While solid‐state materials are commonly classified as covalent, ionic, or metallic, there are cases that defy these iconic bonding mechanisms. Phase‐change materials (PCMs) for data storage are a prominent example: they have been claimed to show “resonant bonding,” but a clear definition of this mechanism has been lacking. Here, it is shown that these solids are fundamentally different from resonant bonding in the π‐orbital systems of benzene and graphene, based on first‐principles data for vibrational, optical, and polarizability properties. It is shown that PCMs and related materials exhibit a unique mechanism between covalent and metallic bonding. It is suggested that these materials be called “incipient metals,” and their bonding nature “metavalent”. 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. This allows the derivation of a characteristic fingerprint of metavalent bonding, composed of five individual components and firmly rooted in physical properties. These findings are expected to accelerate the discovery and design of functional materials with attractive properties and applications, including nonvolatile memories, thermoelectrics, photonics, and quantum materials.

Due to its higher degree of control and its scalability, catalytic chemical vapour deposition is now the prevailing synthesis method of carbon nanotubes. Catalytic chemical vapour deposition implies the catalytic conversion of a gaseous precursor into a solid material at the surface of reactive particles or of a continuous catalyst film acting as a template for the growing material. Significant progress has been made in the field of nanotube synthesis by this method although nanotube samples still generally suffer from a lack of structural control. This illustrates the fact that numerous aspects of the growth mechanism remain ill-understood. The first part of this review is dedicated to a summary of the general background useful for beginners in the field. This background relates to the carbon precursors, the catalyst nanoparticles, their interaction with carbonaceous compounds and their environment. The second part provides an updated review of the influence of the synthesis parameters on the features of nanotube samples: diameters, chirality, metal/semiconductor ratio, length, defect density and catalyst yield. The third part is devoted to important and still open questions, such as the mechanism of nanotube nucleation and the chiral selectivity, and to the hypotheses currently proposed to answer them