Preferential stability of the d-BCT phase in ZnO thin films

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Stoichiometric B4 thin films have formally divergent surface energies, which arise from the intrinsic dipole of the unit cell. Previous density functional theory studies have predicted that below a critical thickness this results in relaxation to the nonpolar planar h-MgO structure. The calculations presented here demonstrate that h-MgO-structured ZnO thin films are themselves unstable with respect to further relaxation to the d-BCT structure, which restores near-tetrahedral local coordination while minimizing the surface dipole. Although the B4→h-MgO relaxation is disfavored for slabs thicker than 20 layers, d-BCT is predicted to be the favored polymorph for slabs up to 54 layers. Nudged elastic band calculations and vibrational analysis indicate that the h-MgO→d-BCT relaxation is spontaneous at nonzero temperatures.
Original languageEnglish
JournalPhysical Review B
Issue number17
Publication statusPublished - 21 Sept 2009


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