Formation of superstructural groups in melt x Na2O - (1-x) B2O3. Ab initio molecular dynamics method

The ab initio molecular dynamics method using the VASP software code was used to simulate a typical alkali borate system x Na 2 O - (100-x) B 2 O 3 in the concentration range x from 0 to 35 mol% Na 2 O in the liquid state at T = 1273 K. We used supercells of 246–250 atoms. Their sizes were determined from the experimental data on the solution density. We heated the system to high temperatures and then cooled it to obtain an equilibrium state free of uncharacteristic structures. Partial pair correlation functions were calculated and it was shown that the positions of the maxima do not depend on the melt composition, which made it possible to determine the bond lengths of different pairs of atoms. A statistical analysis of the boron‑oxygen network, which includes at least 96 % of boron and oxygen atoms, was performed. The basic structural units BO 3 and BO 4 were revealed, and the distribution of non-bridging oxygen atoms between them was analyzed. It was shown that the geometric characteristics of the basic structural units, bond lengths and angles between them, also do not depend on the composition. The BO 3 triangles and BO 4 tetrahedra are close to regular, and their lifetime is, respectively, about 5 ps and 1 ps. Stable superstructural formations, hexatomic rings of four types and their associations, were identified. A comparison of the concentration dependences of various structural parameters with the viscoelasticity curve, previously measured by the vibration method, was carried out, and a high correlation of the latter with the inverse value of the total number of hexatomic rings was found. New types of associations of two rings were found and their classification was proposed. • Alkaline borate melt was studied by ab initio molecular dynamics. • Geometry of base and superstructure groups does not depend on composition. • Non-bridging oxygens are mainly found in trigonal base structures. • Measured viscoelasticity correlates with the reciprocal of the total number of rings. • New types of combinations of two rings were found and a classification was proposed.