Abstract
The irreducible representation for the (unmodulated) soft mode system in NaNO 2 is 1-dim. and hence lacks its antisymmetrized square; nevertheless NaNO 2 exhibits incomm. transitions (prototypic→incomm. ferroic→comm. ferroic). Of such transitions a theory is constructed. It takes one soft B 2 u mode and one hard B 1 g mode into account, so that in this respect it is the same as the theory by Levanyuk et at . But it is different in the method of formulation and many various details. It may be regarded as a correction of the theory by Aizu in 1978. A full account is given of the process of deducing the free energy function. Although the function is ultimately pruned into a simplest one, the qualitative and the quantitative results deduced from that function are found to agree with the experimental results on NaNO 2 , except some part of them.
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