Theoretical Study of the Properties of the Type II Clathrate A^xSn^136, (A = alkali atom; 0 <= x <= 24)

Motivated by recent experimental and theoretical interest in the x dependence of the properties of the Si and Ge-based Type II clathrate materials A^xSi^136 and A^xGe^136 (A = alkali atom) [1,2] we are carrying out a systematic theoretical study of the properties of the Sn-based Type II clathrate system A^xSn^136. Type II clathrates have cage-like lattices in which Si, Ge, or Sn atoms are tetrahedrally-coordinated and sp₃ covalently bonded. The cages can contain ``guests''; usually alkali or alkaline earth atoms. These materials are particularly interesting because of their potential use as thermoelectrics. Recent powder X-ray diffraction experiments have found the very interesting result that, for increasing x in the range 0 <= x <= 8 a lattice contraction occurs, and that x is increased further (8 <= x <= 24), a contrasting lattice expansion results. These observations have motivated us to study the behavior of the lattice constant and other properties as a function of guest concentration in other Type II clathrates. In the present paper, we report preliminary results of a density functional based theoretical study of the properties of K^xSn^136 as a function of x. We present results for the x dependence of the lattice constant as well as for other structural and electronic properties of this material. [4pt] [1] S. Stefanoski and G. Nolas, Cryst. Growth Des. 2011, dx.doi.org/10.1021/cg200756r[0pt] [2] M. Beekman, E. Nenghabi, K. Biswas, C. Myles, M. Baitinger, Y. Grin, G.S. Nolas, Inorg. Chem. 49 2010, DOI: 10.1021/ic1005049