Amorphous tantala and its relationship with the molten state

The structure factors of molten Ta2O5 and Nb2O5 have been measured by high-energy x-ray and pulsed neutron diffraction. These are compared to transmission-mode x-ray diffraction through a self-supported 15-mu m ion-beam sputtered amorphous tantala film. Atomistic models derived from the diffraction data by means of empirical potential structure refinement reveal that tantala and niobia liquids are very close to isomorphous, as confirmed by measurement of a molten mixture, Ta0.8Nb1.2O5. Nonetheless, peak Nb-O bond lengths are about 1% shorter than those for Ta-O, at temperatures, T* = T/T-melt, scaled to the melting points. Mean coordination numbers are n(MO) similar or equal to 5.6(1), n(OM) similar or equal to 2.23(4) in the liquid state, and n(TaO) similar or equal to 6.6(2), n(OTa) similar or equal to 2.63(8) in the solid. The liquids are built from five-and six-fold M-O polyhedra which connect principally by corner sharing, with a minority of edge sharing; a-Ta2O5 on the other hand has a local structure more akin to the crystalline polymorphs, built primarily from six-and seven-fold polyhedra, with a larger degree of edge sharing. The structural differences between liquid and amorphous Ta2O5, coupled with observations of increasing peak bond lengths upon cooling, are consistent with the interpretation that the amorphous film reaches a supercooled liquidlike metastable equilibrium during deposition. In other words, the amorphous film shares a common progenitor state with a hypothetical glass quenched from a fragile melt. In addition, we show that recent classical interatomic potentials do not fully reproduce the diffraction data, and infer that inclusion of attractive (non-Coulombic) Ta-Ta interactions is important, particularly for obtaining the correct degree of edge sharing, coordination numbers, and densities. Nanoscale inhomogeneity of the amorphous film is confirmed by the observation of small-angle x-ray scattering.