Investigation of substitution effects and the phase transition in type-I clathrates Rb xCs 8–xSn 44□ 2 (1.3≤ x≤2.1) using single-crystal X-ray diffraction, Raman spectroscopy, heat capacity and electrical resistivity measurements

Abstract

The substitution of cations in Rb x Cs 8– x Sn 44□ 2(1.3≤ x≤2.1) is reported. The compounds crystallize at room temperature in the space group la 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 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)Sn 44 at T=293 K ( 1), Rb 1.42(8)Cs 6.58(8)Sn 44 at T=293 K ( 2a), Rb 1.46(5)Cs 6.54(5)Sn 44 at T=373 K ( 2b) and Rb 1.32(8)Cs 6.68(8)Sn 44 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.