Effect of grain size on structural transitions in anatase TiO₂: A Raman spectroscopy study at high pressure

Micro-Raman spectroscopy has been used to investigate the structural stability of nanoanatase (TiO₂), of ∼12 nm average grain size, up to pressures of ∼40 GPa in a diamond anvil cell at room temperature. This has been compared to the Raman pressure behavior of bulk anatase, which undergoes a structural transition from the tetragonal structure to the orthorhombic α-PbO₂-type intermediate near ∼5 GPa before transforming to the monoclinic baddeleyite structure at an onset pressure of ∼15 GPa and remains in this phase up to the highest pressure of ∼35 GPa. By contrast, the nanophase anatase maintains its structural integrity up to ∼18 GPa before transforming directly to the baddeleyite structure, which is stable to the highest pressure of this study. The pressure dependence of the four most prominent Raman modes is similar for both the nanophase and bulk-anatase compounds suggesting that they have similar compressibilities, all of these modes exhibiting normal stiffening behavior as the pressure rises. In a separate temperature dependent micro-Raman study to 1000 °C at ambient pressure, the most intense (E_g) mode of both the nanophase and bulk-anatase samples shows unusual stiffening behavior on heating. This mode is associated with O-Ti-O bond-bending vibrations in which O displacements are bigger than the Ti displacements. In both samples, all other prominent Raman modes exhibit anticipated softening when heated. From these temperature and pressure dependences it may be deduced that intrinsic anharmonicity and associated phonon-phonon interactions govern the temperature dependence of the intense E_g mode, whereas either or both dilatation effects or intrinsic anharmonicity determine the behavior of the other modes. The linewidth of the most intense E_g mode in nanoanatase exhibits a sharp decrease from ambient to ∼2 GPa, descending to a broad minimum at ∼5 GPa, in contrast to the behavior of the bulk phase. The linewidth then increases monotonically at P > 5 GPa similar to the behavior of the bulk solid. This unusual behavior of the linewidth in the nanophase material may be explained by the effect of internal pressure acting across the curved surface in the nanosized grains being compensated by the externally applied pressure. The average grain size of ∼12 nm of the nanoanatase is less than the critical size estimated for nucleation and growth of a new structural α-PbO ₂-type phase. This rationalizes how the structural transition to the α-PbO₂-type intermediate phase, which occurs in bulk anatase, has been inhibited in the nanophase material. Instead, the formation of new grain boundaries of compacted nanoparticles of anatase would be the anticipated main pressure-induced structural modification, likely representing a lower free-energy situation than if nucleation of the new α-PbO₂-type intermediate with associated anatase/α-PbO₂-type interfaces occurred. At sufficiently high pressure the free-energy gain from the volume reduction to the baddeleyite phase is shown to offset the interface energy cost involved in the nucleation and growth of this high-pressure phase and, thus, drives the direct anatase→baddeleyite structural transition. Both the nanophase and bulk-sample pressure quench from the baddeleyite to the α-PbO₂-type structure upon decompression from 35-40 GPa to ambient conditions.