Effect of nitrogen sorption mechanisms on the properties of fullerite C60 over a wide range of temperatures

Abstract

X-ray diffractometry is used to study the effect of the adsorption of nitrogen at a pressure of 30 atm and temperatures of 200–550 °C on the structural and thermodynamic properties of fullerite C60. The sorption kinetics of nitrogen at different temperatures are studied, and the lattice parameter is plotted as a function of the time for fullerite to saturate with nitrogen. The sorption mechanism is found to change with increasing saturation temperature. The diffusive filling of lattice octahedral voids by nitrogen atoms at temperatures below 450 °C is supplanted at higher temperatures by a chemical interaction of nitrogen with fullerite molecules leading to the formation of a new molecular compound, fullerite nitride C60Nx. The transition from physisorption to chemisorption of nitrogen by fullerite (the adsorption crossover) takes place at saturation temperatures of 450 > T > 400 °C. When C60 molecules are nitrogenated, the volume of the cubic cell increases dramatically, while the intensity of the x-ray reflections decreases and their width increases rapidly. The limiting distortions of the fcc lattice are determined, as well as the characteristic times for diffusional filling of the lattice voids and for nitrogenation, respectively, during physical and chemical sorption of nitrogen. Nitrogenation of fullerite molecules leads to a significant reduction in thermal expansion of the crystalline material, and suppresses both the orientational phase transition and formation of the glassy state.