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Wurtzite-ZnO is a wide-bandgap polar material with a ferroelectric-switching barrier that is too high to utilize, but the barrier can be reduced and switching observed in substituted materials such as Zn0.5Mg0.5O. Here, we focus on the planar hexagonal structures h-ZnO and h-Zn0.5Mg0.5O that may act as transition states or else metastable intermediates along the switching pathway. Consensus is obtained by considering a range of pure and dispersion-corrected density-functional theory (DFT) computational approaches, as well as ab initio Hartree–Fock (HF), Møller–Plesset perturbation-theory (MP2), and random-phase approximation (RPA) calculations. The MP2 and RPA results are in good agreement and emphasize the need for uniform treatments of both long-range and short-range electron correlation, but methodological limitations and internal variations still limit quantitative accuracy, with, e.g., the RPA singles-excitation correction appearing to be important. Of the DFT methods considered, only r2SCAN/rVV10 is found to be generally reliable. The perceived stability of h-ZnO is found to be strongly influenced by the nature of the dispersion correction, with the consensus being that dispersion interactions are insufficient to stabilize h-ZnO as a metastable phase in infinite crystals. Dispersion forces are found to be most significant for hypothetical planar hexagonal structures constrained to the lattice vectors of the wurtzite phases, and are of similar or greater magnitude than the differences between the density functional methods used and their manifestation of the self-interaction error. In general, our results demonstrate that an accurate treatment of dispersion forces is essential when describing polarization switching and ferroelectric behavior in wurtzite-structured materials.
Zhang et al. (Tue,) studied this question.