Temperature dependent order–disorder multiple phase transitions studies of the hexamethylenetetraminium p-nitrobenzoate crystal

The phase transition behaviour of three temperature polymorphs of hexamethylenetetraminium p-nitrobenzoate (HMTA–PNBA) crystal has been investigated by single crystal X-ray diffraction experiment at three different temperatures with the same crystal. The crystal undergoes multiple phase transitions from monoclinic Cm to Cc (first transition) and Cc to Cc (second transition). These transitions are reversible. Both of the phase transitions are of first order, involving the discontinuity and doubling of the c- and a-, b- axes at critical temperatures (TC) 259.5 and 182.5 K, respectively. The p-nitrobenzoate anions are disordered except for the low temperature phase (100 K), displaying order–disorder phase transformation at transition temperature 182.5 K. Hydrogen bonding plays a pivotal role in these phase transitions as shown by the decrease of hydrogen bonding as the temperature is raised. Both of the phase transitions are explained by Landau phenomenological theory.