Trans-Regime Structural Transition of (In³⁺ + Nb⁵⁺) Co-Doped Anatase TiO₂ Nanocrystals under High Pressure

Chemical co-doping and high pressure reactions have been broadly used to synthesize novel materials or to tune the physicochemical properties of traditional materials. Here, we take In³⁺ and Nb⁵⁺ ions co-doped anatase TiO₂ nanocrystals as an example and report that a combination of both a chemical and a high pressure reaction route is more powerful for the preparation of metastable polymorphs. It is experimentally demonstrated that In³⁺ and Nb⁵⁺ co-doping significantly changes the high-pressure reaction behaviors of anatase TiO₂ nanocrystals (<10 nm) and leads to their trans-regime structural transition in terms of in situ Raman analysis, from an anatase to a baddeleyite-like phase under compressive pressures and then to an α-PbO₂-like structure under decompressive pressures. This abnormal phase transition is attributed to a defect-induced heterogeneous nucleation mechanism. Furthermore, the stiffness of co-doped TiO₂ nanocrystals is significantly enhanced due to the synergistic effects of co-dopants. This research not only proposes a potentially effective strategy to synthesize co-doped metastable polymorphic phases but also suggests one feasible method to improve the mechanical properties of anatase TiO₂ nanocrystals.