Effect of grain size on structural transitions in anatase TiO2:: A Raman spectroscopy study at high pressure -: art. no. 134102

Micro-Raman spectroscopy has been used to investigate the structural stability of nanoanatase (TiO2), of similar to12 nm average grain size, up to pressures of similar to40 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 alpha-PbO2-type intermediate near similar to5 GPa before transforming to the monoclinic baddeleyite structure at an onset pressure of similar to15 GPa and remains in this phase up to the highest pressure of similar to35 GPa. By contrast, the nanophase anatase maintains its structural integrity up to similar to18 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 degreesC 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 similar to2 GPa, descending to a broad minimum at similar to5 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 similar to12 nm of the nanoanatase is less than the critical size estimated for nucleation and growth of a new structural alpha-PbO2-type phase. This rationalizes how the structural transition to the alpha-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 alpha-PbO2-type intermediate with associated anatase/alpha-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 alpha-PbO2-type structure upon decompression from 35-40 GPa to ambient conditions.