Measurements of the heat capacity of diamond with different isotopic compositions

We report measurements of the heat capacity C,, of diamond with several isotopic compositions (C-12(1-x) C-13(x)) 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 C-rho/T-3 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. (C) 2004 Elsevier Ltd. All rights reserved.