Elucidating the Limited Solubility of Y³⁺ in La₂Ti₂O₇: Looking Beyond Radius Ratio Guidelines

Chemically flexible A₂B₂O₇ oxides are attractive candidates for designing structure-specific functional materials by judicious substitution. However, modulating functionality by substitution is governed by the solubility limits of the substituent. Present work reports an a typical 10 mol % solubility of Y³⁺ in La₂Ti₂O₇(LTO:10Y), substantially lower than 60 mol %, anticipated by cationic radius ratio guidelines (r_A/r_B) for A₂B₂O₇. Systematic experimental and theoretical investigations have been performed to justify this limited solubility. The solubility of Y³⁺ in La₂Ti₂O₇ was verified by XRD at the bulk scale and via an Eu probe at the polyhedral level. Whereas XRD shows phase segregation beyond 10 mol % Y³⁺, the 0.5 mol % Eu³⁺-codoped LTO:10Y sample showed typical Eu³⁺ emissions, advocating structural homogeneity. However, the corresponding excitation spectrum revealed that insertion of Y³⁺ in LTO enhanced the energy transfer by the titanium-oxygen polyhedral network, [TiO₆]^8- → Eu³⁺. These observations were rationalized through density functional theory (DFT) calculations, which unveiled yttrium's preference for a low-coordination La site that substantially modifies adjacent TiO₆ polyhedra and influences the energy transfer. Combined experimental and theoretical studies propose that yttrium's preference for a lower coordination number and a shorter/stronger metal-oxygen bond renders site selectivity and simultaneous regulation of the TiO₆ network, limiting its solubility in La₂Ti₂O₇. Thus, it provides supporting evidence for bonding-controlled substitution of yttrium in La₂Ti₂O₇, rendering solubility lesser than that endorsed by radius ratio guidelines.