Abstract
Due to the exceptional property portfolio and technological applications of phase change
materials, mostly chalcogens related to IV-VI and V2-VI3 families, which are in turn related to
pnictogens (group V or 15) and chalcogens (group VI or 16), the nature of the unconventional
chemical bonding in these materials has been debated for almost 70 years. This unconventional
bond, which has been quoted in the literature as resonant, hypervalent, electron-rich
multicenter, three-center-four-electron (3c-4e), and metavalent, is believed to be responsible
for the exceptional properties of phase change materials. In the last decade, two bonding
models, the metavalent and the electron-rich multicenter models, have competed to explain the
nature of this unconventional bond, which we have here renamed as metavalent multicenter
bond (MMB) for the sake of clarity. In this comprehensive work, we address the nature of MMB
and propose that MMB is an electron-deficient multicenter bond (EDMB), related to the
threecenter-two-electron (3c-2e) bond. For that purpose, we explore the pressure-induced
mechanism of MMB formation in the some of the simplest possible systems, pnictogens (As, Sb,
Bi) and chalcogens (Se, Te, Po), with density-functional theory calculations. In the way, we find
that polonium is the only element among chalcogens and pnictogens with crystalline α and β
structures already exhibiting MMBs at RP. We find that the mechanism of MMB formation in
pnictogens (chalcogens) is comprised of three (two) stages, is similar to that of the EDMB
formation in B2H6, in some Zintl phases, intermetallics, and cluster compounds, and in
atomic/polymeric nitrogen and hydrogen at high pressures. On the other hand, the mechanism
of EDMB formation is completely different from that of the 3c-4e bond formation in molecules.
Finally, we propose the simplest geometries of EDMBs that can be found in solids along one,
two, and three dimensions and comment on the validity of the doublet/octet rules in the
hypercoordinated multicenter units with EDMBs.
Supplementary materials
Title
Supplementary Materials
Description
More details about the computational calculations. Additional results to support our theory and the results presented in the main paper.
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