Active-learning search for unitcell structures: A case study on Mg₃Bi_2-xSb_x

Given a concentration, the determination of atomic configuration of a compound is very challenging, although it is the base and prerequisite for the investigation of the properties. Taken the Mg-Bi-Sb alloy system as an example, we have investigated the crystal structure of the Mg₃Bi_2-xSb_x(0 < x < 2) using the revised LAsou method with an active learning strategy. We have explicitly examined the atomistic configurations with three unit-cell sizes (10, 40, and 90 atoms). Referring to the conventional method, our method reduces the computational demand by several order of magnitude. Structural analysis including bond-order parameter, disorder parameter, short-range order parameter, and radial distribution function, revealed that the 90-atom unit cells allowed for more diverse and random Sb distribution in the structure than the 10-atom and 40-atom cell simulations. The XRD spectral analysis confirms that the predicted structures agree well with experiment. Through an illustrative case study of Mg-Bi-Sb system, our work has demonstrated a route to determine the atomic configurations with the information of formula combined with first-principles calculations, which might benefit the atomistic unit-cell modelling of compounds and alloys in computational materials science.