Novel intermetallics with the Ca11 Ga7 structure type

The binary alkaline earth (AII) trielides Ca11 Ga7 and Sr11 In7 crystallize with a singular structure type (cubic, space group Fm3¯m, a≈1600 (AII=Ca) and 1700 (AII=Sr) pm, Z=8). Fourteen new ternary Ca and Sr mixed trielides and triel/tetrelides forming this structure type were synthesized from stoichiometric mixtures of the elements and characterized by single crystal X-ray diffraction. The structures exhibit isolated M(1) atoms (M(2)=Al, Ga, In, Sn, Pb) together with tetrahedral units [M(1)4], which are formed by the triel elements exclusively. Remarkably, one of the four crystallographically independent AII positions is - as in the pure element - coordinated by AII partners in a cuboctahedral arrangement. In the mixed MIII /MIV compounds, a maximum phase range up to a maximum composition limit of A11[M(1)4III][M(2)IV]3 is observed, which nearly corresponds to an ionic description according to the Zintl concept (tetrahedral anions [M4III]8− isoelectronic to white phosphorus P4 besides isolated noble gas isosteric tetrelide anions [MIV]4−). In the case of the Sr/In/Pb and some Ca compounds, the A position that is coordinated by alkaline earth atoms only is statistically substituted by the heavier triel/tetrel element. The stabilities and phase ranges of the compounds, which are influenced by geometric and electronic factors (investigated by FP-LAPW band structure calculations), are discussed.