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Unique atom hyper‐kagome order in Na 4 Ir 3 O 8 and in low‐symmetry spinel modifications
Author(s) -
Talanov V. M.,
Shirokov V. B.,
Talanov M. V.
Publication year - 2015
Publication title -
acta crystallographica section a
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.742
H-Index - 83
ISSN - 2053-2733
DOI - 10.1107/s2053273315003848
Subject(s) - spinel , atom (system on chip) , crystal structure , nanoclusters , crystallography , order (exchange) , atomic orbital , octahedron , lattice (music) , materials science , condensed matter physics , physics , chemistry , quantum mechanics , nanotechnology , finance , computer science , economics , acoustics , embedded system , metallurgy , electron
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 Na 4 Ir 3 O 8 crystal. The formation of an atom hyper‐kagome sublattice in Na 4 Ir 3 O 8 is described theoretically on the basis of the archetype (hypothetical parent structure/phase) concept. The archetype structure of Na 4 Ir 3 O 8 has a spinel‐like structure (space group ) and composition [Na 1/2 Ir 3/2 ] 16 d [Na 3/2 ] 16 c O 32 e 4 . The critical order parameter which induces hypothetical phase transition has been stated. It is shown that the derived structure of Na 4 Ir 3 O 8 is formed as a result of the displacements of Na, Ir and O atoms, and ordering of Na, Ir and O atoms, ordering d xy , d xz , d yz orbitals 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.