Structure and bonding of ternary gallides CaGa1−x(Si/Sn/Al/In)x with the CrB type and related structures

The polar binary alkaline earth (AII) tetrelides AIIMIV are known to crystallize with the CrB type, where the two-bonded M atoms of planar zig-zag chains (6 v.e./M) are surrounded by stretched trigonal prisms of A cations. In contrast, in the only known isotypic trielide, CaGa (with only 5  v.e./M) these prisms are substantially compressed. Hence, the phase widths of mixed gallides/tetrelides AIIGa1−xMxIV (MIV=Si, Sn) forming the CrB structure type and mixed trielides CaGa1−x(Al/In)x of related types were studied synthetically, structurally and by theoretical calculations. All compounds were synthesized from stoichiometric mixtures of the elements and characterized by single crystal X-ray diffraction. DFT band structure calculations were carried out for the binary border compounds. In mixed tetrelides CaGa1−xSix, the phase width of the CrB type reaches from CaSi up to x=0.42; in CaSn the maximum of Ga substitution is 0.41, whereas nearly no tetrel substitution is observed in CaGa. The geometrical changes (inter- and intra-chain M-M distances) during these substitutions (involving reduction of v.e./M) as well as the observed substitution limits and the changes of the bonding character in the planar chains are explained by the results of the band structure calculations. Partial exchange of Ga in CaGa against Al or In results in a series of mixed trielides with large monoclinic commensurately modulated superstructures of the CrB type: these phases, CaGa0.67Al0.33 (space group C2/c, a=2717.7(12), b=1159.9(3), c=607.4(2) pm, β=92.42(3)°, Z=36, R1=0.0374), CaGa0.84In0.16 (space group P21/c, a=1222.9(8), b=1167.9(5), c=612.0(3)  pm, β=95.24(3)°, Z=16, R1=0.0558) and CaGa0.55In0.45 (space group P21/c, a=936.1(6), b=1188.1(9), c=618.5(3) pm, β=101.73(4)°, Z=12, R1=0.0793) show only very small phase widths. In their structures the b axis is unchanged with respect to the CrB type and the enlargement of the a-c basis can be described by a general transformation matrix. In all three structures, the M chains are of wavelike form, varying the inter-chain distances such that Al and In atoms tend to approach each other, whereas Ga tends to form short intra-chain distances while avoiding close inter-chain contacts.