Structure of liquid Al6Si2O13 (3 : 2 mullite)

We report the first measurements of the structure factor, S(Q), and the pai r distribution function, G(r), of Al6Si2O13 (3:2 mullite) in the normal and supercooled liquid states in the temperature range 1776-2203 K, Measuremen ts are obtained by synchrotron x-ray scattering on levitated, laser-heated liquid specimens, The S(Q) shows a prepeak at 2.0 Angstrom (-1) followed by a main peak at 4.5 Angstrom (-1) and a weak feature at 8 Angstrom (-1). Th e G(r) shows a strong (Si,Al)-O correlation at 1.80 Angstrom at high temper ature that moves to 1.72 Angstrom as the Liquid is supercooled. The second and third nearest neighbor peaks at 3.0 and 4.25 Angstrom sharpen with supe rcooling, The short-range structure of the high-temperature liquid is simil ar to the corresponding glasses produced by rapid quenching. Supercooling c auses an increase in the concentration of tetrahedral Si4+ ions, which is m anifested by the large shift in the first peak to lower ionic distance, r, values in G(r), The increase in tetrahedrally coordinated Si4+ ions is offs et by an increase in octahedral Al3+ ions. The clustering of the SiO44- tet rahedral units results in increased viscosity of the liquid at temperatures below the melting point, which is consistent with Al6Si2O13 being a fragil e liquid.