Detectability of core level crossing and electronic topological transformations: the case of Osmium

TL;DR

This paper investigates the electronic and structural changes in osmium under high pressure, demonstrating that observed anomalies are likely due to mechanical effects rather than electronic topological transitions.

Contribution

The study shows that electronic level crossing and topological transitions in osmium can be explained by density functional theory without affecting the c/a ratio, suggesting alternative causes for experimental anomalies.

Findings

Electronic phenomena are reproducible in DFT but do not affect c/a ratio.

Observed c/a anomalies likely result from mechanical yield under stress.

Core-level overlap effects on crystal structure are not conclusively supported.

Abstract

Osmium, the least compressible metal, has recently been observed to undergo abrupt changes in the c/a ratio at extreme pressures. These are claimed to provide evidence for two unusual electronic behaviors: a crossing of the semicore 4f and 5p levels, and an electronic topological transition. We demonstrate that these two electronic phenomena are readily reproduced and understood in density functional theory, but that neither perturbs the trend in c/a ratio against pressure. Hence the observed anomalies in c/a must have another cause. Osmium is also notable for its high yield stress: the c/a anomalies lie well within the differential strains which osmium can support. We propose that observed c/a changes can arise from mechanical yield of crystallites with strong preferred orientation under high deviatoric stress in the experimental data. We discuss what evidence remains for the more general hypothesis that core-level overlap under pressure can have measurable effects on the crystal structure in any material.