Heat Capacity and Anisotropic Thermal Conductivity in Cr₂AlC Single Crystals at High Temperature

The temperature dependences of both heat capacity and thermal conductivity in nanolamellar Cr2AlC single crystals are measured using modulated photothermal radiometry and compared to first-principles calculations. The electronic contribution to the thermal conductivity of Cr2AlC single crystals is computed ab initio by determining the electronic transport coefficients using density functional theory and by solving the Bloch–Boltzmann transport equation with a temperature-dependent relaxation time. The lattice thermal conductivity is predicted by going beyond the quasi-harmonic approximation and considering renormalized second- and third-order force constant matrices, with anharmonic three-phonon scattering, isotopic scattering, and scattering by carbon vacancies. Isotopic scattering does not modify the lattice thermal conductivity. In contrast, even a small concentration of carbon vacancies induces a substantial decrease of the in-plane lattice thermal conductivity. The anisotropy measured in the thermal conductivity, with a ratio of ∼2 over the whole temperature range, is confirmed theoretically. This anisotropy seems to mainly arise from lattice contributions. A similar anisotropy is expected for other MAX phases with identical layered structures.

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