Dynamic Disorder of [BO₃] Groups in the LiBa₁₂(BO₃)₇F₄ and NaBa₁₂(BO₃)₇F₄ Crystals

Based on the ab initio molecular dynamic simulations, for LiBa12(BO3)7F4 and NaBa12(BO3)7F4, the transition into a dynamically disordered state was established. In LiBa12(BO3)7F4, the [BO3] groups and Li atoms located in the channels pass into the dynamically disordered state above ∼700 K. In NaBa12(BO3)7F4, the dynamic disorder of the [BO3] groups occurs at the lower temperature, ∼500 K. Na atoms, in contrast to Li atoms, do not pass into the dynamically disordered state at any temperature. It was shown that the dynamic disorder of the [BO3] groups increases the symmetry of the structure from ordered P42bc to P42/mbc and I4/mcm.