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Amorphous mediated commensurate-incommensurate phase transition in dense potassium

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Original languageEnglish
JournalPhysical Review B
Publication statusAccepted/In press - 14 Nov 2019

Abstract

Solid-solid phase transitions in metals normally procede in an orderly way such that the position of the atoms in the two phases are related by some well-defined transformation path. This leads to particular orientation relationships between the phases. Alkali metals exhibit complex structures under pressure, however, the transformation kinetics and microscopic mechanisms between these are yet to be elucidated. Here, taking potassium as an example, we investigate the phase transformation behavior between close-packed fcc and
incommensurate host-guest structures (KIII). We use multi-scale molecular dynamics, with a machine-learned potential which fully reproduces the phase diagram and known phase transitions of potassium. We find that no straightforward, low energy path exists: previously proposed mechanisms having impossible high transformation barriers. The fcc to KIII transition occurs via a complex intermediate state which persists for many picoseconds. Any
phase transition could occur via an amorphous intermediate, so the free energy of the amorphous state places an upper bound on the transformation barrier, and any mechanism involving an intermediate state of higher free energy than amorphous can be precluded.

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