Finite-size and pressure effects on the Raman spectrum of nanocrystalline anataseTiO2

The Raman scattering behavior of anatase nanocrystals with average diameters of 4, 8, 20, and 34 nm has been compared with bulk crystal data in order to establish size-dependent changes to the phonon spectrum at ambient conditions. Further, the high-pressure behavior of the anatase nanocrystals was examined at room-temperature using in situ Raman scattering data obtained in diamond-anvil cells to a maximum pressure of 41 GPa. The size-dependent changes to the Raman spectrum are best explained in terms of three-dimensional confinement of phonons in finite-sized nanocrystalline anatase. The difference in slopes obtained for the pressure shifts of Raman modes between nanocrystalline and single crystal anatase is in conformity with the observed size-dependent bulk modulus values. The metastability of anatase as a function of pressure is demonstrated to be size dependent, with smaller crystallites preserving the structure to higher pressures. Three size regimes have been recognized for the pressure-induced phase transition of anatase at room temperature: an anatase-amorphous transition regime at the smallest crystallite sizes, an anatase-baddeleyite transition regime at intermediate crystallite sizes, and an anatase--$\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Pb}{\mathrm{O}}_{2}$ transition regime comprising large nanocrystals to macroscopic single crystals. This size-dependent phase selectivity of anatase at high pressures explains the recent contradictory experimental data. A semiquantitative phase diagram for anatase metastability as a function of size and pressure at room temperature is proposed.