Experimental synthesis of possible aluminum nitride polytypes by using DFT calculations

We present the results of a first-principles study on the structural stability and electronic and optical properties of new aluminum nitride (AlN) polytypes. The study includes the following experimentally or theoretically known phases of AlN: wurtzite (WZ), zincblende (ZB), and rocksalt (RS) structures, which complement the pressure-dependent phase diagram of this industrially important compound. In addition to the structures of AlN considered in previous studies, we evaluated the dynamical stability of various novel phases, viz., SiC(4H), ZnS(15R), BeO, 5-5, TiAs, NiAs, MoC, Li2O2, and NiS. These were predicted recently in a high-pressure data mining study of more than 140,000 variations of the AlN structure, which claimed that they were either stable or nearly stable, based on first-principles calculations. Based on the new AlN polytypes, the physical properties of all considered phases were compared, and the common trends and differences were determined. According to the phonon band structure calculations, nine phases of these new polytypes are free from imaginary frequencies. This indicates adequate dynamical stability, and experimental accessibility of the polytypes. Additionally, the calculated cohesive energies of the dynamically stable phases are comparable to those of WZ-AlN and those specified in the available literature. Furthermore, the observed electronic structures and optical properties indicate that the polytypism of AlN can be a practical tool for refining its physical and chemical properties. The new phases show significant potential for use in future electronic and optoelectronic applications of AlN.