Phenomenological coefficients and pressure-temperature phase diagram for the phase transition sequences in tetramethylammonium tetrachlorozincate and tetrachlorocobaltate

Abstract
The experimental free-energy data for (N(CH))ZnCl and (N(CH))CoCl are obtained from Cp measurements in the temperature range 60-350K. The experimental results have been used to determine the values of the phenomenological coefficients in a free-energy expansion proposed by Mashiyama (1980) which describes the phase transition sequence. The calculated coefficients determine the experimental temperature dependence of the incommensurate wavevector as well as the experimental pressure-temperature phase diagram.