Total energy calculation of high pressure selenium: The origin of incommensurate modulations in Se-IV and the instability of proposed Se-II

TL;DR

This paper investigates high pressure selenium phases, explaining incommensurate modulations via phonon anomalies and questioning the stability and structure of the experimentally reported Se-II phase.

Contribution

It provides a theoretical analysis of selenium's high pressure phases, explaining incommensurate modulations and challenging the experimentally reported Se-II structure.

Findings

Incommensurate phases explained by phonon anomalies.

Calculated Se-II structure is metallic, conflicting with experimental semiconducting nature.

Proposed that experimental Se-II structure may be incorrect.

Abstract

We present calculation of the high pressure crystal structures in selenium, including rational approximants to the recently reported incommensurate phases. We show how the incommensurate phases can be intuitively explained in terms of imaginary phonon frequencies arising from Kohn anomalies in the putative undistorted phase. We also find inconsistencies between the calculated and experimental Se-II phase - the calculations show it to be a metastable metal while the experiment finds a stable semiconductor. We propose that the experimentally reported structure is probably in error.