The structure of Y- and La-bearing aluminosilicate glasses and melts: A combined molecular dynamics and diffraction study

To understand the behavior of rare earth elements (REE) in magmatic systems it is important to characterize in a systematic way their incorporation into silicate melts and glasses. Here, we study the structural environment of the REE Y and La in four aluminosilicate glasses and melts with varying REE content, using a combined simulation and diffraction approach. Glasses are investigated by X-ray and neutron diffraction as well as classical molecular dynamics simulations using two different polarizable ion potentials. Structure models of the corresponding melts are derived from classical and first-principles molecular dynamics simulations. We discuss the effect of temperature on coordination numbers and rationalize the structural changes in response to variations in melt/glass composition in terms of cation field strengths. We find robust evidence that REE and Al coordination numbers decrease with increasing REE content in the investigated melts and glasses. Comparing the two classical potentials, one of them is able to reproduce features of the experimental structure factors with a mixture of corner- and edge-sharing Al-O/REE-O polyhedra, whereas the other potential predicts corner-sharing Al-O/REE-O polyhedra only.