High-quality doped and undoped GaN crystallites have been grown by reacting high-purity Ga-NH3 solutions and Ga alloys (Ga-Mg and Ga-Si) with ultra-high-purity ammonia in a horizontal quartz tube reactor at temperatures between 1000 and 1200 degrees C. An initial low-temperature treatment to dissolve ammonia into the Ga melt produces GaN powders with 100% reaction efficiency. The optoelectronic properties of these materials are comparable or superior to the best thin epitaxial films. Doping was achieved by dissolving doping impurities, such as silicon and magnesium, into the liquid metal. The powders synthesized by this method consist mostly of two differently shaped crystallites: large columns with lengths of around 20 mu m and small platelets with diameter between 1 and 3 mu m. All these crystallites have the hexagonal wurtzite structure and all exhibit strong room-temperature luminescence with the characteristic GaN-band-edge as well as silicon- and magnesium-related emissions. These characteristics make these materials strong candidates for optoelectronic applications. (C) 2007 Published by Elsevier B.V.