Abstract
When an atom gains one or more electrons because it is more electronegative than the others, it becomes an anion and therefore has a negative charge that enables it to interact with the other members, creating the chemical bond. The state of the chemical bond within the solid can be, in terms of energy and in the quantum sense of the term, binding, antibinding or non-binding.
By building solids containing several anions of differing electron affinity, electronegativity and polarizability, Coulombic and Van der Walls interactions within a lattice of particular symmetry will contribute to radically altering the electronic distribution within the solid and the charge carried by these anions. Some of the most polarizable anions may therefore adopt a mixed valence state, with increased Van der Walls-type dipole-dipole interactions. Other, more electropositive elements will also be perturbed by the presence of several anions or non-bonding states, often independent and specific to a single atom, appearing around the Fermi level. These are the last occupied electronic levels, the source of instabilities where chemists and physicists scrutinize the properties of an insulator, semiconductor or metal.
From time immemorial, man has moved to help his societies evolve. For the atom, it's the charge q of the ionized element or its partial charge density associated with the probability of the electron's presence in its orbital at a given distance from the nucleus that are key parameters for better understanding the chemical bond. The sociology of atoms thus becomes clear, and the q/ ratio plays a major role. We can thus compare the individual, in a given place and at a given time, including his or her life history, to the atom that ionizes and acquires a charge that may vary firstly as a function of the radial distribution of the electron in its orbital, and then of its immediate and wider environment. In this way, the multiplication of the most electronegative but also polarizable elements within the lattice generates significant variations in partial charges around these elements, which are accompanied by a surprisingly small variation in ionization energy or, more precisely, electron affinity. We have thus seen how the coexistence of more or less electronegative elements such as fluorine, oxygen, sulfur or hydrogen, of varying polarizability and size, will enable antagonistic or competitive effects to increase the density of non-bonding states at the Fermi level. Instabilities of electronic origin appear with these very particular densities of states in matter, where certain orbitals become particularly polarizable or deformable. By examining various compounds with mixed anions and their crystalline structures, such as oxy-fluoro-sulfides, fluoro-hydrides and even fluoro-silicides, where ionic and metallic bonds coexist in the latter case, electronic and/or magnetic instabilities can appear and, where appropriate, give rise to superconductivity phenomena [1]. We will show how to understand these chemical bonds using solid-state chemist's approaches, where we will look at anions and their interactions, leading us to imagine a certain sociology of atoms.
Bibliographical reference :
[1] Vaney J.-B., Vignolle B., Demourgues A., Gaudin É., Durand É., Labrugère C., Bernardini F., Cano A. and Tencé S., "Topotactic fluorination of intermetallics: a novel route towards quantum materials", Nature Communication, vol. 13, 2022, art. 1462, https://doi.org/10.1038/s41467-022-29043-8.
