Pressure-induced structural evolution of apatite-type La _9.33 Si ₆ O ₂₆

Abstract

The pressure-induced structural evolution of apatite-type La 9.33 Si 6 O 26 was systematically studied using in situ synchrotron x-ray diffraction (XRD). The XRD spectra indicated that a subtly reversible phase transition from P6 3 /m to P6 3 symmetry occurred at ∼ 13.6 GPa because of the tilting of the SiO 4 tetrahedra under compression. Furthermore, the La 9.33 Si 6 O 26 exhibited a higher axial compression ratio for the a-axis than the c-axis, owing to the different axial arrangement of the SiO 4 tetrahedra. Interestingly, the high-pressure phase showed compressibility unusually higher than that of the initial phase, suggesting that the low P6 3 symmetry provided more degrees of freedom. Moreover, the La 9.33 Si 6 O 26 exhibited a lower phase transition pressure (P T ) and a higher lattice compression than La 10 Si 6 O 27 . Comparisons between La 9.33 Si 6 O 26 and La 10 Si 6 O 27 provided a deeper understanding of the effect of interstitial oxygen atoms on the structural evolution of apatite-type lanthanum silicates (ATLSs).