Inverse Sandwich Geometry and Stability of B$_7$Y$_2$ Clusters: A DFT Study

In this work, we use density functional theory (DFT) to investigate the structural and electronic properties of B$_7$Y$_2$ clusters -- boron frameworks doped with two yttrium atoms. Our results show that the most stable configuration adopts an inverse sandwich geometry, while higher-energy isomers ($\sim$1.45 eV above) exhibit B$_7$ wheel-like motifs with yttrium at top or bridge sites. To understand the bonding and stability, we analyze the electron localization function (ELF) and localized orbital locator (LOL) maps. The global minimum shows a symmetric, delocalized electron distribution, strong B-B covalent bonding, and partial charge transfer from Y atoms. In contrast, higher-energy isomers show less effective Y-B interactions. ELF and LOL analyses confirm electron delocalization within the boron framework. Mulliken population analysis reveals significant metal-to-boron charge transfer, contributing to the stability of the inverse sandwich structure through synergistic effects of delocalized bonding and charge redistribution.