A High-Temperature Neutron Diffraction and First-Principles Study of Ti_3AlC_2 and Ti_3(Al_(0.8)Sn_(0.2))C_2

摘要

Herein, we report on the temperature-dependent crystal structures of Ti_3AlC_2 and Ti_3Al_(0.8)Sn_(0.2)C_2 in the 373-1273 K temperature range, as determined by Rietveld analysis of high-temperature neutron diffraction time-of-flight data. The compositions are 86(1) wt% Ti_3AlC_2 and 14(1) wt% TiC_(0.92(2))) for the sample with no Sn, and 95(1) wt% Ti_3(Al_(0.8)Sn_(0.2))C_2 and 5 (1) wt% Ti_2AlC for the solid solution with Sn. The average linear volumetric thermal expansion is 8.0(2) × 10~(-6) K~(-1) for Ti_3AlC_2 and 8.2(5) ×10~(-6)K~(-1) for Ti_3(Al_(0.8)Sn_(0.2))C_2. The average linear thermal expansion in the a and c directions, respectively, are 7.6(2) × 10~(-6) K~(-1) and 8.9(2) × 10~(-6) K~(-1) for Ti_3AlC_2. For Ti_3(Al_(0.8)Sn_(0.2))C_2, the respective values are 8.0(5) × 10~(-6) K~(-1) and 8.6(6) × 10~(-6) K~(-1). In the case of the solid solution, the quadratic thermal expansion coefficients are also given. Detailed bond lengths analysis shows that the thermal expansions along the a and c directions are controlled by the thermal expansions of the Ti-C, and Ti-Al bond lengths, respectively. The atomic displacement parameters (ADPs) show that the Al and Sn atoms vibrate with a higher amplitude than the Ti and C atoms. Consistent with first-principles calculations, the ADPs of the Al/Sn site(s) in Ti_3(Al_(0.8)Sn_(0.2))C_2 are lower than the ADPs of Al in Ti_3AlC_2.