Chemical Bonding in Dense AIVXVI and A2VX3VI Chalcogenides: Electron-Deficient Multicenter Bonds in Electron-Rich Elements

Chemical bonding in dense chalcogenides of the AIVXVI and A2VX3VI families has been a subject of significant debate, with competing models trying to explain their unique and exceptional properties, in particular those corresponding to phase change materials. We present a paradigm-shifting advancement by demonstrating that electron-deficient multicenter bonds (EDMBs) occur in the dense chalcogenides of these two families with electron-rich elements, definitively resolving the long-standing controversy between metavalent and hypervalent bonding models—a fundamental question that has made difficult to get a rational materials design for decades. Through a comprehensive theoretical investigation of monochalcogenides (GeS, SnSe, PbTe) and sesquichalcogenides (As2S3, Sb2Se3, Bi2Se3, Bi2Te3) under compression, we reveal a revolutionary unified framework that coherently explains why heavier chalcogenides naturally exhibit phase change, thermoelectric, and topological properties at room pressure while lighter compounds require compression to exhibit these exceptional properties—a phenomenon that has puzzled researchers for years. Our systematic analysis uncovers a universal pattern in EDMB formation following two or three well-defined stages, depending on the specific elements involved, that agrees with the recently published unified theory of multicenter bonding. Our unprecedented application of three local energy densities (kinetic, potential, and total) at bond critical points provides quantitative validation of the EDMB model and offers the first comprehensive explanation for the anomalous sixfold coordination in these compounds that defies the conventional 8-N rule. Most significantly, our work establishes clear structure-property relationships that provide a direct pathway to rational design of new materials with tailored properties.