Uncovering the Thermo-Kinetic Origins of Phase Ordering in Mixed-Valence Antimony Tetroxide by First-Principles Modeling

Phase ordering in the mixed-valence oxide Sb₂O₄ has been examined by density functional theory (DFT) calculations. We find that the ground-state total energies of the two phases (α and β) are almost degenerate and are highly sensitive to the choice of the approximation to the exchange correlation (xc) functional used in our calculations. Interestingly, with the inclusion of the zero-point energy corrections, the α phase is predicted to be the ground state polymorph for most xc functionals used. We also illustrate the pronounced stereochemical activity of Sb in these polymorphs of Sb₂O₄, setting an exception to the Keve and Skapski rule. Here, we find that the actual bonding in the α phase is more asymmetric, while the anomalous stability of the β phase could be rationalized from kinetic considerations. We find a non-negligible activation barrier for this α-β phase transition, and the presence of a saddle point (β phase) supports the separation of Sb(III) over a continuous phase transition, as observed in experiments.