Endohedral Cluster Superconductors in the Mo-Ga-Sn System Explored by the Joint Flux Technique

Superconductivity in Mo4Ga20As with endohedral gallium clusters

We report the discovery and detailed investigation of superconductivity in Mo₄Ga₂₀As. Mo₄Ga₂₀As crystallizes in a space group ofI4/m(No. 87), with the lattice parametersa= 12.86352 Å andc= 5.30031 Å. The resistivity, magnetization, and specific heat data reveal Mo₄Ga₂₀As to be a type-II superconductor withT_c= 5.6 K. The upper and lower critical fields are estimated to be 2.78 T and 22.0 mT, respectively. In addition, electron-phonon coupling in Mo₄Ga₂₀As is possibly stronger than the BCS weak-coupling limit. First-principles calculations suggest the Fermi level being dominated by the Mo-4dand Ga-4porbitals.

Intermetallic Compound Re2Ga9Ge with Re- and Ge-Embedded Gallium Clusters: Synthesis, Crystal Structure, Chemical Bonding, and Physical Properties

Transition metal-based endohedral cluster intermetallic compounds are interesting electron phases, which frequently exhibit superconductivity with a peculiar interplay between the critical temperature and valence electron count. We present a new Re-based endohedral gallium cluster compound, Re₂Ga₉Ge. Its unique crystal structure (P4₂/mmc space group, a = 8.0452(3) Å, c = 6.7132(2) Å) is built by two types of gallium polyhedra: monocapped Archimedean antiprisms centered by rhenium atoms and tetrahedra containing a main-group element inside. The analysis of chemical bonding shows the presence of localized pairwise interactions between the p-block elements and the formation of multicenter bonds with the participation of d-orbitals of rhenium. In the electronic band structure, the Fermi level is located in a narrow pseudogap indicating the optimum band filling and thus explaining the virtual absence of a homogeneity range. The compound exhibits Pauli paramagnetism and metallic properties with unexpectedly low thermal conductivity. A sharp anomaly observed on the magnetic susceptibility and resistivity curves presumably indicates the electronic phase transition accompanied by charge ordering at the characteristic temperature of T * = 271 K in zero magnetic field.

Endohedral cluster intermetallic superconductors: at the frontier between chemistry and physics

When a transition metal combines with an excess of a p-metal, the latter forms endohedral clusters with the number of vertices up to 14. These clusters are the building units of endohedral cluster intermetallic compounds. Although discovered a few decades ago, they have gained renewed interest due to their peculiar crystal and electronic structures and frequently observed superconducting properties. Advances over recent years reveal that endohedral cluster architectures are flexible enough, enabling chemical substitutions and the formation of a series of structurally related phases, where the same clusters can be arranged in different ways. Within the structural series, the superconducting-state parameters, including critical temperature and magnetic field, can be controlled and finely tuned. Herein, we present the most recent results in the chemical properties and superconductivity of endohedral cluster intermetallics and provide an outlook for the field.

Semiconducting and superconducting Mo–Ga frameworks: total energy and chemical bonding

The superconducting Mo₄Ga₂₁ structure type is derived from the electron-precise MoGa₄ cubic framework.

Electronic stabilization by occupational disorder in the ternary bismuthide Li3-x-yInxBi (x ≃ 0.14, y ≃ 0.29)

A ternary derivative of Li₃Bi with the composition Li_3-x-yIn_xBi (x ≃ 0.14, y ≃ 0.29) was produced by a mixed In+Bi flux approach. The crystal structure adopts the space group Fd-3m (No. 227), with a = 13.337 (4) Å, and can be viewed as a 2 × 2 × 2 superstructure of the parent Li₃Bi phase, resulting from a partial ordering of Li and In in the tetrahedral voids of the Bi fcc packing. In addition to the Li/In substitutional disorder, partial occupation of some Li sites is observed. The Li deficiency develops to reduce the total electron count in the system, counteracting thereby the electron doping introduced by the In substitution. First-principles calculations confirm the electronic rationale of the observed disorder.