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Molar Heat Capacity of Potassium Alum KAl(SO4)2 ·12H2O at Low Temperatures: Revisited with Determination of the Debye Temperature

Two earlier published experimental data [C. H. Shomate, J. Am. Chem. Soc. 67, 765 (1945); D. G. Kapadnis and R. Hartmans, Physica 22, 173 (1956)] of the molar heat capacity Cp of potassium alum, KAl(SO4)2·12H2O, are revisited. From analysis of the temperature dependence of Cp [D. G. Kapadnis and R. Hartmans, Physica 22, 173 (1956)] below 20 K by using the experimental values of Cp represented as a plot of Cp/T = γ+AT2, the Sommerfeld parameter γ (electronic heat capacity coefficient) and the slope A (lattice heat capacity coefficient) are estimated as γ = 0 JK −2mol −1 and A = 3.35×10 −3 JK −4mol −1, respectively. This indicates that only acoustic phonons contribute to the heat capacity below 4 K. The Debye T3-law is found to hold good to 4 K, i.e., to T < θD/20, where θD is the calorimetric Debye temperature calculated as θD = 83.4 K, indicative of the strength of the chemical bonding in the system.