Simulation of structural phase transition in NaNO3 and CaCO3

The order-disorder phase transitions in NaNO3 and CaCO3 are simulated by molecular dynamics. The simulations are based on the potentials calculated from the Gordon–Kim modified electron gas formalism extended to molecular ions. We successfully reproduced the transition temperature Tc and the abnormally large c axis thermal expansion observed in experiment. The phase transitions in NaNO3 and CaCO3 were found to be initiated by ±60 and ±180° reorientation of the NO3 and CO3 ions about the c axis. The orientations of NO3 and CO3 ions are continuous with six preferred calcite-type orientations above the phase-transition temperature.