Metavalent or resonant bonding is a recently proposed new class of bonding that is intermediate between p-type covalent bonding and metallic bonding. It characterizes a new family of materials known as “incipient metals” [1]. Metavalent bonding occurs in materials where there is a deficiency of valence electrons in the unit cell to form a large number of bonds, such as in octahedrally-coordinated rocksalt-related structures occurring in GeTe, SnTe, PbSe, PbTe, Sb2Te3, Bi2Te3, Bi2Se3, Sb, Bi and AgSb2Te4. The main characteristics of metavalent bonding are: i) a much higher cation coordination than that assumed with the 8-N rule; ii) very high Born effective charge and optical dielectric constant as compared to typical covalent materials; iii) high mode Grüneisen parameters of phonons and low wavenumbers of optical phonons as compared to typical covalent materials; and iv) a moderately high electrical conductivity caused by thesmall bandgap which stems from the partial delocalization of electrons between several bonds

This work has been financially supported by Spanish MINECO under projects FIS2017-83295-P and MAT2016-75586-C4-1/2/3-P and by Generalitat Valenciana under project PROMETEO/2018/123-EFIMAT. Supercomputer time has been provided by the Red Española de Supercomputación (RES) and the MALTA cluster. JAS acknowledges “Ramón y Cajal” fellowship (RYC-2015-17482) and E.L.d.S acknowledges Marie Sklodowska-Curie grant No. 785789-COMEX from European Union’s Horizon 2020 research and innovation programme. We also thank ALBA synchrotron for funded experiments.

57th European High Pressure Research Group Meeting on High Pressure Science and Technology (EHPRG2019) in Prague, 1-6 september 2019