The specific heat of tetramethylammonium salts

New measurements of the (N(CH3)(4))(2)MnBr4 specific heat by adiabatic calo rimetry around the ferro- paraelastic phase transition shown by the crystal around 276 K are compared with previous calorimetric studies on similar te tramethylammonium bromide compounds. The thermodynamic behaviour of the tri bromides and tetrabromides derivatives together with the influence on the p hase transition parameters of the cation and halogen molecular substitution s are examined. The thermal relaxation experiments permit to study the beha viour of the crystals thermal conduction as a function of the temperature. Finally, the Landau theory for second order phase transitions is used to de scribe the thermodynamic behaviour of some of these crystals.