Influence of grain size, surface energy, and deviatoric stress on the pressure-induced phase transition of ZnO and AlN

Abstract

The high pressure behavior of aluminum nitride (AlN) and zinc oxide (ZnO) nanocrystals was studied up to 30 GPa using second harmonic generation. ZnO (10 nm) crystals transform to the high pressure B1 phase at 16.6 GPa, ≈5 GPa higher than the corresponding value for a bulk sample. The transition of AlN (20 nm) and AlN (100 nm) occurred at 14 and 21.5 GPa at lower values than for bulk samples. Under non-hydrostatic pressure conditions, the transition pressures of ZnO (10 nm) and AlN (100 nm) decrease to 12.5 and 18 GPa, respectively. We determined the surface energy ( and ) of the B1 polymorphs. We show that the main reason for the size-dependent decrease of the transition pressure of AlN nanocrystals is due to the higher surface energy of the B4 phase relative to the surface energy of the B1 phase. We predict that it is possible to quench or synthesize pure B1 AlN to ambient conditions if the grain size is less than 8.5 nm.

Keywords:

• high pressure
• phase transition
• surface energy
• nanocrystals
• aluminum nitride
• zinc oxide

Acknowledgment

Financial support from the DFG, Germany, within SPP1236 (project Ba-4020) and the FOKUS program of the Goethe University is gratefully acknowledged.

Notes

^†This contribution is part of the final report summarizing the results of the DFG-priority project SPP1236, which was funded by the Deutsche Forschungsgemeinschaft from 2006–2013.