This study investigated ammonothermal synthesis of nanocrystalline gallium nitride (GaN) in supercritical ammonia with acidic mineralizers NH4X (X = Cl, Br, I) at 400-500 degrees C. Results showed that three types of acidic mineralizers could effectively accelerate the formation of GaN. The mixed hexagonal/cubic phase fractions and lattice parameters of nanocrystalline GaN were calculated by the Rietveld refinement method. SEM showed an agglomerate of nanocrystalline GaN. A considerable amount of GaN was synthesized using NH4Cl as the mineralizer, however, there was no yield using NH4Br or NH4I at 400 degrees C. For acidic mineralizers, both hexagonal structures (wurtzite) and cubic structures (zincblende) were obtained in ammonothermal synthesis by XRD and Raman measurement. GaN synthesized with NH4Br and NH4I showed mixed phases of hexagonal-GaN (h-GaN) and cubic-GaN (c-GaN) at 450-500 degrees C. In the case of NH4Cl mineralizer, GaN only exhibited mixed phases of h-GaN and c-GaN at 500 degrees C, but pure h-GaN at 400-450 degrees C. Based on the results, NH4Cl favored pure h-GaN, and NH4Br and NH4I favored c-GaN at 400-450 degrees C. (C) 2008 Elsevier B.V. All rights reserved.