Commensurate-incommensurate phase transition of dense potassium simulated by machine-learned interatomic potential

The alkali metal potassium exhibits complex structures under pressure, including both commensurate and incommensurate phases, however, the transformation kinetics and microscopic mechanisms between these are yet to be elucidated. Here, we investigate the phase transformation behavior between close-packed fcc and incommensurate host-guest structures (KIII). We use multiscale 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: The previously proposed displacive transformation mechanisms have impossibly high kinetic barriers. The fcc-KIII transition occurs in a complex and diffusive manner, and involves an intermediate amorphization process during the nucleation of the product phase. Our findings may provide further insight into phase transition theory.