О б ч и с л ю в а л ь н е м а т е р і а л о з н а в с т в о _______________________________________________________________________________ УДК 539.2:539.32 https://doi.org/10.15407/materials2024.08-09.001 Ab-initio investigations of icosahedral boron compounds with Al, Mg, C, O, Si atoms L. I. Ovsiannikova*, N. M. Rozhenko I. M. Frantsevich Institute for Problems of Materials Science of NAS of Ukraine Omeliana Pritsaka str., 3, 03142, Ukraine, Kyiv *E-mail: l.ovsiannikova@ipms.kyiv.ua The geometry and cohesive energy of isolated clusters - fragments of icosahedral boron compounds — with Al, Mg, C, O, Si substitution atoms have been calculated within the framework of DFT electron density theory using the Gamess software package. Electron density distribution between atoms has been investigated. The bulk modulus of the B12 cluster has been calculated on the basis of quantum chemical calculations and a thermodynamic series of cluster hardness has been constructed: HB22O2 > HВ22C2 > HВ24 > HВ22Si2 > HB22Al2 > HB22Mg2. The calculated bulk modulus and hardness values based on the results of the first-principles study of the clusters are in good agreement with the experimental data for compounds with similar chemical compositions. The technique is applicable to the prediction of the choice of substitutional atoms in icosahedral boron groupings. Keywords: boron, boride, isolated cluster model, bulk modulus, hardness. Introduction Computer-aided design plays an important role in the development of the theoretical basis for creating composite materials based on high-boron compounds, the performance characteristics of which will be higher than those of existing ones. Due to the tendency of boron atoms to form three-center electron-deficient B—B bonds, a stable state of crystalline boron is achieved through the formation of complex structural motifs, including B12 icosahedra [1]. They are the structural blocks of both allotropes of elemental boron [2] and complex boride phases [3], which demonstrate high hardness, low density and thermal endurance, which opens up prospects for the search for new materials based on them [4, 5]. One direction of such a search is higher metal borides [6, 7]. Icosahedrons В12 act as structural elements of high-boron compounds, which are characterized by an increase in hardness with increasing boron content. With increasing boron content, the proportion of B—B covalent bonds