Short- and medium-range structure of amorphous zircon from molecular dynamics simulations

We have simulated the structure of amorphous zircon using classical molecular dynamics simulations with a partial-charge model. We present detailed structural characterizations of the simulated high and low density amorphous zircon, and compare our results with available neutron diffraction, EXAFS, NMR and other experimental results. The results show that amorphization leads to polymerization of the silicon-oxygen network and the formation of regions rich in zirconium. The average n value of Q(n) species is 1.6-1.8, where a considerable percentage of the oxygen ions (around 20%) have only zirconium in the first coordination shell in amorphous zircon. The Zr-O bond length (around 2.10 angstrom) is shorter and the oxygen coordination number around zirconium smaller (6-7) than in crystalline zircon, in good agreement with the EXAFS results. The calculated static neutron structure factors show reasonable agreement with experimental result. The medium range structures were characterized by the first sharp diffraction peak and primitive ring analysis, as well as from Qn distribution, oxygen environment and cation-cation pair distribution. It is found that most of the silicon-oxygen tetrahedra form branched chains and rings of various sizes, while the zircon-oxygen polyhedra form a percolated network different from the crystalline percolated structure.