GaV 4 S 8 was prepared by direct heating of the elements at 1123 K. The crystal structures were refined from single-crystal data at room temperature (RT-GaV 4 S 8 , GaMo 4 S 8 -type, F4 h3m, a ) 9.661(1) Å, Z ) 4) and by the Rietveld method at 20 K (LT-GaV 4 S 8 , R3m, a ) 6.834(1) Å, R rh ) 59.66(2)°, Z ) 1). Magnetic measurements show weak paramagnetism with a temperature-dependent magnetic moment between 1.5µ B (50 K) and 1.9µ B (300 K) compatible with one unpaired electron per V 4 cluster. Ferromagnetic ordering was detected below T C ) 10 K. DC electrical conductivity measurements between 30 and 320 K reveal semiconducting behavior. GaV 4 S 8 undergoes a structural phase transition at 38 K from cubic to rhombohedral symmetry, as known for GaMo 4 S 8 , but with an opposite change of the rhombohedral angle R rh . This behavior is explained by self-consistent band structure calculations and simple cluster orbital schemes for GaV 4 S 8 in comparison with GaMo 4 S 8 . The results prove that the directions of the structural deformations are determined by orbital interactions and depend on the electron count in the tetrahedral metal cluster units.
The crystal structures of the tetrahedral cluster compounds GaNb(4)S(8) and GaTa(4)Se(8) were determined by single-crystal X-ray diffraction. They crystallize in the cubic GaMo(4)S(8) structure type (F3m), which can be derived from the spinel type by shifting the metal atoms off the centers of the chalcogen octahedra along [111]. Electrical resistivity and magnetic susceptibility measurements show that the electronic conduction originates from hopping of localized unpaired electrons (S = (1)/(2)) among widely separated Nb(4) or Ta(4) clusters, and thus these materials represent a new class of Mott insulators. Under high pressure we find that GaNb(4)S(8) undergoes a transition from the Mott insulating to a superconducting state with T(C) up to 4 K at 23 GPa, similar to GaNb(4)Se(8) and GaTa(4)Se(8). High-pressure single-crystal X-ray studies of GaTa(4)Se(8) reveal that the superconducting transition is connected with a gradual decrease of the octahedral distortion with increasing pressure. DFT band structure calculations show that weakly coupled cluster orbitals are responsible for a high density of states at the Fermi level. The correct insulating magnetic ground state for GaNb(4)S(8) with mu(eff) = 1.73 mu(B) is for the first time achieved by the LDA+U method using U = 6 eV and rhombohedral symmetry.
Electronic conduction in GaM4Se8 (M=Nb,Ta) compounds with the fcc GaMo4S8-type structure originates from hopping of localized unpaired electrons (S=1 / 2) among widely separated tetrahedral M4 metal clusters. We show that under pressure these systems transform from Mott insulators to a metallic and superconducting state with T(C)=2.9 and 5.8 K at 13 and 11.5 GPa for GaNb4Se8 and GaTa4Se8, respectively. The occurrence of superconductivity is shown to be connected with a pressure-induced decrease of the MSe6 octahedral distortion and simultaneous softening of the phonon associated with M-Se bonds.
Eu2SnS4, Sr2SnS4, and Sr2GeSe4 have been synthesized by heating the elements or binary precursors at 1073 K and their crystal structures were determined by single crystal methods. Eu2SnS4 crystallizes with a new structure type (Pnma, a = 11.187(2), b = 8.768(2), c = 7.538(2)Å, Z = 4), consisting of distorted [SnS4]4— tetrahedra and sevenfold coordinated Eu2+ ions. Sr2SnS4 and γ‐Sr2GeSe4 are isostructural and form a new structure type likewise (Ama2, Z = 4, Sr2SnS4: a = 9.977(1), b = 10.311(2), c = 7.243(1)Å, Sr2GeSe4: a = 10.284(2), b = 10.543(2), c = 7.411(1)Å) with more regular tetrahedral anions and strontium coordinated by seven and eight chalcogen atoms, respectively. Eu2SnS4 is a Curie‐Weiss paramagnet (7.80(2) μB/Eu; θ = 4.2(2) K) and orders antiferromagnetically at TN = 5.5(2) K with a magnetization of 6.56(5) μB/Eu at 5.5 T. The divalent nature of europium is also evident from 151Eu Mössbauer spectra which show a single signal at an isomer shift of —12.2(1) mm/s at 78 K. Full magnetic hyperfine field splitting (19.5(2) T at the europium nuclei) is observed at 4.2 K. The 119Sn spectrum shows one signal at 1.25(9) mm/s subject to quadrupole splitting of 0.76(2) mm/s. At 4.2 K a transferred hyperfine field of 3(1) T is detected at the tin site.
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