A new monoclinic structure type for ternary gallide MgCoGa2

A new ternary derivative of the Laves phases in the Mg-Co-Ga system

Crystal structures of MgCoGa, Mg0.74CoGa0.52 and Mg0.49CoGa0.15 phases from the Mg-Co-Ga system were investigated using single-crystal diffraction. These structures belong to the family of so-called Laves phases. Hexagonal MgCoGa crystallizes as a disordered phase within the MgZn2 structure type. The orthorhombic structure of Mg0.74CoGa0.52 is a distortion variant of MgZn2 and URe2 structure type, and the structural relation is demonstrated in terms of a Bärnighausen formalism group-subgroup transformation scheme. The structure of trigonal phase Mg0.49CoGa0.15 is strongly disordered, as is shown by the presence of adjacent atomic sites which cannot be occupied simultaneously. In Mg0.49CoGa0.15, two subcells (A and B) were obtained in a ratio of 9:1. Subcell A is closely related to MgZn2-type.

MgMn4Ga18: a novel three-shell gallium cluster structure

The new ternary gallide MgMn4Ga18 (magnesium tetramanganese octadecagallium) was synthesized and its crystal structure determined by means of single-crystal X-ray diffraction. The MgMn4Ga18 structure can be described as that of a three core–shell cluster compound. The Mg atoms are surrounded by 16 adjacent Ga atoms, [MgGa16], and the respective coordination polyhedron is an octadecahedron. This [MgGa16] octadecahedron is encapsulated inside a [Ga32] icohexahedron, which is in turn encapsulated inside a [Ga40] pentacontaoctahedron. As a result, a three core–shell cluster, [MgGa16@Ga32@Ga40], is identified. Electronic structure calculations were performed by means of the TB-LMTO-ASA program and additionally confirm the existence of the core–shell packing of the clusters.

New ternary MgCo2Ga5 and MgNi2Ga5 gallides

The crystal structure of new isostructural compounds MgCo2Ga5 and MgNi2Ga5 has been investigated using single-crystal X-ray diffraction. Both compounds represent a new type of structure: orthorhombic, space group Pnnm, oP16, with the following lattice parameters: a = 6.2700(2) Å, b = 6.6946(2) Å, c = 6.0789(2) Å (for MgCo2Ga5) and a = 6.2693(3) Å, b = 6.6968 Å, c = 6.0794 Å (for MgNi2Ga5). The MgCo2Ga5 and MgNi2Ga5 are closely related to the tetragonal structure of CoGa3 which crystallizes in the ht-IrIn3 type. The orthorhombic structures of MgCo2Ga5 and MgNi2Ga5 are derived from CoGa3 via a translationengleiche symmetry reduction of index 2. The symmetry reduction from P42/mnm to Pnnm causes that the 4c site splits into two sites 2c and 2d. The gallium atoms together with cobalt or nickel form 3D-nets with channels, in which magnesium atoms are inserted. The formation of these polyatomic nets is confirmed by distribution of electron localization function (ELF) and charges of atoms.