New ternary MgCo₂Ga₅ and MgNi₂Ga₅ gallides

Abstract 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.