Atomic order and metallic nanoclusters in Na4Ir3O8

Abstract

A theory for forming Ir-atomic nanoclusters in a dielectric matrix of Na₄Ir₃O₈ structure and spinel-like structures is suggested. The atomic order in the Na₄Ir₃O₈ structure is investigated by group-theoretical methods of phase transition theory. The critical irreducible representation τ , generating appearance of enantiomorphic P4₁32 (P4₃32)-phases from high symmetry spinel-like phase with space group Fd3m, is six dimensional irreducible representation k₁₀(τ₁) (in Kovalev designation). Ir and Na atoms form an intriguing atom ordering, giving rise to a network of corner shared Ir triangles, called a hyperkagome lattice. It is shown that inside the hyperkagome lattice, there are closed metal contours of chemical bonds formed by Ir-clusters – decagons. Unusual physical properties of solid solutions on the basis of Na₄Ir₃O₈ are expected. The existence of hyperkagome lattices in six types in ordered spinel structures is theoretically predicted.