Measurements of the heat capacity of diamond with different isotopic compositions

We report measurements of the heat capacity Cp of diamond with several isotopic compositions (12C1−x13Cx) in the 20-280 K temperature range. The results, in particular the derivative of the specific heat with respect to the average isotopic mass, are compared with recent ab initio calculations. Agreement is good for T>150 K. At lower temperatures, the samples with isotopic compositions different from the natural one display an anomalous increase in Cp/T3 with decreasing temperature which is attributed to the presence of metallic inclusions stemming from the metal used as a solvent in the high-pressure high-temperature growth process.