Determination of Crystal-Field Splitting Induced by Thermal Oxidation of Titanium

The electronic structure of transition-metal oxides is a key component responsible for material's optical and chemical properties. Specifically for metal-oxide structures, the crystal-field interaction determines the shape, strength, and occupancy of electronic orbitals. Consequently, the crystal-field splitting and resulting unoccupied state populations can be foreseen as modeling factors of the photochemical activity. Herein, we study the formation of crystal-field effects during thermal oxidation of titanium in an ambient atmosphere and range of temperatures. The X-ray absorption spectroscopy is employed for quantitative analysis of average t_2g-e_g crystal-field splitting (Δoct) and relative t_2g/e_g bands occupancy. The obtained result shows that Δoct changes as a function of temperature from 1.97 eV for a passive oxide layer created on a Ti metal surface at room temperature to 2.41 eV at 600 °C when the material changes into the TiO₂ rutile phase. On the basis of XAS data analysis, we show that the Δoct values determined from L₂ and L₃ absorption edges are equal, indicating that the 2p_1/2 and 2p_3/2 core holes screen the t_2g and e_g electronic states in a similar manner.