Dynamic evolution of Sr during high-temperature phase transition in Sr-doped TiO2: Insights from In Situ XRD and DFT analysis

The high‐temperature anatase-to-rutile phase transition in TiO₂ has been extensively investigated; however, the evolution of dopant ions during this process, and their effects on the crystal lattice and electronic structure, remain poorly understood. Herein, we combined semiquantitative Rietveld refinement of high-temperature in situ X-ray diffraction (XRD) data with Raman spectroscopy, X-ray photoelectron spectroscopy, and density functional theory calculations to systematically elucidate the evolution of Sr-doped TiO₂ during its phase transition from 750 °C to 950 °C. At 750 °C, only the SrTiO₃ (110) reflection was observed, while rutile emerged at 800 °C; at higher temperatures, anatase and rutile coexisted with SrTiO₃. Interstitial and substitutional Sr models in anatase and rutile were constructed based on the unit‐cell parameters of pure TiO₂. For the first time, the Sr evolution mechanism during the phase transition was clarified through differential charge density maps, Mulliken charge analyses, and Ti–O bond length calculations. The three-stage evolution mechanism includes: (i) interstitial and substitutional solid-solution coexistence, (ii) substitution-dominated lattice expansion, and (iii) solid-solution saturation followed by exsolution to form SrTiO₃. This study provides theoretical guidance for tuning TiO₂ phase transition temperatures, grain size, and defect structures through Sr doping and provides deeper insights into the high-temperature behavior of metal ions in TiO₂ systems.