First highly transparent Gd2Sn2O7 pyrochlore ceramics with high refractive index: Al2O3 additive roles on structural features, sintering behaviors, and optical performances

Highly transparent novel Gd ₂Sn ₂O ₇ pyrochlore ceramics, for the first time, were successfully fabricated by means of oxygen-assisted pressureless sintering. First-principles calculations and experimental test analysis revealed that the polycrystalline Gd ₂Sn ₂O ₇ bulk material had a direct bandgap of ∼3.8 eV, low phonon energy of ∼502 cm ⁻¹ , a high refractive index of ∼2.02 at 633 nm, a large Abbe`s number of ∼32.7, optical basicity of 1.011 ± 0.015, and an oxygen ion polarizability of 2.53 ± 0.06 Å ³. Al ₂O ₃ was observed to be a beneficial additive for Gd ₂Sn ₂O ₇ ceramic sintering, with an optimum concentration of 0.3 at.%. Al ³⁺ substantially penetrated the octahedral interstice formed by Sn−O polyhedral to yield the interstitial Al i · · · defect, leading to lattice expansion. The point defect effect and solute drag mechanism jointly contributed to sintering densification. The grain boundary of the pure Gd ₂Sn ₂O ₇ ceramic was quite clean with a width of ∼1.0 nm, while a 0.3 at.% Al ³⁺ dopant broadened the grain boundary to ∼10.8 nm. The best Gd ₂Sn ₂O ₇ ceramic sample exhibited a high transparency of ∼75.1 % at 633 nm and a fine average grain size of ∼3.2 μm. Upon UV excitation, the bulk samples presented green emission centering at ∼470 nm deriving from the oxygen-vacancy defect. The Al ₂O ₃ dopants enhanced the photoluminescence intensity and shortened the fluorescence lifetime. The cathodoluminescence spectra emitted a white light similar to sunlight, which covered almost the full visible spectrum. The cathodoluminescence intensity also increased as the acceleration voltage increased, while the emission intensity from the grain boundary was slightly higher than that from the grain interior at each voltage level.