High-pressure polarized Raman spectra of Gd2 (MoO4 )3 : phase transitions and amorphization

Polarized Raman spectra of a single crystal of gadolinium molybdate [Gd2(MoO4)3] were obtained between 1 atm and 7 GPa. Using a mixture of alcohols as the 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. The first and second transitions are reversible, while amorphization is not. In alcohol, amorphization is observed above 6.5 GPa. With argon as the pressure-transmitting medium, amorphization is progressive and begins above 3 GPa. The spectral changes with pressure affect the high wavenumber bands attributed to symmetric and antisymmetric MoO4 stretching modes as well as the very low wavenumber modes such as librations of the tetrahedra. This means that both short-range and long-range organizations of the tetrahedra are involved in these phase transitions. The amorphization mechanism and its dependence on the pressure-transmitting medium are discussed, and the steric hindrance between polyhedra is believed to be the most relevant mechanism. The TO and LO low wavenumber modes of A1 symmetry, observed in the Y(ZZ)Y and Z(YY)Z 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 behavior by decreasing from 53 to 46 cm−1 at 2 GPa. The softening of these modes is related to the orientation change of tetrahedra observed by ab initio calculations when the volume of the cell is decreased. These orientation changes can explain the wavenumber decrease of the MoO stretching modes above 2 GPa, which indicates an increase of Mo coordination. Copyright © 2010 John Wiley & Sons, Ltd.