Titanium - L3,2 and O-K electron energy loss near-edge structures of TixOy phases: fingerprints to the valence state and site geometry

Titanium (Ti) at chemical conditions on the Earth's surface occurs in the oxidation state of 4+. Understanding its incorporation into high-pressure minerals would provide important insight into the oxidation state and element partitioning in the Earth's mantle. Thus we have studied the electronic structure of Ti in various oxidation states and crystallographic environments, establishing a fingerprint technique for the study of mantle derived samples. To this avail we have combined experimental electron energy-loss near-edge structures (ELNES) with band structure computations. The Ti L-edge and O K-edge spectra of six titanium oxides (TiO2 rutile, Ti2O3, TiO, Ti3O5, Ti4O7 and Ti5O9 with valence states - Ti4+, Ti3+, Ti2+ - either in a single or in mixed valence state) have been measured using Electron Energy Loss Spectroscopy (EELS). Band structure computations were based on electron density functional theory (DFT), as implemented in the Linearized Augmented Plane Wave method (WIEN2k) with the Generalized Gradient Approximation (GGA) to exchange and correlation. These first principle calculations of the band structure allowed to compute the transition energies of the O K ELNES. The Ti L3,2 ELNES of rutile provides a fingerprint for six-fold coordinated Ti4+ in tetragonal distortion. In Ti2O3 the Ti is six-fold coordinated with a valence state of 3+. In the three mixed valence state oxides (Ti3O5, Ti4O7 and Ti5O9) the ratio Ti4+:Ti3+ is different and ELNES spectra show different energies and features. In TiO the Ti is octahedrally coordinated with a valence state 2+: as the coordination polyhedron shows no distortion the spectrum can be used as fingerprint for six-fold coordinated Ti2+. We observe the following spectral changes as function of increasing valence state: (i) the crystal-field splitting and (ii) the energy separation between the Ti 3d and Ti 4s/4p states become larger, and (iii) the Ti 4s/4p band-width decreases. These observations are proven by our computed transition energies. The crystal-field splitting depends on the magnitude of covalent bonding. As the number of d electrons increases from TiO2 to TiO covalent bonding between Ti and O is weakened. For all spectra we obtain good agreement between experimental measurements and computed results. The results of our study substantiate that Ti L-edge and O K-edge spectra provide key information on valence state and site geometry. We have recently verified the application of the data to Ti-bearing mantle minerals synthesized in multi-anvil experiments.