Polar binary Zn/Cd-rich intermetallics: Synthesis, crystal and electronic structure of A(Zn/Cd) 13 (A = alkali/alkaline earth) and Cs 1.34Zn 16

Abstract

The binary alkali/alkaline earth (A) zinc and cadmium (M) compounds that form the NaZn 13 structure type (cubic, space group F m 3 ¯ 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 A IZn 13 are only formed with the lighter alkali elements sodium to rubidium ( A I = Na : a = 1227.3(2) pm, R 1 = 0.0205 ; A I = K : a = 1237.9(3) pm, R 1 = 0.0154 ; A I = Rb : a = 1245.19(1) pm, R p = 0.0250 ), whereas the cadmium compounds A I Cd 13 are stable only for the heavier alkali metals potassium to cesium ( A I = K : a = 1379.2(4) pm, R 1 = 0.0118 ; A I = Rb : a = 1384.5(2) pm, R 1 = 0.0139 ; A I = Cs : a = 1392.0(3) pm, R 1 = 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 ( A II ) zinc compounds with calcium, strontium and barium ( A II = Ca : a = 1215.4(1) pm, R 1 = 0.0274 ; A II = Sr : a = 1222.2(2) pm, R 1 = 0.0154 ; A II = Ba : a = 1235.8(2) pm, R 1 = 0.0227 ) show statistical defects at the Zn(2) position centering the Zn(1) icosahedra. Both the geometric and the electronic stability of the NaZn 13 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 NaZn 13 type ceases to exist in the binary system Cs–Zn. In this case a phase Cs 1.36Zn 16 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, R 1 = 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 ( P c n 2 ( 0 0 g ) s s o , a = 1760 ( 5 ) pm , b = 720.8 ( 14 ) pm , c = 264.2 ( 5 ) pm, w R main = 0.0550 , w R satellites = 0.140 ) shows a strong occupational and a weak positional modulation of Cs in the channels formed by the Zn partial structure.