Newly Formulated Phenomenological Theory of Incommensurate Transitions in a Substance like NaNO₂

Abstract

The irreducible representation for the (unmodulated) soft mode system in NaNO₂ is 1-dim. and hence lacks its antisymmetrized square; nevertheless NaNO₂ exhibits incomm. transitions (prototypic→incomm. ferroic→comm. ferroic). Of such transitions a theory is constructed. It takes one soft B_2u mode and one hard B_1g mode into account, so that in this respect it is the same as the theory by Levanyuk et al. 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 NaNO₂, except some part of them.