Raman spectroscopy and density functional theory analyses of the melt structure in a Li2B4O7 crystal growth system

Abstract

Melt structure, a fundamental and challenging subject for borate crystal growth, has not been solved for many years. In this paper, a new method has been employed to study the Li₂B₄O₇ melt structure. High-temperature Raman spectroscopy has been used to investigate the structural evolvement from a Li₂B₄O₇ crystal to a Li₂B₄O₇ melt. Based on the investigation, a model was proposed to describe the Li₂B₄O₇ melt. The melt is made up of polymer-like boron–oxygen chains; the minimal repeated unit is the B₄O₆Ø₂²⁻ (Ø = bridging oxygen) group which is formed by a B₃O₄Ø₂⁻ six-membered ring and a BOØ₂⁻ triangle linked by a bridging oxygen atom. DFT calculations have verified the melt structure and provided accurate assignments for the vibrational bands present in the Li₂B₄O₇ melt Raman spectrum.