THE VIBRATIONAL-SPECTRA AND DECOMPOSITION OF ALPHA-CALCIUM NITRIDE (ALPHA-CA3N2) AND MAGNESIUM NITRIDE (MG3N2)

alpha-Ca3N2 has been characterized by X-ray powder diffraction and its structure confirmed by a crystal structure refinement with the Rietve ld method. Ca3N2 and the isostructural Mg3N2 crystallize in the anti-b ixbyite structure of the mineral (Mn, Fe)(2)O-3 in the body-centered s pace group of Ia3(T-h(7)), and the general appearance of their infrare d and Raman spectra resembles that of the sesquioxides belonging to th e same space group. The decomposition of M3N2 (M = alpha-Ca, Mg) into M(OH)(2) and NH3, when exposed to the atmosphere, is reported. The pre sence of NH4+ vibrational bands in the decomposition products is expla ined in terms of the existence of Bronsted acid centers on the surface of the solid. During the initial stages of decomposition, infrared ba nds characteristic of adsorbed NH3 were observed, showing that Lewis c enters also exist on the surface. The decomposition product Mg(OH)(2) has weaker proton-donating centers than Ca(OH)(2). The kinetics of the decomposition of Mg3N2, which is a much slower reaction than that of alpha-Ca3N2, has been studied with FT-IR microspectrometry, and it is shown that the formation of Mg(OH)(2) is a three-dimensional diffusion process while the proton donation by the hydroxide to adsorbed NH3 to form NH4+ ions is a quasi-first-order reaction. (C) 1998 Academic Pre ss.