Neutron Powder Diffraction Studies of Order-Disorder Phase Transitions and Kinetics

Abstract

One of the major applications of neutron diffraction in mineral sciences has been in the study of order-disorder processes and phase transitions. Neutron scattering methods provide unique insights into the origins and mechanisms of these processes, and have enabled mineralogists to develop new models of phase transformation behavior. The time-temperature dependence of processes such as phase transformations, exsolution, cation ordering and disordering in minerals has considerable potential geophysical, geochemical and petrological importance. Let’s take atomic ordering as an example. Order-disorder transformations and similar structural phase transitions are typically some of the most efficient ways a mineral can adapt to changing temperature or chemical composition. Disorder of distinct species across different crystallographic sites at high temperature provides significant entropic stabilization of mineral phases relative to low-temperature ordered structures. Positional or orientational disordering can have similar drastic effects. For example, the calcite-aragonite phase boundary shows a significant curvature at high temperature due to disorder of CO3 groups within the calcite structure, associated with an orientation order-disorder phase transition (Redfern et al. 1989). This leads to an increased stability of calcite with respect to aragonite over that predicted by a simple Clausius-Clapeyron extrapolation of the low pressure-temperature thermochemical data. Understanding of the structural characteristics of the phase transition in calcite developed in tandem with studies of the analogous transition in nitratine, NaNO3, but it was not until Dove and Powell (1989) carried out high-temperature neutron diffraction experiments on powdered calcite that there was direct experimental evidence linking the thermodynamic and structural nature of the transition in CaCO3. The key to the success of their study was the fact that neutrons penetrate the entire volume of samples held at extreme conditions (in this case very close to the melting temperature and under a confining CO2 pressure). Cation ordering …