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

TL;DR

This study re-refines the crystal structure of the metastable planar hexagonal phase of ZnO nanocrystals, confirming its existence under ambient conditions and aligning experimental data with theoretical predictions.

Contribution

It provides the first detailed re-refinement of ZnO nanocrystals' structure, clarifying previous controversies and supporting the metastable phase's stability at room temperature.

Findings

Lattice parameters at room temperature are a = 3.45 ± 0.02 Å and c = 4.46 ± 0.02 Å.

Refined values are larger than previous reports and match computational predictions.

Confirms the existence of a metastable planar hexagonal phase in ZnO nanocrystals.

Abstract

The planar hexagonal phase of ZnO, known as h-ZnO, g-ZnO, {\alpha}-ZnO, the Bk structure, the 5-5 phase, the {\alpha}-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 to be stabilized by external pressure, but critical is its possible existence 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 lattice parameters at room temperature of a = 3.45{\pm}0.02 {\AA} and c = 4.46{\pm}0.02 {\AA}. These values are 0.35 {\AA} and 0.80 {\AA}, 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.