New insights into the BiB3O6 melt structure

Abstract

The structure of borate melt has long been a fundamental unsolved problem in condensed matter physics, materials chemistry. Here, we use high temperature Raman spectroscopy and first principles calculations to study the structural change before and after the melting of a BiB₃O₆ crystal to obtain the BiB₃O₆ melt structure. A transformation of the 4-fold coordinated boron atoms to 3-fold coordinated boron atoms and a destruction of the atomic configuration around the Bi³⁺ ions were found when the crystal melted. Considering the growth habit of the BIBO crystal, we proposed a polymer model for the first time to describe the melt structure. The BiB₃O₆ melt is made up of special [B₃O₃Ø₃] (Ø = bridging oxygen) structural units, which further polymerize into [B₃O₃Ø₃]_n chains by sharing oxygen atoms; the boron–oxygen chains are surrounded by the Bi³⁺ ions. Density functional theory (DFT) calculations were carried out to simulate the melt Raman spectrum based on the structural unit. The calculated BiB₃O₆ melt Raman spectrum shows good agreement with the experimental spectrum. The main vibrational bands were assigned. The strongest band below 400 cm⁻¹ is mainly attributed to the wagging vibration of the side BØO₂ triangle as a whole; the band located around 630 cm⁻¹ is assigned to the bending vibrations of the [B₃O₃Ø₃]_n chain; the bands in the range of 1200–1500 cm⁻¹ arise from the stretching vibrations of B–O (or B–Ø) bonds in the boron–oxygen triangles.