Structural and magnetic phase transitions of theV4-cluster compoundGeV4S8

Abstract

The phase transitions of the strongly correlated tetrahedral V 4 -cluster compound GeV 4 S 8 have been studied by low-temperature powder x-ray diffraction, magnetic susceptibility, and specific heat measurements. The crystal structure is cubic at room temperature ͑space group F43m͒ and transforms to orthorhombic ͑space group Imm2͒ at T S = 30 K. A Jahn-Teller distortion reduces the symmetry of the V 4 -cluster from 43m to mm2. The second transition at 18 K is the onset of antiferromagnetic ordering without symmetry change but with a certain increase in the distortion. The latter reflects a strong magnetoelastic coupling at T N . Specific heat anomalies at 30 and 18 K confirm the two phase transitions.

Structure–property relationships of iron arsenide superconductors

“…Such behaviour is indeed known from materials with highly correlated electrons with relation to superconductivity, [66][67][68] and currently discussed in terms of orbital ordering. 69 On this account, lifting of the d xz /d yzdegeneracy by Coulomb repulsion leads to uneven occupation of these orbitals that make the orthorhombic structure energetically more favourable.…”

Low-temperature Crystal Structures and Magnetic Properties of the V4-Cluster Compounds Ga1-xGexV4S8

“…A report by Chudo et al about a rhombohedral (R3m) distortion of GeV 4 S 8 was recently disproved [11]. In general, we have observed suppressions of the structural transitions in GaMo 4 S 8 -type compounds with mixed metal site occupations like Ga x V 4−y Cr y S 8 [18], GaNb 4−x Mo x S 8 [19] and GaV 4−x Mo x S 8 [24].…”

“…Fig. 6 shows the Rietveld fit of Ga 0.5 Ge 0.5 V 4 S 8 at 10 K. The structural data of the low-temperature phase GeV 4 S 8 (space group Imm2) [11] have been used as initial parameters, and the refinements resulted in very similar structural parameters for Ga 0.5 Ge 0.5 V 4 S 8 . As for the pure germanium compound, the distortion is mainly caused by the change of the ideal V 4 tetrahedron into a butterflylike structure, where one V-V edge is shortened by 6.5 pm, and the opposite edge is elongated by 2.5 pm.…”

“…In order to check for symmetry reduction, we have recorded X-ray powder patterns of all samples at 10 K. Broadening or even splitting of some reflections was observed, best visible for the (440) reflection close to 54 • . We have shown earlier [11] that the splitting of this reflection in two components (220) and (208) indicates a rhombohedral distortion (R3m), whereas three components (400), (224), and (040) are only consistent with an orthorhombic lattice (Imm2). Fig.…”

“…Among them, the different properties of GaV 4 S 8 and GeV 4 S 8 are remarkable. By adding only one electron to the V 4 cluster, the magnetic ordering changes from ferro-to antiferromagnetic, and the structural distortion of GeV 4 S 8 leads to orthorhombic symmetry (space group Imm2) [11,12] instead of rhombohedral as in GaV 4 S 8 [9]. In this paper, we report the synthesis, crystal structures and magnetic properties of the solid solutions Ga 1−x Ge x V 4 S 8 in order to study the transition between these different behaviors in more detail.…”