Chemical bonding topology of superconductors

Unique atom hyper-kagome order in Na4Ir3O8and in low-symmetry spinel modifications

Group-theoretical and thermodynamic methods of the Landau theory of phase transitions are used to investigate the hyper-kagome atomic order in structures of ordered spinels and a spinel-like Na4Ir3O8crystal. The formation of an atom hyper-kagome sublattice in Na4Ir3O8is described theoretically on the basis of the archetype (hypothetical parent structure/phase) concept. The archetype structure of Na4Ir3O8has a spinel-like structure (space group Fd\bar 3m) and composition [Na1/2Ir3/2]16d[Na3/2]16cO32e4. The critical order parameter which induces hypothetical phase transition has been stated. It is shown that the derived structure of Na4Ir3O8is formed as a result of the displacements of Na, Ir and O atoms, and ordering of Na, Ir and O atoms, orderingdxy,dxz,dyzorbitals as well. Ordering of all atoms takes place according to the type 1:3. Ir and Na atoms form an intriguing atom order: a network of corner-shared Ir triangles called a hyper-kagome lattice. The Ir atoms form nanoclusters which are named decagons. The existence of hyper-kagome lattices in six types of ordered spinel structures is predicted theoretically. The structure mechanisms of the formation of the predicted hyper-kagome atom order in some ordered spinel phases are established. For a number of cases typical diagrams of possible crystal phase states are built in the framework of the Landau theory of phase transitions. Thermodynamical conditions of hyper-kagome order formation are discussed by means of these diagrams. The proposed theory is in accordance with experimental data.

Ti2Rh6B – a new boride with a double perovskite-like structure containing octahedral Rh6 clusters

Single crystals of Ti2Rh6B were synthesized by arc-melting the elements in a water-cooled copper crucible under an argon atmosphere. The new silver-like compound with metallic luster crystallizes in space group Fm-3m (no. 225) with a = 7.8191(5) Å, V = 478.05(5) Å3, and Z = 4. The refinement converged to R 1 = 0.0169 and wR 2 = 0.0486 for all 96 unique reflections and 7 parameters. The structure can be described as a defect double perovskite, A2BB'O6, where the A site is occupied by titanium, the B site by boron, the O site by rhodium but the B' site is vacant leading to the formation of perfectly octahedral Rh6 clusters and BRh6 units. Rh—B, Rh—Ti, and Rh—Rh interactions are observed. According to density-functional (LMTO) electronic structure calculations, the strongest bonding occurs for the Rh—B contacts, and the Rh—Rh bonding within the clusters is more than two times stronger than in the BRh6 units.