Defects in Anatase TiO₂ Single Crystal Controlled by Heat Treatments

Abstract

Optical absorption and electron paramagnetic resonance (EPR) spectra were measured for synthetic anatase TiO₂ single crystals annealed under oxygen or hydrogen atmosphere at the desired temperatures. The as-grown crystal changed its color from pale blue through yellow to colorless by annealing under oxygen pressure. On the other hand, the contact of the colorless crystal with hydrogen gas resulted in the change of its color to pale blue or dark blue depending on annealing temperature. On repetitive heat-treatment under oxygen atmosphere, the dark blue crystal became dark green, yellow and colorless. Since the change of the crystal colors reflects difference in the defect states, repetitive control of the defect states was established in this work. The EPR spectra revealed absence of paramagnetic species in colorless crystal and presence of the conduction electrons in pale blue, dark blue and dark green crystals. The EPR spectra of pale blue crystal were asymmetric because of the existence of conduction electrons and its line shape was analyzed on the basis of normal skin effect. The yellow crystal shows complex EPR spectra. Models for various defects in these crystals are proposed.