Low-dimensional sublattice melting by pressure:: Superionic conduction in the phase interfaces of the fluorite-to-cotunnite transition of CaF2

The pressure-induced phase transformation of calcium fluoride (CaF2) from the cubic-fluorite-type structure (Fm (3) over barm) to the orthorhombic cotunnite (PbCl2) type structure (Pnma) is investigated from transition path sampling molecular dynamics simulations. Starting from an artificially prepared transformation route connecting fluorite to cotunnite, subsequent trajectory rectification evolved to a distinct picture of the favored mechanism. The latter is characterized by nucleation and growth of the new phase. The overall transformation mechanism was identified as a symmetry-lowering step from the cubic to the orthorhombic atomic configuration which is caused by the reorganization of one half of the octahedral voids. At the interface between the cubic and the orthorhombic structure, a pressure-induced local melting of the fluoride sublattice is observed. This produces defects that allow for the reorganization of the calcium sublattice which eventually leads to the recrystallization of the fluoride ions fixating one of the stable structures. Variation of the thermodynamic parameters shows that the mechanism is conserved over the experimentally relevant range, however with an increasing tendency towards incomplete transformation on lowering the temperature, in accordance with experiments.