High-pressure structural study of a-Mn: solving a three decades-old mystery

TL;DR

This study combines high-pressure experiments and density functional theory calculations to resolve a long-standing question about the stability of a-Mn, revealing its extended stability up to 220 GPa and explaining the phase transition mechanisms.

Contribution

It demonstrates the extended stability of a-Mn under high pressure and explains the discrepancy with previous studies through electron transfer and ionic bonding effects.

Findings

a-Mn remains stable up to 220 GPa

The hcp-Mn phase becomes more stable above 160 GPa

Electron transfer stabilizes a-Mn via ionic bonding

Abstract

Manganese, in the a-Mn structure, has been studied using synchrotron powder x-ray diffraction in a diamond anvil cell up to 220 GPa at room temperature combined with density functional calculations (DFT). The experiment reveals an extended pressure stability of the a-Mn phase up to the highest pressure of this study, in contrast with previous experimental and theoretical studies. On the other hand, calculations reveal that the previously predicted hcp-Mn phase becomes lower in enthalpy than the a-Mn phase above 160 GPa. The apparent discrepancy is explained due to a substantial electron transfer between Mn ions, which stabilizes the a-Mn phase through the formation of ionic bonding between monatomic ions under pressure.