Experimental Growth of New 6-fold Symmetry Patterned Microcrystals of AlN: Equilibrium Structures and Growth Mechanism

For the first time, we experimentally detected the formation of 6-fold symmetry patterned microcrystals of AlN (aluminum nitride), such as simple prisms, stellar and sectored plates, and stellar and fernlike stellar dendrites. These crystals were formed during combustion of a solid mixture of Al + kAlF₃ (k = 0.05-0.1 mol) under a nitrogen atmosphere (P_N2 = 2.5 MPa). The combustion temperature recorded with a thermocouple was in the 1700-1900 °C range. We examined the physical mechanisms governing the formation of AlN crystals, and we treated this problem as a case study of the dynamics of crystal growth from the liquid phase. Particular attention was given to the basic theoretical understanding of the subject, utilizing first-principles density functional theory calculations. The Wulff construction method was applied to identify equilibrium structures of nano- and microscale AlN crystals. Energy minimization arguments were used to show that certain crystal planes of AlN are more probable than others, giving the crystal 6-fold symmetry. The integrated experiments and computations form the basis for our proposal of the underlying mechanisms for nucleation and growth of the AlN crystals.