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.
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