Quantum and anharmonic thermodynamics of BC 3 monolayer

Abstract

We investigate the structural and thermodynamic properties of a BC3 monolayer by combining classical Monte Carlo and path-integral Monte Carlo simulations in the isothermal–isobaric ensemble at zero external pressure. Quantum nuclear motion expands the bond network by about 0.5% at 40 K, enhances out-of-plane fluctuations, and drives negative thermal expansion through the anharmonic coupling between in-plane stretching and flexural modes. The zero-point vibrational energy reaches 0.1704(2) eV/atom, and the quantum contribution to the vibrational energy remains comparable to the classical one up to about 352 K. A species-resolved analysis shows that boron behaves as a comparatively rigid structural component, whereas carbon hosts the dominant anharmonic relaxation channels; in particular, the crossover from the carbon relaxation-dominated regime occurs near 471 K in the quantum simulations. Negative thermal expansion persists up to about 856 K and 1140 K along the armchair and zigzag directions, respectively. These results identify BC 3 as a quantum anharmonic membrane whose finite-temperature response is strongly shaped by the interplay of substitutional chemistry, flexural dynamics, and nuclear quantum effects.