Topochemical Control in the Solid-State Conversion of Cyclotrigallazane into Nanocrystalline Gallium Nitride

Pyrolysis of powdered samples of cyclotrigallazane, [H₂GaNH₂]₃, under a variety of conditions led to nanocrystalline gallium nitride. Thermogravimetric analysis established that the primary weight loss event occurred at 150 °C and mass spectral analysis verified the production of hydrogen with lesser amounts of ammonia. The final weight in the TGA experiments (achieved above 500 °C) and the elemental analyses established that the formula of the powder was GaN_0.83. Transmission electron microscopy (TEM) and X-ray line broadening established that the particles were approximately 60 Å in diameter, and highresolution TEM measurements revealed lattice fringes in the images. Modeling the X-ray diffraction data using the Debye equation established that the nanocrystalline GaN had neither the pure wurtzite nor the pure zinc blende structure. The best fit to the data was found for a random arrangement of stacking planes with an equal amount of cubic and hexagonal planes. The nanocrystalline material slowly converted into the known wurtzite phase at 900 °C. Synthesis of bulk GaN at 600 °C from Ga₂O₃ and NH₃ produced only the pure wurtzite phase. The synthesis of metastable samples of gallium nitride comprised of such a high percentage of the cubic phase was proposed to result from a topochemical reaction in which the hydrogen elimination (with concomitant Ga-N bond formation) occurs along a reaction coordinate established by the crystal and molecular structure of [H₂GaNH₂]₃.