Synthesis of La2Ti2O7 Nanoscale Powder and Ceramics Based on It by Sol–Gel Synthesis and Spark Plasma Sintering

The use of ceramics as matrices for immobilization of radionuclides for the purpose of their safe long-term disposal or useful application is studied with an emphasis placed on phase stability, structural integrity, hydrolytic stability, etc. This work implements a combined approach based on the sol–gel citrate synthesis of nanosized La2Ti2O7 powder and its subsequent spark plasma sintering to produce dense ceramics. The phase composition and structure of the nanosized La2Ti2O7 powder and the ceramic samples obtained in the temperature range of 900–1300°C were studied by powder X-ray diffraction and SEM. It was shown that the conditions of synthesis of the powder ensure the formation of nanosized crystalline La2Ti2O7 grains, consolidation of which under spark plasma heating is accompanied by a change in the phase composition of La2Ti2O7 from a single-phase monoclinic structure to orthorhombic structure with a LaTiO3 impurity at temperatures above 1200°C. It was found that a change in the ceramic structure is accompanied by the formation of non-porous and defect-free monolithic samples. This type of change was shown to lead to an increase in the relative density (81.3–95.7%) and compressive strength (78–566 MPa) of the ceramics. However, the hydrolytic stability decreases, as indicated by increasing La3+ leaching rate from 10–7 to 10–5 g/(cm2 day). The obtained results are useful for the systematic study of materials suitable for immobilization of radioactive waste in ceramics.