Crystal structural, thermal, and mechanical properties of Yb2+xTi2−xO7−x/2 solid solutions

Abstract

Crystal structural, thermal and mechanical properties of Yb2+xTi2−xO7−x/2 solid solutions annealed at 1673 ​K were investigated for x from 0.05 to 1.02. From Rietveld refinements and electron density distributions determined by the maximum-entropy method for X-ray powder diffraction data, crystal structures for x ​≲ ​0.53 and 0.63 ≲ x ​≲ ​0.8 were identified as Yb2Ti2O7-based pyrochlore and Yb2TiO5-based defect fluorite structures, respectively, while defect fluorite and Yb2O3-based bixbyite structures were found to coexist at x ​≈ ​1.02. Anisotropic distributions of the electron densities on cation sites became more noticeable for compositions near the phase boundary. Adopted model of pyrochlore structure was validated through the evaluation of defect formation energies and the comparison with theoretical values by first-principles calculations. Thermal expansion coefficients, thermal conductivities, and Young’s moduli of sintered samples with x ​≈ ​0.05, 0.53, 0.63 and 0.69 depended strongly on x, which is ascribed to changes in structural features such as densities and oxygen-vacancy concentrations.