Models for the Order—Disorder Transition in NaH3(SeO3)2

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Abstract
The entropy of the Makita—Miki model for NaH3(SeO3)2 is shown to be exactly solvable by transformation into a Slater KDP model problem. However, arguments are presented to show that the model is unsatisfactory. Two additional models are introduced. The first model is reduced to a dimer problem and is solved exactly. The solution is interesting in the context of critical phenomena, but it indicates that the model is not a good one for NaH3(SeO3)2. The second model is readily solved in the limit of high temperature using results for the two-dimensional Ising model. Although this model is too difficult to solve exactly at finite temperature except for one limiting case, it is encouraging that analogous models exhibit multiple phase transitions.