The structure of crystals, glasses, and melts along the CaO-Al2O3 join: Results from Raman, Al L- and K-edge X-ray absorption, and 27Al NMR spectroscopy

Abstract

Calcium aluminate glasses are important materials where AlO_4/2⁻ is the only network former. Aluminum in crystals or glasses between CaO and Al₂O₃ can have different environments as a function of the CaO/Al₂O₃ ratio. Using X-ray absorption at the Al K- and L-edges, Raman and ²⁷Al NMR spectroscopies, we have determined the structural surroundings of Al in glasses, crystals, and melts in this binary system. Aluminum is in octahedral coordination at high-Al₂O₃ content (>80 mol%) and essentially in fourfold coordination with 4 bridging O atoms (BOs) at Al₂O₃ contents between 30 and 75 mol%. At around 25 mol% Al₂O₃, Al is in tetrahedral coordination with two BOs. The presence of higher-coordinated species at high-Al₂O₃ contents and their absence at low Al₂O₃ imply different viscous flow mechanisms for high- and low-concentration Al₂O₃ networks.