Phase transitions and pressure effects in simple pyridinium salts

Structural phase transitions in the pyridinium iodide [C(5)NH(6)](+.)I(-) and bromide [C(5)NH(6)](+.)Br(-) crystals were studied by calorimetry, volumetric dilatometry and dielectric methods. The pressure dependences of the transition temperatures were determined by dielectric constant and differential thermal analysis measurements under hydrostatic pressures of up to 1 GPa. Analysis of the experimental data indicated that the p-T phase diagrams of simple pyridinium salts could include tri-critical points in the pressure range above I GPa. The effect of external hydrostatic pressure is correlated with the chemical pressure that appears due to the change in the anionic radius. A new model of the phase transition is proposed that includes antiferroelectric order and six-fold disorder of the pyridinium cations, respectively, in the low- and high-temperature phases.