ANHARMONIC EFFECTS IN THERMODYNAMIC PROPERTIES OF SOLIDS .4. HEAT CAPACITIES OF NACL KCL AND KBR BETWEEN 30 AND 500 DEGREES C

The adiabatic calorimeter described previously has been improved by automation of the adiabatic shield control. The calorimeter has been used to measure the heat capacities of NaCl, KCl and KBr in the temperature range 30 to 500° C with estimated accuracies of better than ±0·3% for T less, similar 430° C and ±0·5% at the highest temperatures. The precision of the results, as measured by their average deviation from a smooth curve, is about 0·15%. Contributions to the heat capacity arising from the thermal creation of vacancies are discussed in relation to the increasing positive curvature of the Cp(T) plots above 600-700° K. Comparison with the results of ionic conductivity experiments shows that in all three cases the contribution from vacancy formation to the apparent excess Cp is a very small fraction (similar 1%) of the total, which is primarily a higher-order anharmonic effect.