Tunable p-p Interactions Along Linear Chains Control Metavalent Bonding in Chalcogenide Heterostructures

We present an analysis of heterostructures of alternating layers of rocksalt structures of metavalently bonded (MVB) group-IV (MX) and ionic chalcogenides (IX) and show that bonding heterogeneity across their semiconductor-insulator interface arises from the tunable p-p interactions. Using Born dynamical charge ( $Z^{*}$ ) as the key atomistic descriptor of MVB, we demonstrate that metavalent bonds parallel to the interfacial plane are unaffected by proximal ionic bonds as they constitute periodic (-X-M-)_n chains. In contrast, the bonds perpendicular to the interface (-X-M-X-I-X-) are spectacularly affected: characteristic $Z^{*}$ of the MVB evolve to the bulk behavior within about a nm of the interface. With comparative analysis with I = Sr, Cd, and Sn, we establish that interatomic p-p orbital interactions along linear atomic chains with 5 contiguous (-X-M-) pairs govern the emergence, persistence, and suppression of MVB. The discontinuity at the interface has long-range effects on the out-of-plane component of $Z^{*}$ of ions, which are explained using a simple model of electrostatic screening. Energies of p orbitals of anions and cations, and their interactions along (-X-M-)_n chains are shown here to be essential to MVB, which result in the tunability of thermoelectric properties of interfacial hetero-structures of chalcogenides.