Abstract
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, ), 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, ). 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.
Highlights
Clathrates of the group 14 elements have already been known for five decades, being named in analogy to the gas hydrates G8(H2O)36 and G24(H2O)136 (G = Xe, Cl2, CH4, etc.) [1]
A24(M,Tt)136 are 24:136 or 1:5.67 compared to the ratio for type-I clathrates A8(M,Tt)46 of 8:46 or 1:5.75. The cavities of the former one have a larger difference in size; a directed synthesis should be possible with choosing different guest atoms combined with the ―right‖ M-substituted framework
We carried out investigations in the systems Cs–Na–Ga–Si, Rb–Na–Ga–Si, and Rb–Na–Zn–Si, and the resulting type-I and type-II clathrates are presented in this report
Summary
A24(M,Tt)136 are 24:136 or 1:5.67 compared to the ratio for type-I clathrates A8(M,Tt) of 8:46 or 1:5.75 The cavities of the former one have a larger difference in size; a directed synthesis should be possible with choosing different guest atoms combined with the ―right‖ M-substituted framework. Based on this hypothesis, we carried out investigations in the systems Cs–Na–Ga–Si, Rb–Na–Ga–Si, and Rb–Na–Zn–Si, and the resulting type-I and type-II clathrates are presented in this report
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