CALORIMETRIC AND PHOTOACOUSTIC INVESTIGATION OF KNO3 PHASE-TRANSITIONS

Results of a thermodynamic study of the phase transitions of powdered KNO3 at two impurity concentrations (99.999% pure and 99.9% pure) are reported. Transition onset and peak temperatures have been measured be tween 350 and 420 K using both heat flow and photoacoustic calorimetry . The transition temperatures and characteristics observed with both t echniques show excellent agreement. Specific-heat values calculated fr om scanning and stepped temperature heat-flow data, utilizing both pos itive and negative temperature increments, compare well with previousl y reported values. Enthalpies for the II-I, I-III, and III-II phase tr ansitions were measured as 5.065, 2.603, and 2.084 kJ/mol, respectivel y, for 99.999% pure KNO3 powder using heat-flow calorimetry. The trans ition enthalpies and temperatures measured for 99.9% pure powder were slightly lower for all three transitions. Entropy changes for the II-I , I-III, and III-II transitions were 12.53, 6.61, and 5.30 J/mol K, re spectively. Enthalpy, entropy, and Gibbs free-energy curves are presen ted with emphasis on the temperature range over which the ferroelectri c phase transition occurs. The experimental data and calculated thermo dynamic functions indicate that the ferroelectric phase III in bulk KN O3 is a metastable state at atmospheric pressure, which was not observ ed to exist below 350 K contrary to previous reports.