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Researchers from Skoltech, Ural Federal University and the M.N. Mikheev Institute of Metal Physics of the Ural Branch of the Russian Academy of Sciences presented a fundamental theoretical description of the important properties of chemical bonding in materials, suggesting a way to determine atomic charges and isolate ionic and covalent components in the energy of chemical bonding. The scientists' conclusions serve as a convincing justification for the traditional concepts of chemical bonding based on the concept of electronegativity. However, in the course of the study, an anomalous case in the form of a semiconductor compound, boron phosphide, was also revealed that contradicts chemical intuition. The results of the study are published in The Journal of Chemical Physics. Since the beginning of the 20th century, scientists have been trying to explain the properties of molecules and crystals through chemical bonding — the property of atoms to attract each other due to the redistribution of external electrons. It has been found that there are different types of connections. For example, in a covalent bond, two atoms share a common electron pair, which holds them together. Subsequently, it turned out that there could be more than two atoms. Such a bond between many atoms is called metallic. The opposite case is also known, when two atoms can have two or more common electron pairs. In the case of a covalent polar bond, the total electron pair shifts towards a more electronegative atom, and then two oppositely charged ions are formed, which are held together by an ionic bond. "Consider the bond that holds atoms together in a material. Most of the properties of this material will depend on the degree of covalence or ionization of this bond, or in other words, on the charges of atoms," says co—author of the study, head of the Skoltech Materials Design Laboratory, Honored Professor Artyom R. Oganov. "The problem is that over the last century, so many ways to determine atomic charges have been proposed that scientists have not yet come to a consensus on how best to do this and which numerical values of the charge are considered correct." There are a number of approaches in which the atomic charge is determined through the properties of a material that can be measured, for example, the amount of energy spent on breaking a bond. Other approaches are based on the use of the mathematical operation of integrating the distribution of electron density over the volume of an atom, but even in this case, an unambiguous definition has not yet been found. There are also several approaches that use the wave functions of atoms — the basic characteristics of atoms in problems of quantum mechanics. There are also many ways to analyze the wave function, which cannot be measured directly in an experiment. "Our method, the main creator of which is the classic of modern solid state physics Vladimir Anisimov, uses a formal mathematical description of the so—called Vanier functions, which allow us to describe the chemical bond in a crystal or molecule based on electronic orbitals as close as possible to atomic ones," adds Professor Oganov. "Using our method, it is possible to determine atomic charges in a non-empirical way and isolate covalent and ionic components in the binding energy. The results obtained are in good agreement with chemical intuition. The only exception was the crystal of boron phosphide, whose atomic charges turned out to be inverted." It is known that a similar inversion pattern is observed in a carbon monoxide molecule consisting of one oxygen atom and one carbon atom. Since oxygen has a higher electronegativity than carbon, it is logical to assume that it will attract a common electron pair more strongly: oxygen will have a partially negative charge, and carbon will have a partially positive charge. However, in reality, the opposite is happening. A similar situation arises in the case of boron and phosphorus atoms forming a crystal of boron phosphide BP in a one-to-one ratio. Despite the fact that phosphorus is more electronegative than boron, the latter eventually receives a partially negative charge. Why is this happening? This phenomenon is explained by the fact that in both cases, the transfer of electrons, which is unfavorable from an energy point of view, makes it possible to obtain a stronger covalent bond, thereby more than compensating for energy losses. Due to the charge inversion, a very strong triple bond between oxygen and carbon is formed in the carbon monoxide molecule. If the atoms had not made this "energy sacrifice", then the bond would have been only double. In the same way, in boron phosphide, phosphorus gives one electron to boron, as a result of which these two atoms have four valence electrons, with which they create covalent bonds with their neighbors in the crystal structure. Otherwise, both boron and phosphorus would have only three bonds per atom, which is less advantageous from an energy point of view. Curiously, the charge inversion in boron phosphide was predicted twenty years ago in the so-called dynamic Born charges, which from a physical point of view have a completely different nature. The method proposed by scientists is universal, which means that it will help to better understand the nature of the chemical bond in a wide variety of chemical compounds. The research was supported by a grant from the Russian Science Foundation No. 19-72-30043. Image: In a crystal of boron phosphide, each atom has bonds with four neighboring atoms. In the absence of charge inversion, atoms would have only three bonds each. The authors of the study proposed a fundamentally new theoretical model that allows taking into account charge inversion. The image was created by the Deep Dream Generator neural network / provided by the Skoltech press service Information and image provided by the Skoltech press service Information taken from the portal "Scientific Russia" (https://scientificrussia.ru /)