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 La-9.71(Si5.81Mg0.18)O-26.37 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 P6(3)/m, P6(3) and P (3) over bar the former is correct for La-9.71(Si5.81Mg0.18)O-26.37. 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. (C) 2009 Elsevier Inc. All rights reserved.