Structure of molten titanium dioxide

Abstract

The x-ray structure factor of molten TiO 2 has been measured for the first time, enabled by the use of aerodynamic levitation and laser beam heating, to a temperature of T = 2250(30) K. Ti-O coordination number in the melt is close to n TiO = 5.0(2), with modal Ti-O bond length r TiO = 1.881(5) Å, both values being significantly smaller than for the high temperature stable Rutile crystal structure (n TiO = 6.0, r TiO = 1.959 Å). 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 Ti 4+ and Al 3+. 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. New interatomic potentials, suitable for modelling molten TiO 2 , are introduced, given the inability of existing MD models to reproduce the diffraction data. These new potentials have the additional great advantage of being able to predict the density and thermal expansion of the melt, as well as solid amorphous TiO 2 , in agreement with published results. This is of critical importance given the strong correlation between density and structural parameters such as n TiO . The large thermal expansion of the melt is associated with weakly temperature dependent structural changes, whereby simulations show that n TiO = 5.85(2) -(3.0(1) x 10 -4 )T (K, 2.75 Å cut-off). The TiO 2 liquid is structurally analogous to the geophysically relevant high pressure liquid silica system at around 27 GPa. We argue that the predominance of 5-fold polyhedra in the melt implies the existence of as yet undiscovered TiO 2 polymorphs, based on lowerthan-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.

Rare‐earth titanate melt structure and glass formation

“…This is supported by the longer Ti‐O modal bond lengths in the RE titanate, Figure , as well as by the XANES observations. Furthermore, both La and Ba modifier oxides appear to suppress n TiO compared to its value in pure molten TiO 2 , which is again supported by comparison of the modal bond lengths, Figure .…”

“…The pyrometer temperature was also corrected for reflection losses from a CaF 2 window and lens that were in the optical path. The diffraction data reduction and definitions of the scattering functions used are given elsewhere . The measured structure factors, S ( Q ), represented as functions of Q = (4 π sin ϑ )/ λ , with 2 ϑ the scattering angle, and λ = 0.12370 Å the X‐ray wavelength, were Fourier transformed with Q max = 20 Å −1 , without modification (window) function.…”

In‐situ determination of the HfO2–Ta2O5‐temperature phase diagram up to 3000°C

“…In‐situ, high temperature, synchrotron XRPD experiments were performed on the hafnium tantalate beads at the Advanced Photon Source (APS), Argonne National Laboratory, at Beamline 6‐ID‐D. Hafnium tantalate sintered beads were levitated in a stream of argon mixed with 21% oxygen (to simulate air) in a conical nozzle levitator (CNL) system . The levitated sample rotates on axis while being heated using the beam from a 400 W sealed tube CO 2 laser (10.6 μm; Synrad FSi401SB; Mukilteo, WA).…”

Phase behavior of empirical potentials of titanium dioxide

“…Although in principle, we could also investigate the melting behaviour of the various crystal polymorphs studied here by computing the Gibbs energy of the fluid phase by thermodynamic integration from a suitable perfect gas, the TiO 2 melt has been shown not to be very well represented by the MA potential, 5 and so we do not focus on the very high temperature behaviour of the system here.…”

“…Using the Clapeyron equation [Eq. (5)] and the relation ∆ trs H = T trs ∆ trs S, we can determine the entropy change from the gradient of the phase coexistence curve. We can also compute the entropy difference at the point of coexistence between two phases readily using the free energy and enthalpy calculations that were required to compute the chemical potentials to begin with.…”