Re-refinement of the structure of the planar hexagonal phase of ZnO nanocrystals

The planar hexagonal phase of ZnO, known as h-ZnO, g-ZnO, α-ZnO, the B_k structure, the 5-5 phase, the α-BN phase, etc., has P6₃/mmc symmetry and is implicated in ferroelectric switching mechanisms for wurtzite-ZnO. It is well known in thin films on substrates and can be stabilized by external pressure, but its possible existence is critical in high-purity nanocrystals under ambient conditions. Indeed, a crystal structure has been reported, but this work remains controversial as first-principles calculations predict very different structural properties. Herein, the original experimental data is re-refined, through phase-shift determination and Morlet wavelet transformation, that molecular dynamics simulations associate with a P6₃/mmc structure with unit-cell parameters at room temperature of a = 3.45±0.02 Å and c = 4.46±0.02 Å. These values are 0.35 Å and 0.80 Å, respectively, larger than those previously reported and in good agreement with computational predictions. This confirms that ZnO nanocrystals can form a metastable planar hexagonal phase. It provides key information pertaining to polarization switching in ZnO, its derivatives, and general wurtzite-structured materials.