Emerging - orbital coupling between non--block main-group elements Mg and I at high pressure.

orbital coupling, which increases anisotropic and directional bonding, commonly occurs between -block transition metals. Here, we report an unexpected - orbital coupling in the non--block main-group element compound MgI based on first-principles calculations. The unfilled orbitals of Mg and I atoms under ambient conditions become part of the valence orbitals and couple with each other under high pressures, resulting in the formation of highly symmetric I-Mg-I covalent bonding in MgI, which forces the valence electrons of Mg atoms into the lattice voids to form interstitial quasi-atoms (ISQs). In turn, the ISQs highly interact with the crystal lattice, contributing to lattice stability. This study greatly enriches the fundamental understanding of chemical bonding between non--block main-group elements at high pressures.