Free energy phenomenological parameters for the phase transition sequences of incommensurate (N(CH3)4)2BCl4compounds with B = Co, Zn y Cu

From specific heat measurements we have determined the free energy behaviour of A2BX4 organic compounds (A=N(CH3)4; B=Co, Zn, Cu and × = Cl) in the 50-330K temperatures range. The complex phase transition sequence observed in these compounds is well described by the use of a phenomenological model for phase transitions1. The good agreement found with the experimental data have allowed to specify the numerical values of the phenomenological coefficients which account not only for the stability of the phases in a wide temperature range, but also for a good description of the wave vector and for the order parameter in the incommensurate phases2. The gradual disappearance of the ferroelectric phase in Co and Zn compounds by the influence of the pressure3 as well as the presence of two different incommensurate phases in the Co compound is also studied by a suitable pressure dependence of the phenomenological parameters. This method suggests a simple general procedure in order to determine the pressure-temperature phase diagrams.