Sn_20.5□_3.5As₂₂I₈: A Largely Disordered Cationic Clathrate with a New Type of Superstructure and Abnormally Low Thermal Conductivity

Abstract

Sn(20.5)As(22)I(8), a new cationic clathrate, has been prepared by using an ampoule technique. According to the X-ray powder diffraction data, it crystallizes in the face-centered cubic space group F23 or Fm(-)3 with a unit-cell parameter of a=22.1837(4) A. Single-crystal X-ray data allowed solution of the crystal structure in the subcell with a unit-cell parameter of a(0)=11.092(1) A and the space group Pm(-)3n (R=5.7 %). Sn(20.5)As(22)I(8) (or Sn(20.5) square(3.5)As(22)I(8), accounting for the vacancies in the framework) possesses the clathrate-I type crystal structure, with iodine atoms occupying the cages of the cationic framework composed of tin and arsenic atoms. The crystal structure is strongly disordered. The main features are a random distribution of vacancies, and shifts of the tin and arsenic atoms away from their ideal positions. The coordination of the tin atoms has been confirmed by using (119)Sn Mössbauer spectroscopy. Electron diffraction and high-resolution electron microscopy (HREM) analyses have confirmed the presence of the superstructure ordering, which results in a doubling of the unit-cell parameter and a change of the space group from Pm(-)3n to either F23 or Fm(-)3. Analysis of the crystal structure has led to the construction of four ordering models for the superstructure, which have been corroborated by HREM, and has also led to the identification of disordered regions originating from overlap of the different types of ordered domains. Sn(20.5)As(22)I(8) is a diamagnetic semiconductor with an estimated band gap of 0.45 eV; it displays abnormally low thermal conductivity, with the room temperature value being just 0.5 W m(-1) K(-1).

Semiclathrates of the Ge–P–Te System: Synthesis and Crystal Structures

“…It provides a rare example of a superstructure that combines strong disorder with an eight-fold increase in cell volume and retention of cubic symmetry. [20] The origin of the formation of the clathrate superstructure seems to be individual to each particular case. Even though the preferred coordination and environment of the framework atoms are known, it is difficult to predict from a general consideration not only the type of superstructure, but also the possibility of its occurrence.…”

Thermoelectric Power Generation by Clathrates

“…In both cases, no degradation of the initial sample was observed, proving that densiication does not change composition and structure of clathrates and that concomitant thermoelectric measurements are performed on the samples of desired nature. depending on concentration of indium and corresponding vacancies in clathrate framework [16,31]. Similarly, stoichiometric Si 30 P 16 Te 8 is not good electrical conductor with room-temperature value of 63.3 S m −1 [32].…”

“…The only reason for such diference is reported to be tremendous disorder in crystal structure of As-based compound, leading to signiicant scatering of charge carriers on laws of crystal structure [31].…”

“…Recently, it was shown that of-center displacement of guest atoms adds signiicantly to glasslike character of thermal conductivity; in particular, thermal conductivity of Sr 8 Ga 16 Ge 30 turns from crystalline-like to glass-like upon increasing of-center displacement of guest atoms siting on 6d site [43]. Clathrate Sn 20.5 As 22 I 8 displays combination of eightfold cubic superstructure of type-I clathrates with vacancies and mixed occupancies of sites within the framework [31]. In response to structural features, this compound exhibits very low thermal conductivity with roomtemperature value slightly over 0.4 W × m [44].…”

Distribution of phosphorus and arsenic atoms in the solid solution Sn24As x P19.3-x I8 with the structure of clathrate-I

“…More than 30 cationic clathrates that crystallize in the clathrate I structure type have been syn thesized in recent years, and the thermoelectrical proper ties of some representatives of this family were examined. 7, 9 In particular, it was assumed on the basis of analysis of their crystal structure that the electric conductivity and thermal conductivity of these compounds can be varied independently of each other by introducing various sub stituents into the host framework or into positions of the guest. The properties of the phases are influenced to most extent by fine peculiarities of their crystal structure, in cluding the specific features of vacancy ordering and dis tributions of diverse atoms over independent crystallo graphic sites.…”