Unveiling the structure of calcium borate glass by neutron diffraction, MAS-NMR, Raman and first-principles calculations

Abstract

Calcium borate glass is a challenging system, due to the rich variety of borate units and alterable boron coordination number, and hence they are also of particular interest for academic study. In this study, neutron diffraction, solid-state magic angle spinning nuclear magnetic resonance (MAS-NMR) and Raman spectroscopy were combined with first-principles calculations, experience potential structure refinement (EPSR) simulations to investigate the structure of xCaO·(1-x)11B2O3 glass. Property data such as density and glass transition temperature were also measured to study the impact of structural changes on the properties of glasses. The structure of calcium borate glass was analyzed using various methods including structural unit, atomic pair bond length, coordination number, ring structure, chain structure and cavity distribution. Most notably, the atomic labeling method plays a key role in the structural analysis process. Through this method some new conclusions were obtained. For example, the increase of CaO content mainly leads to the conversion of BO to NBO in the glass structure, and has little effect on the content of BO3 and BØ4-. This result further leads to an increase in Ca-NBO interactions. Interestingly, the bond length distribution of Ca-NBO and B-NBO is shorter than that of Ca-BO and B-BO, so the glass structure becomes denser. The structural reason why the density of calcium borate glass is less than the crystal density was also revealed. This study contributes to unveiling the structure of calcium borate glass and exploring the correlation between the structure and properties of calcium borate glass, which provides a valuable reference for developing more valuable amorphous materials.