Incipient Metals: materials that exhibit a unique mechanism between covalent and metallic bonding. Data for a diverse set of 58 materials show that metavalent bonding is not just a superposition of covalent and metallic cases, but instead gives rise to a unique and anomalous set of physical properties. [DOI: 10.1002/adma.201803777]

The main characteristics of metavalent bonding are: (i) cation coordination much higher than that assumed with the 8-N rule; (ii) high Born effective charges that are much larger than the valence of the atoms; (iii) higher optical dielectric constants than typical covalent materials; (iv) high mode Gru ¨neisen parameters of phonons and lower wavenumbers of optical phonons than typical covalent materials, thus revealing a high lattice anharmonicity causing a very small thermal conductivity; and (v) a moderately high electrical conductivity caused by a very small bandgap. All these features stem from the partial delocalization of electrons that are shared between several bonds. Due to these characteristics, materials featuring metavalent bonding have been named ‘‘incipient metals’’ because they show characteristics close to those of metals. In fact, they exhibit extraordinary properties that make them ideal candidates for phase-change materials, thermoelectric materials, and topological insulators. [DOI: 10.1039/c9cp06298j]