Crystal structure and electron density in the apatite-type ionic conductor La9.71(Si5.81Mg0.18)O26.37

Crystal structure and electron density in the apatite-type ionic conductor La9.71(Si5.81Mg0.18)O26.37 have been investigated at 302, 674 and 1010 K by Rietveld refinement and a whole-pattern fitting approach based on the maximum-entropy method (MEM) using synchrotron X-ray powder diffraction data. Second harmonic generation measurements indicated that the space group of this material is centrosymmetric. Among the possible hexagonal groups P63/m, P63 and P3¯ the former is correct for La9.71(Si5.81Mg0.18)O26.37. Rietveld refinements suggested an oxygen interstitial site (0.03,0.15,0.85) near the hexagonal axis. MEM analyses revealed that the Si0.97Mg0.03 atom has covalent bonds with four adjacent oxygen atoms to form a tetrahedron. The oxygen O4 atom located at the 2a site (0.0,0.0,1/4) exhibited large atomic displacement parameters along the c axis and electron density mapping also indicated the wide distribution consistent with migration of oxygen ions in this direction.

Graphical abstractEquielectron density surfaces at 1.0 Å−3 of apatite-type La9.71(Si5.81Mg0.18)O26.37 at 302 K. O4 atoms have large distribution along the 〈001〉 direction (white arrows).Figure optionsDownload full-size imageDownload as PowerPoint slide