Investigation of substitution effects and the phase transition in type-I clathrates Rb_xCs_8-xSn₄₄□₂ (1.3≤x≤2.1) using single-crystal X-ray diffraction, Raman spectroscopy, heat capacity and electrical resistivity measurements

The substitution of cations in Rb_xCs_8-xSn₄₄□₂(1.3≤x≤2.1) is reported. The compounds crystallize at room temperature in the space group la over(3, -) d adopting the type-I clathrate 2×2×2 superstructure with partly ordered framework vacancies (□), whereas at higher temperatures they transform to the primitive, more disordered modification (space group Pm over(3, -) n). The guest atom distributions in the Sn cages on the Rb: Cs ratios is studied by means of single-crystal X-ray diffraction for Rb_2.1(1)Cs_5.8(1)Sn₄₄ at T=293 K (1), Rb_1.42(8)Cs_6.58(8)Sn₄₄ at T=293 K (2a), Rb_1.46(5)Cs_6.54(5)Sn₄₄ at T=373 K (2b) and Rb_1.32(8)Cs_6.68(8)Sn₄₄ at T=293 K (3). The structural order-disorder phase transition influences the electrical resistivity. The hysteresis observed for the electrical resistivity in combination with the symmetric shape of the specific heat anomaly suggests that the transformation is of first-order type and is characterized by an entropy change of about 2.5 J mol^-1 K^-1. The Raman spectrum for the low-temperature modification of 2 is also reported.