On the heat capacities of Ta2AlC, Ti2SC, and Cr2GeC

Herein we report on the heat capacities c(p) of bulk predominantly single-phase polycrystalline samples of Ti2SC and Cr2GeC in the 3-1500 K temperature range and Ta2AlC in the 3-260 K range. At temperatures up to 10 K the main contributors to c(p) for Ta2AlC and Cr2GeC are electronic, with electronic coefficients gamma of 7.13 and 26.12 mJ/mol K-2, respectively. The latter is exceptionally high and is a record for this family of layered ternary carbides and nitrides also known as the MAX phases. In Ti2SC another low-temperature contribution-in addition to a gamma of 3.8 mJ/mol K-2-is manifested by an upturn in c(p)/T observed at the lowest temperatures. This feature, appearing as a Schottky-like anomaly, has a local maximum near 4.5 K and an intensity of similar to 1.9x10(-2) J/mol K. A defect concentration of similar to 3x10(21) /mol presumably on the S-sublattice, is proposed as the origin of a two-level energy system responsible for this anomaly. As in previous work on these compounds, the lattice contributions to c(p) in all compounds are analyzed using the Debye and Einstein model approximations. The main effect of increasing the atomic number of the transition metal is a reduction in Debye temperature. (C) 2008 American Institute of Physics.