Polarized Raman spectra of single crystal gadolinium molybdate, Gd₂(MoO₄)₃ were obtained between 1atm and 7 GPa. Ab initio calculations were performed on the isostructural europium compound for reproducing pressure induced structural changes. Using a mixture of alcohol and water as a pressure transmitting medium, YY, ZZ, XY components of scattering matrices were measured. The ZZ spectra were also obtained in argon. Five phase transitions and amorphization were identified. In alcohol, amorphisation is observed above 6.5 GPa. In argon the amorphisation is progressive and begins above 3 GPa and thus the chemical decomposition is not considered to drive the amorphization. The spectral changes with pressure concern as well high-frequency bands attributed to symmetric and antisymmetric MoO4 stretching modes as the very low frequency modes such as librations of tetrahedra. This means that both tetrahedra short range and long-range organisation are involved in these phase transitions. The disordering mechanism is strongly dependent on the pressure-transmitting medium. The TO and LO low frequency modes of A₁ symmetry, observed in the Y(ZZ)Y and c(bb)c geometries respectively below 50 cm^-1, soften continuously through the first three phases when increasing pressure. The strong A₂ mode observed in the Z(XY)Z spectra exhibits the same anomalous behaviour by decreasing from 53 down to 46 cm^-1 at 2 GPa. The softening of these modes, probably due to librations of tetrahedra, is related to the orientation change of tetrahedra observed by ab initio calculations when the volume cell is decreased. Symmetry coordinates for these modes are identical to cooperative orientation changes observed in ab initio results. These orientation changes can explain the frequency decrease of Mo-O stretching modes above 2 GPa, which indicates an increase of Mo coordination. Among the different amorphization mechanisms, the steric hindrance between polyhedra is believed to be the most relevant.

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