Transparent Lu ₃NbO ₇ bodies prepared by reactive spark plasma sintering and their optical and mechanical properties

Transparent lutetium niobate (Lu ₃NbO ₇) bodies were prepared by reactive spark plasma sintering using Lu ₂O ₃ and Nb ₂O ₅ powders. Fully dense Lu ₃NbO ₇ bodies with density greater than 99.5% of the theoretical were obtained at 1300-1650 °C. The grains steadily grew from 0.1 to 0.6 μm with increasing sintering temperature from 1200 to 1450 °C and significant grain growth from 2.2 to 9.2 μm occurred at 1550-1650 °C. The Lu ₃NbO ₇ body sintered at 1450 °C showed the highest transmittance of 63% at 550 nm after heat treatment at 850 °C in air for 6 h. Fully dense, submicron-size transparent Lu ₃NbO ₇ exhibited Vickers hardness of ∼13.0 GPa and indentation fracture toughness of 1.0 MPa m ^1/2.