The incommensurate phase in tetramethylammonium tetrachloromanganate

Highly accurate specific heat capacity measurements were used in order to study the phase transition sequence of tetramethylammonium tetrachloromanganate (TMATC-Mn) between 50 and 330 K. This study allowed the authors to confirm that the incommensurate phase that had been previously found at pressures higher than 38 MPa is also present at atmospheric pressure. At this pressure the phase transition sequence is given by I-(292.6 K)-II-(292.2 K)-III-(267.0 K)-IV-(175.0 K)-V, where phase II is incommensurate and covers a very narrow temperature interval. Heating and cooling measurements allowed them to study the thermal hysteresis associated with the lock-in (II-III) phase transition. The other phase transitions were also characterised within the authors' general calorimetric study. The position of the compounds of the general family (N(CH3)4)2MX4 in its common pressure-temperature phase diagram could be related to the thermodynamic function values of their phase transition sequences.