Speaker
Description
High-entropy transition-metal diborides (HEDBs; HETMB$_2$) retain the hexagonal AlB$_2$ structure while introducing severe multication disorder, raising practical questions about how large and how random a supercell must be for reliable first-principles predictions. Here, density functional theory calculations (ABINIT, PBE, norm-conserving pseudopotentials) are used to connect electronic structure to elastic stiffness, with the bulk modulus determined from near-equilibrium pressure--volume equations of state. Disorder is sampled using multiple cation configurations and two representative model sizes: an ordered 36-atom supercell for systematic composition scans and a 144-atom supercell with random site occupancy for convergence testing. Although larger cells promote like-cation clustering, the key density-of-states features and computed bulk moduli are essentially unchanged with supercell size and specific cation arrangement within numerical uncertainty. Across binary diborides and designed four-component HEDBs, the average metal-sublattice $d$-electron occupancy, $n_{\mathrm{dest}}$, emerges as the primary descriptor: it sets the Fermi-level position relative to the pseudogap and correlates with a monotonic increase in stiffness. Residual variations at fixed $n_{\mathrm{dest}}$ arise from metal-size dispersion, 4$d$/5$d$ substitution, and transition-metal vacancies, with vacancies providing a plausible route to experimentally reduced stiffness in real ceramics.
