The 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, NaNO 3 , were studied using Rietveld structure refinements based on synchrotron powder X-ray diffraction. Nitratine transforms from at T c = 552(1) K. A NO 3 group occupies, statistically, two positions with equal frequency in the disordered phase, but with unequal frequency in the partially ordered phase. One position for the NO 3 group is rotated by 60° or 180° with respect to the other. The occupancy of the two orientations in the phase is obtained from the occupancy factor, x , for the O1 site and gives rise to the order parameter, S = 2 x − 1, where S is 0 at T c and 1 at 0 K. The NO 3 groups rotate in a rapid process from about 541 to T c , 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 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.