Heat capacity of niobium and tantalum carbides NbCy and TaCy in disordered and ordered states below 300K

The low-temperature heat capacity of nonstoichiometric niobium and tantalum carbides in disordered and ordered states is measured by an adiabatic calorimetry method. It is found that in the temperature interval from 4 to 300 K the investigated ordered carbides have a higher heat capacity than the disordered carbides of the same composition. The effect of ordering on the heat capacity of nonstoichiometric carbides is interpreted in terms of the order parameter functional (OPF) method. Changes in heat capacity due to ordering in niobium and tantalum carbides are calculated. The degree of long-range order in ordered carbides is estimated by comparing of the calculated and experimental results.