Ab Initio Molecular Dynamics Analysis of the Structural Units and Their Stability in the Na2O–B2O3 Melt

—The aim of this work is to develop a computational–theoretical technique for a detailed study of the geometry and the statistical characteristics of the local structural groups in complex liquids such as alkaline-borate systems, which tend to form a bulk boron–oxygen network. This technique was applied to the 30Na 2 O–70B 2 O 3 melt at T = 1273 K. Ab initio molecular dynamics, implemented in the VASP program code, was used for a supercell consisting of 250 atoms. The ion coordinates obtained at each step were used to get statistically significant information about the detailed structure of the melt. An original computer program was developed to determine partial atomic radial distribution functions, to analyze the nearest coordinations around each kind of atoms found in the model, and to determine the types and number of stable groupings, the bond lengths, and the bond angles. In addition, the tetrahedricity criterion is calculated for the BO 4 and BB 4 units. The basic structural units turned out to be almost regular triangles (include ~80% of boron atoms) and tetrahedra (~19% of boron atoms) with a boron ion at the center and oxygen ions at the vertices. These simple structures form a boron–oxygen network connected by common (bridging) oxygen atoms and it includes almost all boron atoms. Superstructural units, namely, combinations of three or more basic structures, are found. For example, two triangles and one tetrahedron form rings of six alternating boron and oxygen atoms. In addition, the existence of rings formed from four basic structural units has been detected; however, unlike six-atom rings, they are not planar formations. The proposed technique makes it possible to obtain almost any data on the structural features of systems of this type, in particular, to answer an important question about the number of bridging and non-bridging oxygen atoms. Bridging oxygen atoms in the system under study turned out to account for about 86%. The proposed approach correctly takes into account covalent and ionic bonds in liquid systems based on network-forming oxides and modifier oxides, which makes it possible to study the change in the local structural characteristics as a function of concentration and temperature and to explain the behavior of various physicochemical properties.