High-pressure effects on the Raman spectrum and the force constants of the rare-earth aluminium garnets (RE3Al5O12)

The pressure dependence of the Raman-active modes of the rare-earth aluminium garnet compounds (RE 3 Al 5 O 12 , RE: Tb, Dy, Tm, Yb, Lu) has been measured at room temperature and discussed in terms of the rigid-ion model (RIM). The Raman modes in the high-frequency region (ω>650 cm -1 ) exhibit systematically larger pressure slopes (4-5.6 cm -1 GPa -1 ) than the rest, which have slopes ranging between -0.1 and 3.3 cm -1 GPa -1 . The values of the Γ i ( = ∂ln ω i /∂ P ) parameters show that the corresponding bonding in the RE 3 Al 5 O 12 family is nearly of the same nature and order of magnitude. No pressure-induced phase transitions could be detected up to 25 GPa. Using the RIM, the pressure dependence (up to 10 GPa) of the first-neighbour stretching and bending force constants for the Al-O (in the octahedra and tetrahedra) and RE-O bonds (in the dodecahedra) of the Yb 3 Al 5 O 12 , Dy 3 Al 5 O 12 and Tm 3 Al 5 O 12 has been calculated. The calculated force constants show that the corresponding bonds in the samples investigated are of comparable strength and their pressure variation is quite similar. Furthermore, it has been found that they increase according to the sequence dodecahedra → octahedra → tetrahedra and that the compressibility follows the same trend, while the electronic configuration of the dodecahedral cation might play a role in the total compressibility.