄ is a unique compound showing successive phase transitions: cubic to tetragonal (c < a) at ~140 K, from tetragonal to orthorhombic accompanied by ferrimagnetic transition at ~110 K and from orthorhombic to tetragonal (c > a) at ~70 K with decreasing temperature. It is suggested that these phase transitions originate from the orbital degrees of freedom of both Fe²⁺ ions at A-site (tetrahedral site) and V³⁺ ones at B-site (octahedral site), however, the origin remains controversial. In the present study, we investigate the substitution effect of Fe²⁺ with Co²⁺ having no orbital degrees of freedom to clarify the role of the orbital degree of Fe²⁺ at the A-site. We carried out magnetization and specific heat measurements and synchrotron powder diffraction experiments by the Debye-Scherrer camera at the beamline BL-8B at Photon Factory in KEK. For x ≤ 0.1, the successive structural transitions similar to that observed in FeV₂O₄ occur although the transition temperature of cubic-to-tetraLT transition rapidly decreases with increasing x. For 0.2≤ x ≤ 0.6, the only structural transition from cubic to tetragonal (c < a) was observed, however, the transition temperatures were somewhat different from the ferrimagnetic transition ones. On the other hand, for x ≥ 0.7, the crystal structure remains cubic down to 10 K similar to that of CoV₂O₄. These structural properties are discussed in terms of the orbital states of Fe²⁺ ions obtained by the normal mode analysis, and they are compared with the results of the specific heat and magnetization measurements.

Orbital Order and Structural Phase Transitions in Vanadium Spinel FeV₂O₄

Orbital degrees of freedom plays an important role in condensed matter physics because it is strongly related with phase transitions and induces the fascinating physical properties. A spinel oxide FeV₂O₄ is one of the peculiar examples because this compound has double orbital degrees of freedom at both Fe²⁺ and V³⁺ ions. Furthermore, this material represents exotic physical properties [1,2], i.e.; multiferroic, large magnetostriction, and successive structural transitions with decreasing temperature: cubic - tetragonal (c < a: tetraHT, 138K) - orthorhombic (orthoHT, 108 K) - tetragonal (c > a: tetraLT, 68 K). However, the origin of structural transitions and physical properties is controversial until now. In order to clarify the origin, we have performed synchrotron x-ray diffraction experiments at low temperatures at beamline BL02B2 (for the powder samples) in SPring-8 and BL-4C (for the single crystal) of the Photon Factory, KEK. Furthermore, we have carried out the magnetization and the specific heat measurements using polycrystalline samples and single crystal of FeV₂O₄. We have firstly found another orthorhombic phase (orthoLT) below 30 K in the polycrystalline sample of FeV₂O₄, shown in figure 1. The Rietveld analysis was performed, and the overall qualities of fittings were fairly good. In order to investigate the details of the orbital state of Fe²⁺ and V³⁺ in FeV₂O₄, we have performed the normal mode analysis, which is based on static displacements of the tetrahedron of FeO₄ and octahedron of VO₆. In the orthoLT phase, we found the orbital order of Fe²⁺ ions, which is mixture of 3z²-r² and y²-z² orbitals, without change of orbital order of V³⁺ ions. This result indicates that the origin of the orthoLT phase is derived from the competition between cooperative Jahn-Teller effect and relativistic spin-orbit coupling of Fe²⁺ ions. We also discuss the origins of the other phase transitions considering the orbital state of V³⁺ and Fe²⁺ ions, and then the orbital dilution effect, where the structural and magnetic properties are investigated by using powder samples substituted for Fe²⁺ and V³⁺ ions by other ions (Mn²⁺ and Fe³⁺) with no orbital degrees of freedom.

Nanoporous Structural Science Developed by the MEM/Rietveld Method

Wide-spread functionalization research of Metal Organic Frameworks(MOFs) has brought rapid increase in variety of materials since the beginning of structural study in nanoporous of MOFs were made by SR(Synchrotron Radiation) powder diffraction using the MEM(Maximum Entropy Method)/Rietveld Method(Kitaura et al, 2002). The MEM/Rietveld method has successfully applied to refine the structural position of absorbed molecules and to investigate a bonding nature between the molecules and MOF's pore walls. Noise-resistance electron density mapping with incomplete data set was a key advantage of MEM to visualize unmodeled feature of molecules in nanoporous. Since then, the charge density studies by the MEM/Rietveld Method have uncovered various ordering structure of absorbed molecules into nanoporous more and more(Takata, 2008). Those findings ignited trends to design the nanoporous as the space to be functionalized. Recently, the MEM/Rietveld method has been further developed as the method to map an electrostatic potential and electric field(Tanaka 2006). This technique is making a progress in structural science of MOFs since the visualized electrostatic potential in the nanoporous ought to provide information of interplay between the molecule and the pore walls. The talk will present the recent progress and challenges of the MEM/Rietveld method to the structural science of the MOFs.