A quantum-chemical study of the mechanism of ionic conductivity in alkali borate glasses

The incorporation of alkali metal oxide molecules into a continuous random network of vitreous boron oxide is considered within the cluster approximation at a semiempirical MNDO level. It is found that the oxygen atom of the Li2O molecule easily forms an additional bond with the threefold-coordinated boron atom with the formation of the BO4 tetrahedron. Different types of metal cation motion in alkali borate glasses are treated. It is revealed that the M+ cation in neutral systems is predominantly located near the edges of a BO4- tetrahedron on the outside of the B-O-B corner fragments and can readily change its position. The M+ cation in the neighborhood of the pentaborate grouping makes a complete turn without considerable expenditure of energy. The vibrations with a change in the rotation angle from 180° to 270° are possible in tetraborates, and, only in the neighborhood of triborate, one position is substantially more stable than the other positions. The cation can migrate from center to center along chains of a continuous glass network on the outside of the B-O-B corner fragments. In the pentaborate and tetraborate centers, the cation can migrate into another chain and change its direction of motion.