New Type-I and Type-II Clathrates in the Systems Cs-Na-Ga-Si, Rb-Na-Ga-Si, and Rb-Na-Zn-Si

Systematic studies in the systems Cs-Na-Ga-Si, Rb-Na-Ga-Si, and Rb-Na-Zn-Si yielded the novel type-I clathrates with refined compositions Cs6Na2Ga8.25Si37.75(3), Rb6.34Na1.66(2)Ga8.02Si37.98(3), and Rb5.20Na2.80(4) Zn3.85Si42.15(2) (cubic, Pm (3) over barn), as well as the type-II clathrates with formulae Cs8Na16Ga22.7Si113.3(1), Rb8.4Na15.6(1) Ga19.6Si116.4(1), and Rb8Na16Zn8.4Si127.6(1) (cubic, Fd (3) over barm). In each system, the type-I and -II compounds are always co-crystallizing, irrespective of the reaction conditions. The structures derived from single-crystal X-ray diffraction confirm complete ordering of Cs and Na atoms, and nearly complete ordering of the Rb and Na guest atoms. The framework-building Si atoms are randomly substituted by Ga or Zn atoms on all framework sites with notable difference in the substitution patterns between the type -I and type -II structure. This, and other details of the crystal chemistry are discussed in this paper.