Searching for Entropically Stabilized Phases: The Case of Silver Iodide

D Mendels and J McCarty and PM Piaggi and M Parrinello, JOURNAL OF PHYSICAL CHEMISTRY C, 122, 1786-1790 (2018).

DOI: 10.1021/acs.jpcc.7b11002

Computational crystal-phase prediction means have become an important tool in materials science for the acceleration of material discovery. Material phases stabilized by substantial entropic effects, however, cannot be accounted for with today's widely used methods. We propose a new search method for such phases in the framework of metadynamics using surrogates for entropy and enthalpy as collective variables. Applying the method to AgI at low pressure, the method is found to be effective in discovering the material's entropically stabilized superionic phase as well as its enthalpically stabilized,8 phase. Through calculating the material's free-energy surfaces at different temperatures, we additionally estimate the system's melting temperature as well as the transition temperature between the beta and superionic phases. The calculation of the free energy is accelerated by the use of a path collective variable defined in the space of our surrogates for entropy and enthalpy.

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