[formula omitted] phase diagram of Na[formula omitted]C[formula omitted]O[formula omitted] at pressures up to 100 GPa

Abstract

In this work, a detailed search for stable structures of the Na2C2O5 composition was carried out using first-principles calculations and modern crystal structure prediction approaches. It was found that the Na2C2O5 compound is formed as a result of the reaction Na2CO3 + CO2 at a pressure of 4.5 GPa at 0 K and is stable in the P21 structure. This structure is consistent with recent experiments on the synthesis of Na2C2O5. The structure of Na2C2O5-P21 belongs to a new class of compounds, namely pyrocarbonates, and is characterized by the presence of [C2O5] pyro-groups formed by [CO3] triangles bonded through a common oxygen atom. Unlike alkaline earth metals, stable structures characterized by the presence of [CO4] tetrahedra have not been found for Na2C2O5. The calculated P–T phase diagram indicates the stability of Na-pyrocarbonate relative to the decomposition reaction in the pressure range 4.5–52.7 GPa at 0 K (6–58.2 GPa at 1000 K). Thus, for the first time, the lower limit of stability of Na-pyrocarbonate has been determined, which at 0 K is the same as that of the recently discovered Li-pyrocarbonate. Calculations of the phonon spectrum indicate that Na-pyrocarbonate is dynamically stable at atmospheric pressure.