Molecular dynamics study of structural changes in berlinite

Fractional coordinates and anisotropic temperature factors of atoms in berlinite, AlPO4 with the quartz topology, were successfully simulated in a molecular dynamics simulation (MDS) at high temperatures. Time-dependent and time-averaged atomic order parameters were analyzed in detail with the aid of spectral densities calculated from trajectory data. These parameters show characteristic behavior of the order-disorder regime for a structure change, where atoms spend most of the time oscillating around the alpha(1)-sites (or alpha(2)-sites) in the low temperature alpha-phase, but oscillate over both sites in the higher temperature alpha-phase and the beta-phase. In the spectral density functions calculated for atom order parameters, a nearly zero-frequency excitation, which is accompanied by the emergence of large-scale al and alpha(2) clusters, appears at the Gamma point of the Brillouin zone below the transition point T-o, and increases in intensity up to T-o. A low-lying optic branch along Gamma-M, which is strongly temperature dependent in the small q-region, is another characteristic of the spectral density functions for the beta phase. The spectrum at Gamma continuously reduces its frequency from 0.6 THz at temperatures far above T-o to nearly 0 THz at temperatures approaching T-o from above. The dynamical behavior in beta berlinite rapidly but continuously changes from that in less oscillatory clusters in the vicinity of T-o to that in the typical beta phase at temperatures departing from T-o.