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

Abstract

The pressure dependence of the Raman-active modes of the rare-earth aluminium garnet compounds (RE3Al5O12, RE:Tb, Dy, Tm, Yb, Lu) has been measured at room temperature anddiscussed in terms of the rigid-ion model (RIM). The Raman modesin the high-frequency region (ω>650 cm-1) exhibitsystematically larger pressure slopes(4-5.6 cm-1 GPa-1) than the rest, which haveslopes ranging between -0.1 and 3.3 cm-1 GPa-1. Thevalues of the Γi ( = ∂ln ωi/∂P) parameters show that thecorresponding bonding in the RE3Al5O12 family isnearly of the same nature and order of magnitude. No pressure-induced phase transitions could be detected up to 25 GPa. Usingthe 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 thedodecahedra) of the Yb3Al5O12,Dy3Al5O12 and Tm3Al5O12 has beencalculated. The calculated force constants show that thecorresponding bonds in the samples investigated are of comparable strength and their pressure variation is quitesimilar. Furthermore, it has been found that they increaseaccording to the sequence dodecahedra → octahedra → tetrahedra and that the compressibility follows the same trend,while the electronic configuration of the dodecahedral cationmight play a role in the total compressibility.