The R3c → R3m transition in nitratine, NaNO3, and implications for calcite, CaCO3

The temperature dependences of the crystal structure and superstructure intensities in sodium nitrate, mineral name nitratine, NaNO3, were studied using Rietveld structure refinements based on synchrotron powder Xray diffraction. Nitratine transforms from R3c to R3m at Tc = 552( 1) K. A NO3 group occupies, statistically, two positions with equal frequency in the disordered R3m phase, but with unequal frequency in the partially ordered R3c phase. One position for the NO3 group is rotated by 60 degrees or 180 degrees with respect to the other. The occupancy of the two orientations in the R3c phase is obtained from the occupancy factor, x, for the O1 site and gives rise to the order parameter, S = 2x - 1, where S is 0 at Tc and 1 at 0 K. The NO3 groups rotate in a rapid process from about 541 to Tc, where the a axis contracts. Using a modified Bragg- Williams model, a good fit was obtained for the normalized intensities ( that is, normalized, NI 1/ 2) for the ( 113) and ( 211) reflections in R3c NaNO3; and indicates a second- order transition. Using the same model, a reasonable fit was obtained for the order parameter, S, and also supports a second- order transition.