A single-crystal-growth study of GaN was performed between 2000-2400 degrees C at 6.5 GPa using GaN powder as a starting material. The average crystal size was about 100 mu m and the X-ray rocking curve showed a peak width less than 30 arcsec, suggesting a rather low dislocation density. Polycrystalline AlxGa1-xN (0 <= x <= 1) alloys were synthesized by a solid-phase reaction at 1800 degrees C and 6.0 GPa. The lattice constants, a and c, of the hexagonal lattice varied continuously with the nominal composition, indicating that the AlxGa1-xN alloy was formed over the entire composition range. The phase diagram was determined for InN in a high T-P span up to 20 GPa and 2000 degrees C by in situ diffraction measurements with brilliant X-rays from SPring-8, a third generation synchrotron radiation facility. The wurtzite-rocksalt phase boundary was located around 10 GPa with a steep negative dT/dP. InN decomposed into In and N-2 between 700 and 800 degrees C in the wurtzite phase region. The decomposition temperature increased rapidly in the rocksalt phase region, reaching 1700 degrees C at 18 GPa. Melting was not observed in the T-P span investigated and consequently, the possibility of single-crystal growth from the melt was ruled out for InN. (c) 2006 Elsevier B.V. All rights reserved.