The phase-transition behavior of both calcite and the isostructural compound NaNO 3 have long been thought to be anomalous. In particular, the beta critical exponent for the orientational order-disorder transitions takes on a value close to tricritical behavior (beta = 0.25) in both materials, and in NaNO 3 two crossovers to regimes where beta = 0.22 and then beta = 0.41 have been reported as T c is approached. The most significant puzzle was why both materials should appear to be tricritical under ambient conditions of both pressure and the conjugate field. The experimental work on these materials is reanalyzed in the light of recent progress in understanding two-dimensional magnetic ordering. It is shown that the experimental results are fully consistent with the two-dimensional XY model. Unlike the tricritical model, this gives a simple physical explanation for the disordering process and observed critical exponents. In particular, it supports other recent experimental findings from calcite and NaNO 3 that the orientational order-disorder occurs through continuous planar rotations of the carbonate and nitrate groups, rather than discrete jumps.

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