Crystal Structures, Electronic Properties, and Pressure-Induced Superconductivity of the Tetrahedral Cluster Compounds GaNb₄S₈, GaNb₄Se₈, and GaTa₄Se₈

Abstract

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.

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.…”

Electric‐Field‐Induced Resistive Switching in a Family of Mott Insulators: Towards a New Class of RRAM Memories

“…A direct consequence of this low gap value is the high sensitivity to external perturbation such as hydrostatic pressure which can induce an insulator to metal transition in the AM 4 X 8 compounds [12,13] . In that respect, the [ 14 ] .…”

Electric‐Pulse‐driven Electronic Phase Separation, Insulator–Metal Transition, and Possible Superconductivity in a Mott Insulator

“…GaTa 4 Se 8 is a rare example of a stoichiometric Mott insulator that undergoes a pressure-induced insulatorto-metal and superconductor transition (at 11.5 GPa). [6,7] This is possible because of the relatively small value of the Mott-Hubbard gap (calculated to be around 0.1 eV), [6] in comparison with the large gaps (several eV) of usual inorganic Mott insulators. [3] The reasons for this must be sought in its particular crystal structure: GaTa 4 Se 8 adopts a deficient spinel cubic structure of the GaMo 4 S 8 type (see Fig.…”

“…This peculiar topology leads to the formation of molecular-like electronic states within the clusters, and GaTa 4 Se 8 is considered as a special class of Mott insulator in which the correlations take place not on the scale of single atoms but, instead, on the scale of small clusters. [6] This difference of scale reduces the magnitude of Coulomb repulsion and makes this compound easier to tune through the MIT. Recently, a fascinating new route to achieve the insulatormetal transition has emerged.…”