Synthesis and crystal structure of mercury-substituted type-I clathrates A₈Hg₄Sn₄₂ (A = K, Rb, Cs)

The mercury-substituted type-I clathrates A₈Hg ₄Sn₄₂,with A = K, Rb or Cs, were obtained by fusion of the pure elements at high temperatures. The crystal structures of the compounds were refined from single-crystal X-ray diffraction data. They crystallize in the space group Pm3̄n (No. 223), Z = 1 with a = 12.1255(4) Å for K ₈Hg₄Sn₄₂ (1), a = 12.1838(4) Å for Rb₈Hg₄Sn₄₂ (2) and a = 12.2130(4) Å for Cs₈Hg₄Sn₄₂ (3). The 3D framework of four-bonded atoms defines two types of polyhedral cages of different size that are fully occupied by the alkali-metal atoms. All three compounds are considered as formally charge-balanced Zintl phases without any homogeneity range. Differential thermal analysis (DTA) indicates that the stability of the clathrates significantly depends on the size of the encaged cations. The thermal stability of the title compounds and the binary phases A₈Sn ₄₄ (A = K, Rb, Cs) is discussed. Temperature-dependent magnetic measurements for compound 3 show also the expected diamagnetic behaviour.