On the heat capacities of Ta2AlC, Ti2SC, and Cr2GeC

• Published in: Journal of applied physics 2008-07, Vol.104 (2)
• Main Authors: Drulis, Monika K., Drulis, H., Hackemer, A. E., Leaffer, O., Spanier, J., Amini, S., Barsoum, M. W., Guilbert, T., El-Raghy, T.
• Format: Article
• Language: English
• ISSN: 0021-8979; 1089-7550
• DOI: 10.1063/1.2956511

Herein we report on the heat capacities cp of bulk predominantly single-phase polycrystalline samples of Ti2SC and Cr2GeC in the 3–1500K temperature range and Ta2AlC in the 3–260K range. At temperatures up to 10K the main contributors to cp for Ta2AlC and Cr2GeC are electronic, with electronic coefficients γ of 7.13 and 26.12mJ∕molK2, 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 γ of 3.8mJ∕molK2—is manifested by an upturn in cp∕T observed at the lowest temperatures. This feature, appearing as a Schottky-like anomaly, has a local maximum near 4.5K and an intensity of ∼1.9×10−2J∕molK. A defect concentration of ∼3×1021∕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 cp 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.