Abstract
The x-ray structure factor of molten has been measured, enabled by the use of aerodynamic levitation and laser beam heating, to a temperature of . The Ti-O coordination number in the melt is close to , with modal Ti-O bond length , both values being significantly smaller than for the high temperature stable rutile crystal structure . The structural differences between melt and crystal are qualitatively similar to those for alumina, which is rationalized in terms of the similar field strengths of and . The diffraction data are used to generate physically and chemically reasonable structural models, which are then compared to the predictions based on various classical molecular dynamics (MD) potentials. Interatomic potentials, suitable for modeling molten , are introduced, given the inability of existing MD models to reproduce the diffraction data. These potentials have the additional advantage of being able to predict the density and thermal expansion of the melt, as well as solid amorphous , in agreement with published results. This is of critical importance given the strong correlation between density and structural parameters such as . The large thermal expansion of the melt is associated with weakly temperature dependent structural changes, whereby simulations show that . The liquid is structurally analogous to the geophysically relevant high pressure liquid silica system at around 27 GPa. We argue that the predominance of fivefold polyhedra in the melt implies the existence of as-yet-undiscovered polymorphs, based on lower-than-octahedral coordination numbers, which are likely to be metastable under ambient conditions. Given the industrial importance of titanium oxides, experimental and computational searches for such polymorphs are well warranted.
2 More- Received 29 July 2014
- Revised 27 August 2014
DOI:https://doi.org/10.1103/PhysRevB.90.094204
©2014 American Physical Society