Kinetics and mechanism of the beta- to alpha-CuAlCl4 phase transition: A time-resolved Cu-63 MAS NMR and powder X-ray diffraction study

The beta and alpha phases of CuAlCl4 have been characterized by solid-state Al-27 and Cu-63 magic angle spinning nuclear magnetic resonance. The very short spin-lattice relaxation times of the copper spins, and the sensitivit y of the I = (3)/(2) Cu-63 nucleus to the small differences in the local st ructure of Cu in the two phases, allowed Cu-63 spectra to be acquired in ve ry short time periods (1 min), in which the beta and alpha phases were clea rly resolved. This time resolution was exploited to follow the phase transi tion from the pseudohexagonal close-packed beta -CuAlCl4 into the pseudocub ic close-packed alpha -CuAlCl4, which occurs above 100 degreesC. In situ ti me-resolved Cu-63 MAS NMR and synchrotron X-ray diffraction experiments wer e used to measure the kinetics of this phase transition as a function of te mperature. The transformation was shown to be a first-order phase transitio n involving no intermediate phases with an activation energy of 138 kJ/mol. The kinetic data obey a first-order Avrami-Erofe'ev rate law. A one-dimens ional growth mechanism is proposed that involves a combination of Cu+ ion s elf-diffusion and a translational reorganization of the close-packed anion layers imposed by the periodic rotations of [AlCl4](-) tetrahedra. This bet a to alpha phase transformation can be induced at ambient temperatures by l ow partial pressures of ethylene.