Effects of the order–disorder phase transition on the physical properties of A8Sn44□2 (A = Rb, Cs)

Abstract

Multi-temperature synchrotron powder diffraction and differential thermal analysis have revealed an enantiotropic order-disorder phase transition in Rb 8 Sn 44 , 2 and Cs 8 Sn 44 , 2 at 353 and 363 K, respectively. The low-temperature phases crystallize with a 2 Â 2 Â 2 superstructure of the conventional type-I clathrate. At higher temperatures both compounds show a phase transition with less ordering of the framework vacancies (,). Rb 8 Sn 44 has a Debye temperature of 152(1) K whereas the Rb atoms in the large cages have Einstein temperatures of 81(1) and 54(1) K for the displacement perpendicular and parallel to the host-structure hexagons. Thermoelectric properties of Rb 8 Sn 44 have been measured from 2 to 400 K. The Seebeck coefficient decreases rapidly above the transition temperature. This is explained by changes in the band structure following the phase transition. The 119 Sn Mo ¨ssbauer spectra for both compounds have also been investigated and their analysis suggests that the transformation occurs without altering the local environment of the tin atoms, but with a variation of the vacancy concentration in the domains of the crystal.

Investigation of substitution effects and the phase transition in type-I clathrates Rb Cs8–Sn44□2 (1.3≤x≤2.1) using single-crystal X-ray diffraction, Raman spectroscopy, heat capacity and electrical resistivity measurements

“…In contrast to the earlier reports a partial ordering of the vacancies occurs under formation of 2 Â 2 Â 2 superstructure (space group Ia3d) [4]. An enantiotropic order-disorder phase transition from the a form (Ia3d) to the high-temperature b form (Pm3n) with lower ordering of the vacancies occurs at 353 K and at 363 K for Rb 8 Sn 44 and Cs 8 Sn 44, respectively [5,6].…”

“…Single crystals of superstructure reflections of Rb 1.42 Cs 6.58 Sn 44 disappeared at 373 K and the full data collection at this temperature resulted in the composition Rb 1.46 Cs 6.54 Sn 44 (2b) in the primitive space group Pm3n (Tables 1-3). This high-temperature modification resembles well the structure of b-A 8 Sn 44 (A ¼ Rb, Cs) [5,6].…”

“…[6] the temperature-dependent resistivity (r(T)) measurements of 2 and Rb 8 Sn 44 were performed on singlecrystals employing a physical property measurement system (PPMS) of quantum design and a standard four-probe dc method with a constant dc current of 0.1 mA. The r(T) data were collected in the temperature range 250-400 K during the respective cooling and heating sequences in a rate of 0.7 K min -1 .…”

“…It was found that the phase transition affects significantly the electrical transport properties of Rb 8 Sn 44 , as the more disordered modification exhibits higher electrical conductivity and Seebeck coefficient (in absolute value) [6]. However, the spark plasma sintering of the clathrate into a pellet caused partial decomposition to b-Sn which exhibits metallic properties.…”

Synthesis, Structure, and Properties of Clathrate Si30.3(8)P15.7(8)Se7.930(3)

“…Se2 (8c), Se3 (4b), and Se4 (4a)w ere surrounded by 20 vertex atoms located in the 12-fold (Si)a nd 8-fold (Si/P) positions. In contrast, the superstructures of Cs 8 Sn 44 [21] and Ba 8 Ge 43 (Ia3 d) [22] and [Sn 14 In 10 P 21.2 ]I 8 (P42/m) [23] formed for the partial or complete ordering of vacancies within the framework. Green pental dodecahedra formed by twelveo range Si2 and eight deepb lue P3 atoms embraced the red Se3 atom.…”