Ba5Ga6: a phase with octahedral clusters of gallium

The crystal structure of Ba5Ga6 was determined by single-crystal diffractometry. It is hexagonal, hP22, space group P6c2 (n. 188), a = 7.771(2) A \u30a, c = 14.376(4) A \u30a, Z = 2, refined with anisotropic displacement factors, R = 0.076 with 358 reflections, and contains a disordered Ba position with partial occupation. The structure is characterized by octahedral Ga6 clusters, isolated and surrounded by a cuboctahedron of twelve Ba atoms. The barium cuboctahedra, by sharing of edges and faces, are condensed in a tridimensional framework. Application of Wade's rules to the Ga6 naked cluster gives an excess of two electrons per formula unit. The compound Ba5Ga6 replaces the phase "BaGa" given in the original Ba-Ga phase diagram

Examples of linear structures of intermetallic compounds described as intergrowth of segments of simple basic structures

Segments of the basic structures BaAl4, AlB2, W, Po, Cu3Au, CaF2 or hybrids obtained combining two segments of these types are used with the mechanism of intergrowth to describe nine linear structures of binary and ternary intermetallic compounds formed by rare earth or alkaline earth metals. Such a representation allows to display relationship among the structures. All examined compounds with one exception contain AlB2 fragments, that means rare earth or alkaline earth face-sharing trigonal prisms centred by the small atoms. Depending on the segments between which the AlB2 fragment is comprised, the atoms centring the prisms show a different behaviour in their waist contacts

Crystal structure of Nd3Au4 and Nd17Au36

The crystal structures of two phases in the neodymium-gold system were determined by single crystal X-ray diffractometer methods, '/.(MoKx = 0.7107 A. Nd3Au4(hR42) is trigonal, R3 (n. 148), a = 13.739(3) A, c = 6.088(2) A, Z = 6, R = 0.083 with 405 reflections, isotypic with Pu3,Pd4. Nd,7Au36(tP 106) is tetragonal, P4/nmm (n. 129), a = 15.835(4) A, c = 9.278(2) A, Z = 2, R = 0.082 with 540 reflections. The structure of Nd17Au36, which represents a new type, can be described as packing of bicapped square antiprisms with vertices occupied by five Nd and five Au atoms and centered by part of the gold atoms. This kind of polyhedron is identical to that of aluminium in the pyramidal site of the BaAl4 structure. In the Nd \u2014Au system the phase Nd17Au36 replaces the phase given as \u201cNdAu2\u201d. \ua9 1994, R. Oldenbourg Verlag. M\ufcnchen. All rights reserved