Analysis of dynamic phase transition processes can be used to identify factors that cause some compounds to adopt crystal structures that are noncentrosymmetric (NCS). NCS structures are required for a variety of useful materials properties. We have examined a two-step, temperature driven reconstructive phase transition in the compound KNaNbOF5 using empirical dynamic simulations, in-situ diffraction studies, and theoretical calculations. We used cryogenic temperature (~15K) and variable-pressure (1-10 GPa) diamond anvil cell single crystal diffraction experiments on the NCS, Pna21 phase of KNaNbOF5 to develop a set of empirical pairwise interatomic interaction functions of the complex five element system. Our investigation lead to the discovery of a dynamically disordered high temperature crystal structure and the mechanistic origin of the NCS phase stabilization upon cooling. The reconstructive transition going from the P4/nmm phase to the high temperature Cmcm phase is believed to result from a loss of O/F site ordering caused by rigid-type rotations of octahedral [NbOF5]2- anionic units. The high temperature phase was found to exist in a dynamically disordered state involving two locally stable phases in the potential energy landscape with Pbcm and Pnma space group symmetries.

Dynamic origins of noncentrosymmetry in KNaNbOF5

253rd National Meeting (2017)

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