Molecular dynamics simulations and electronic structure calculations of type I and type II silicon clathrates

Abstract

The ability of Group IV elements to catenate underlies the possibility for a breadth of metastable allotropes with novel properties. Two such allotropes are type I and type II Si clathrates, materials which are characterized by their formation around guest atoms (typically Na) and their resultant cage structure. These clathrates act as metals when completely filled with Na, yet upon removal of the guest species they become wide band gap semiconductors. Using both plane-wave density functional theory (DFT) and molecular dynamic (MD) simulation methods, an exploration of the electronic, thermodynamic and dynamic (nucleation and growth) properties of these phases is undertaken. The driving force behind this work is the hypothesis that if synthesis of type I and type II Si clathrates could be achieved with noble gas guests in lieu of Na, then the semiconducting properties exhibited by the empty systems would be maintained and the energy expenditures associated with the forceful removal of Na diminished. By these means, two distinct investigations are reported: (1) the viability of noble gases as guests and their effect on the electronic nature of clathrates via quantum mechanical calculations, and (2) the development of an order parameter for the visualization and quantification of nucleation and growth in type II using MD simulations. Of the former, it was found that type II Si clathrate with Ne guests is within the range of thermodynamic stability while having a direct band gap of ~1.4 eV. With respect to the latter, an order parameter which successfully distinguishes between melted and ordered phases is reported.