Formation and Stability of Ordered and Disordered Ba-substituted Calcite Phases.

Calcite has the ability to host large amounts of intracrystalline inclusions, a phenomenon that is known to be the case in biominerals and has been demonstrated in bio-inspired synthetic systems. In this study, we focused on barium as the inclusion. Highly substituted Ba-calcite possesses disordered carbonate orientations, characteristic of R-3m symmetry. We show that calcite undergoes an ordered-disordered transition, in which the carbonate groups undergo progressive loss of their rotational order with increasing amounts of incorporated Ba, and reach complete rotational disorder for a critical amount of Ba. We characterize this transition and propose a theoretical model justifying the influence of Ba. Moreover, the disordered R-3m Ba-substituted calcite has been previously identified as a high-temperature phase or as a highly metastable room-temperature phase. We challenge those descriptions by successfully synthesizing it under slow-rate conditions and by studying its thermal behavior, and we conclude that the fully disordered Ba-calcite is stable, whereas the transitional, partially disordered Ba-calcite phase is metastable.