Phase transitions and pressure effects in simple pyridinium salts

Abstract

Structural phase transitions in the pyridinium iodide [C5NH6]+.I−and bromide [C5NH6]+.Br−crystals were studied by calorimetry, volumetric dilatometry and dielectric methods.The pressure dependences of the transition temperatures were determined bydielectric constant and differential thermal analysis measurements under hydrostaticpressures of up to 1 GPa. Analysis of the experimental data indicated that thep–Tphase diagrams of simple pyridinium salts could include tri-critical points in thepressure range above 1 GPa. The effect of external hydrostatic pressure iscorrelated with the chemical pressure that appears due to the change in theanionic radius. A new model of the phase transition is proposed that includesantiferroelectric order and six-fold disorder of the pyridinium cations, respectively,in the low- and high-temperature phases.