Lattice Dynamics Study of Nanocrystalline Yttrium Gallium Garnet at High Pressure

This work reports an experimental and theoretical lattice dynamics study of nanocrystalline Y3Ga5O12 (YGG) garnet at high pressures. Raman scattering measurements in nanocrystalline Tm3+-doped YGG garnet performed up to 29 GPa have been compared to lattice dynamics ab initio calculations for bulk garnet carried out up to 89 GPa. Good agreement between the theoretical vibrational modes of bulk crystal and the experimental modes measured in the nanocrystals is found. The contribution of GaO4 tetrahedra and GaO6 octahedra to the different phonon modes of YGG is discussed on the basis of the calculated total and partial phonon density of states. Symmetries, frequencies, and pressure coefficients of the Raman-active modes are discussed. Moreover, the calculated infrared-active modes and their pressure dependence are reported. No pressure-induced phase transition has been observed in nano-YGG up to 29 GPa. This is in agreement with theoretical results, which show a mechanical instability of YGG above 84 GPa, similar to what occurs in Gd3Ga5O12.