Abstract
The pressure-induced structural transformation of rare earth, non-stoichiometric silicates, (REE9.33(SiO4)6O2, RE = La, Ce, Nd, Eu, and Gd) with the apatite structure type, were investigated by X-ray diffraction, photoluminescence, far-infrared spectroscopy, and DFT calculations. A pressure-induced degradation of symmetry from P6 3 /m to P6 3 occurs with increasing pressure. The transition is due to the tilting of SiO4 tetrahedra and reduced symmetry constraints on one of the O atoms in the tetrahedron. The critical transition pressure increased from ~13 GPa in La9.33(SiO4)6O2 to ~25 GPa in Gd9.33(SiO4)6O2 with the decrease in lanthanide cation size. The high-pressure phase shows an unexpectedly low value for the bulk modulus over a narrow pressure range (below ~30 GPa), as compared with the low-pressure phase, especially for the structure with larger rare earth elements. High-pressure studies of alkaline earth-doped samples (Nd8 A 2(SiO4)6O2 where A = Ca, Sr) showed that the pressure for the phase transition is mainly related to the size of lanthanides that occupy the large channels along the c axis of the apatite structure type.
Published Version
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