Small- and Wide-Angle X-ray Scattering Studies of Liquid-Liquid Phase Separation in Silicate Melts

Incongruent liquid-liquid phase separation is a common phenomenon in molten oxides, but there are few structural studies as a result of the high temperatures involved, ca. >1600 degrees C. Here, we present a high-energy X-ray combined small-/wide-angle scattering (SAXS/WAXS) method that can directly probe non-equilibrium phase diagrams. The Porod exponent in the SAXS signal is extracted, which measures the roughness of the interface formed between different phases. Problematic volatilization of silica in Al2O3-, CaO-, and MgO-bearing silica-rich liquids was minimized using levitation, laser heating, and rapid time-resolved measurements (lasting tens of seconds), to probe the kinetics of the phase separation process. The WAXS measurement simultaneously probes the local atomic structure of the liquid state and shows that the primary structural changes are associated with the SiO2 network. Cation coordination numbers extracted from pair distribution functions support the model that the origin of phase separation in binary silicate systems is due to coulombic repulsions between poorly screened cations. For CaO-SiO2, the liquid-liquid phase separation is found to correlate with a pause in the temperature-dependent atomic disorder associated with SiO4 tetrahedra.