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

Abstract

The solid solution Sn 24 As x P 19.3-x I 8 in the range of compositions x from 0 to 16 was synthe sized. The crystal structure was refined for seven compositions of the solid solution at -100 °C. All samples crystallize in the cubic space group Pm3 -n with the unit cell parameters a = 10.9358(2)-11.1495(2) Å and belong to the clathrate I structure type. The distribution of the phosphorus and arsenic atoms over two independent crystallographic positions within the clathrate framework was analyzed. The difference in the structure of the substituted clathrates with the cationic and anionic clathrate frameworks is considered.An important problem of creating highly efficient com pact cooling devices appeared at present due to the vigor ous development of computer technique and electronics and manufacturing application of alternative energy sourc es. 1 One of the methods for the solution of the problem is the search for new materials for the creation of thermo electric cooling units. 2 Such materials should combine high electric conductivity and low thermal conductivity, because the efficiency of the unit depends on the ratio of these parameters. 3,4 Among compounds with these prop erties, clathrates belonging to a large family of inclusion compounds evoke much interest. 5,6 Their crystal structure is a three dimensional lattice containing guest atoms or molecules in the cavities. 6 The semiconducting com pounds, whose structures are based on four coordinated silicon, germanium, or tin atoms and which contain either positively charged ions of alkali metals, alkaline earth metals, or europium and negatively charged ions of iodine or tellurium, are also classified as clathrates. 7,8 In the lat ter case, the positive charge is distributed over the clath rate lattice, due to which the compounds were named cationic clathrates. 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. It was shown that compound Sn 24 P 19.3 I 8 (space group Pm3 -n, a = 10.954 Å) has vacancies in the phosphorus position and displays the high electric con ductivity, 10-12 whereas for Sn 20.5 As 22 I 8 (space group Fm3 -, a = 22.1837(4) Å) vacancies are observed in the tin sublat tice, and their partial order induces the formation of the superstructure with the 8 fold increase in the unit cell volume. This superstructure is a complicated superposi tion of ordered d...