Preferential occupation of alkaline-earth metal sites in a seemingly disordered solid solution of Ca11-xSrxSb10: Single crystal structures of Ca8.63(5)Sr2.37Sb10 and Ca3.66(7)Sr7.34Sb10

Two new antimony based intermetallic phases, Ca8.63(5)Sr2.37Sb10(1) and Ca3.66(7)Sr7.34Sb10(2), crystallizing in Ho11Ge10 structure type (tetragonal, I4/mmm) have been synthesized and characterized. Although both Ca11Sb10 and Sr11Sb10 are known to be isostructural (Ho11Ge10 structure type) and hence all Ca sites should be accessible to Sr as well, it appears that certain sites are preferentially ordered by Ca in the mixed (Ca/Sr)11Sb10 compounds reported here. The crystal structure of Ca8.63(5)Sr2.37Sb10 and Ca3.66(7)Sr7.34Sb10 has been solved from single crystal X-ray data using direct methods and refined using full-matrix least-squares method. The structure can be described as bonded network of A-Sb (A=Ca, Sr) with Sb existing as isolated Sb3-, diantimony Sb24- and Sb44- square units. Simple valence electron count reveals these compounds to be Zintl phases. It is found that the larger Sr or Sr/Ca ions preferentially occupy sites that are closer to the diantimony anions as compared to the smaller Ca ions