Mechanism of negative thermal expansion in LaC2 from first-principles prediction

•The NTE behavior along c-axis was reproduced in LaC2 based on DFT calculations.•The mechanism of NTE was clarified firstly and numerically.•A tiny NTE effect was predicted in a-axis quantitatively.

Based on density functional theory and quasiharmonic approximation, the coefficients of thermal expansion (CTE) and negative thermal expansion (NTE) mechanism of tetragonal LaC2 are studied. Numerical results show that there is an obvious NTE parallel to c  -axis, and the CTE is approximately αc=−1.67×10−6 K−1αc=−1.67×10−6 K−1, which coincides with the experimental data −1.0×10−6 K−1−1.0×10−6 K−1. In particular, a tiny NTE phenomenon along a  -axis below 10 K has been predicted. The vibrational modes EuEu and EgEg at Γ(0,0,0)Γ(0,0,0), and other three modes at M(0.5,0.5,0)M(0.5,0.5,0) and Z(0,0,0.5)Z(0,0,0.5), give rise to negative Grüneisen parameters and therefore contribute to the NTE along a- and c-axis. Additionally, the bulk CTE was calculated to be positive, our CTE values and temperature intervals agree well with the presented experiments.