Structure of Liquid Al ₆ Si ₂ O ₁₃ (3:2 Mullite)

Abstract

We report the first measurements of the structure factor, S ( Q ), and the pair distribution function, G ( r ), of Al 6 Si 2 O 13 (3:2 mullite) in the normal and supercooled liquid states in the temperature range 1776–2203 K. Measurements are obtained by synchrotron X‐ray scattering on levitated, laser‐heated liquid specimens. The S ( Q ) shows a prepeak at 2.0 Å −1 followed by a main peak at 4.5 Å −1 and a weak feature at 8 Å −1 . The G ( r ) shows a strong (Si,Al)–O correlation at 1.80 Å at high temperature that moves to 1.72 Å as the liquid is supercooled. The second and third nearest neighbor peaks at 3.0 and 4.25 Å sharpen with supercooling. The short‐range structure of the high‐temperature liquid is similar to the corresponding glasses produced by rapid quenching. Supercooling causes an increase in the concentration of tetrahedral Si 4+ ions, which is manifested by the large shift in the first peak to lower ionic distance, r , values in G ( r ). The increase in tetrahedrally coordinated Si 4+ ions is offset by an increase in octahedral Al 3+ ions. The clustering of the SiO 4 4− tetrahedral units results in increased viscosity of the liquid at temperatures below the melting point, which is consistent with Al 6 Si 2 O 13 being a fragile liquid.