Structure and properties of rare earth silicates with the apatite structure at high pressure

The pressure-induced structural transformation of rare earth, non-stoichiometric silicates, (REE_9.33(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 La_9.33(SiO₄)₆O₂ to ~25 GPa in Gd_9.33(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.