The Vibrational Spectra and Decomposition ofα-Calcium Nitride (α-Ca3N2) and Magnesium Nitride (Mg3N2)

α-Ca3N2has been characterized by X-ray powder diffraction and its structure confirmed by a crystal structure refinement with the Rietveld method. Ca3N2and the isostructural Mg3N2crystallize in the anti-bixbyite structure of the mineral (Mn, Fe)2O3in the body-centered space group ofIa3(T7h), and the general appearance of their infrared and Raman spectra resembles that of the sesquioxides belonging to the same space group. The decomposition ofM3N2(M=α-Ca, Mg) intoM(OH)2and NH3, when exposed to the atmosphere, is reported. The presence of NH+4vibrational bands in the decomposition products is explained in terms of the existence of Brønsted acid centers on the surface of the solid. During the initial stages of decomposition, infrared bands characteristic of adsorbed NH3were observed, showing that Lewis centers also exist on the surface. The decomposition product Mg(OH)2has weaker proton-donating centers than Ca(OH)2. The kinetics of the decomposition of Mg3N2, which is a much slower reaction than that ofα-Ca3N2, has been studied with FT-IR microspectrometry, and it is shown that the formation of Mg(OH)2is a three-dimensional diffusion process while the proton donation by the hydroxide to adsorbed NH3to form NH+4ions is a quasi-first-order reaction.