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Abstract
Some examples of calorimetric measurements in several crystals containing at least one tetramethylammonium unit in compounds of (N(CH3)4)2BX4 and (N(CH3)4)BX3 families are presented. They show the possibilities of using adiabatic calorimetry in the precise characterization of complex phase transition sequences, as well as accurate calculations of the phase transition thermodynamic functions and the specific heat behaviour around the critical temperatures. In addition, complementary experimental information such as spectroscopic, elastic and thermal expansion data establish the different contributions to the specific heat in a crystal. In particular, anharmonic quantities, such as the isothermal compressibility and the Gruneisen parameter, can be calculated. Finally, the suitability of phenomenological models for a good description of the various phase transitions will be briefly discussed.