Pressure-induced structural evolution of apatite-type La_9.33Si₆O₂₆

The pressure-induced structural evolution of apatite-type La_9.33Si₆O₂₆ was systematically studied using in situ synchrotron x-ray diffraction (XRD). The XRD spectra indicated that a subtly reversible phase transition from P6₃/m to P6₃ symmetry occurred at~13.6 GPa because of the tilting of the SiO₄ tetrahedra under compression. Furthermore, the La_9.33Si₆O₂₆ exhibited a higher axial compression ratio for the a-axis than the c-axis, owing to the different axial arrangement of the SiO₄ tetrahedra. Interestingly, the high-pressure phase showed compressibility unusually higher than that of the initial phase, suggesting that the low P6₃ symmetry provided more degrees of freedom. Moreover, the La_9.33Si₆O₂₆ exhibited a lower phase transition pressure (P_T) and a higher lattice compression than La₁₀Si₆O₂₇. Comparisons between La_9.33Si₆O₂₆ and La₁₀Si₆O₂₇ provided a deeper understanding of the effect of interstitial oxygen atoms on the structural evolution of apatite-type lanthanum silicates (ATLSs).