Polar binary Zn/Cd-rich intermetallics: Synthesis, crystal and electronic structure of A(Zn/Cd)13 (A = alkali/alkaline earth) and Cs1.34Zn16

The binary alkali/alkaline earth (A) zinc and cadmium (M) compounds that form the NaZn13 structure type (cubic, space group Fm3¯c), which were known only from indexed X-ray powder diagrams, have been synthesized from stoichiometric mixtures of the elements and have been structurally characterized using a combination of single crystal data and the Rietveld method. The alkali zinc phases AIZn13 are only formed with the lighter alkali elements sodium to rubidium (AI=Na: a  = 1227.3(2) pm, R1=0.0205;AI=K: a  = 1237.9(3) pm, R1=0.0154;AI=Rb: a  = 1245.19(1) pm, Rp=0.0250), whereas the cadmium compounds AI Cd13 are stable only for the heavier alkali metals potassium to cesium (AI=K: a  = 1379.2(4) pm, R1=0.0118;AI=Rb: a  = 1384.5(2) pm, R1=0.0139;AI=Cs: a  = 1392.0(3) pm, R1=0.0381). In the crystal structures M-centered M(2)M(1)12 icosahedra are connected via strong exobonds (forming tetrahedral stars, stella quadrangula) to form a 3D net, in the interstices of which the alkali metals are located in a snub cube 24 coordination. In contrast, the alkaline earth (AII) zinc compounds with calcium, strontium and barium (AII=Ca: a  = 1215.4(1) pm, R1=0.0274;AII=Sr: a  = 1222.2(2) pm, R1=0.0154;AII=Ba: a  = 1235.8(2) pm, R1=0.0227) show statistical defects at the Zn(2) position centering the Zn(1) icosahedra. Both the geometric and the electronic stability of the NaZn13 structure type, which can be assessed from the pseudo-bandgap calculated using FP-LAPW-DFT methods, are discussed. In contrast to the aforementioned systems, the NaZn13 type ceases to exist in the binary system Cs-Zn. In this case a phase Cs1.36Zn16 with a new structure type but very similar stoichiometry emerges. This compound is the first and only known phase in the system Cs-Zn and crystallizes with a new incommensurably modulated structure: its average structure (orthorhombic, space group Imma, a=264.2(5)  pm,b=720.8(14)  pm,c=1760(5)  pm, R1=0.0198) already indicates the representative structural features: in a 3D net of Zn atoms tubular channels filled by the Cs atoms are formed. This Cs filling is, due to geometric requirements, not commensurate with the Zn host structure resulting in a modulated structure with satellite reflections that can be indexed with a q→ vector of (0, 0, 0.2755). The final refinement in the superspace group (Pcn2(00g)sso,a=1760(5)  pm,b=720.8(14)  pm,c=264.2(5)  pm, wRmain=0.0550,wRsatellites=0.140) shows a strong occupational and a weak positional modulation of Cs in the channels formed by the Zn partial structure.