The characterisation of the “X” crystal structure in the Stillinger-Weber potential

Domagoj Fijan,Mark Wilson

doi:10.1016/j.cplett.2017.07.083

Abstract

A recently observed crystal structure is characterised by reference to the calculated diffraction patterns. The “X” crystal is related to the β-Sn crystal structure via an atom displacement and a reduction in symmetry from tetragonal to orthorhombic. A further crystal polymorph is identified which retains the tetragonal symmetry of the β-Sn structure but involves a related atom displacement. The unit cells and space groups are reported. The two crystals are labelled as o-X and t-X respectively. The transformation mechanisms from the β-Sn crystal structure to the two structures are identified. No routes to these crystals from other polymorphs are found.

Highlights

There has been enormous levels of experimental and theoretical effort devoted to probing the phase behaviour of systems dominated by local tetrahedral environments under ambient conditions
Computer simulation offers an alternative method for exploring the phase space both as a function of temperature and pressure and as a function of the potential model itself
Whilst electronic structure methods [9] offer potentially the most accurate results relatively simple potential models, in which the system energy is expressed as a function of the atom positions, offer a flexible and efficient method for exploring the morphology space

Summary

Introduction

There has been enormous levels of experimental and theoretical effort devoted to probing the phase behaviour of systems dominated by local tetrahedral environments under ambient conditions. Systems such as Si and Ge appear homologous in that (broadly speaking) their phase diagrams map onto each other. Recent investigations have uncovered yet more complexity and richness in the phase diagrams of silicon and the related mono-water (mW) models [21] These models employ a Stillinger-Weber (SW) potential in which the system energy is effectively expressed as the sum of 2- and 3-body terms [28]. A full understanding of the phase diagrams of systems such as Si and Ge is still potentially lacking

The ‘‘X’’ crystal

Determination of the space group and primitive cell

The diffraction pattern

Transformation mechanism

Attempted transformations to the X phase

Conclusions