Fabrication of transparent polycrystalline Y2Ti2O7 pyrochlore via direct solid-state reaction sintering

Transparent Y2Ti2O7 pyrochlore was fabricated via the direct solid-state reaction of Y2O3 and TiO2 by hot pressing at 1400 °C under 20 MPa in vacuum, followed by annealing at 700–1000 °C for 4–10 h in air. A small amount of excess Y2O3 was added to the stoichiometric composition to promote densification via the formation of more oxygen vacancies. The black-colored samples obtained after hot pressing exhibited transparency upon annealing owing to the restoration of oxygen ions in the oxygen vacancies, where the transmittance depended significantly on the annealing conditions. The highest transmittances of 55.1 and 74% were obtained at 800 and 2400 nm, respectively, for the Y2Ti2O7 annealed at 800 °C for 8 h. The bandgap energy was 3.75 eV for the transparent Y2Ti2O7, while that of the black Y2Ti2O7 without annealing was 0.29 eV due to the defect level formed by the oxygen vacancies. A model was proposed to explain the transparency behavior of Y(2+2X)Ti2O(7-X) owing to the variations in the atmosphere and excess Y2O3 in terms of oxygen vacancies and ions present in a unit cell.