Structural investigation of La9.33Si6O26- and La9AESi6O26+δ-doped apatites-type lanthanum silicate (AE=Ba, Sr and Ca) by neutron powder diffraction

Abstract Crystalline structures of La 9.33 Si 6 O 26 and La 9 AE Si 6 O 26+δ ( AE =Ba, Sr and Ca) doped apatites-type lanthanum silicates were investigated by X-ray and neutron powder diffraction at room temperature. The results obtained after different models testing show that the apatite structure is best described using the P 6 3 space group. The loss of the mirror symmetry perpendicular to the ionic conduction channel direction results from heterogeneous La 3+ / AE 2+ distribution of the sites so-called “4 f ”. The Rietveld refinements do not show splitting of the conduction oxygen ( 0 , 0 , 1 4 ) site but rather a very large spread of the nuclear density associated to this site. This effect is more pronounced for the La 9 AESi 6 O 26+δ -doped compounds. Large anisotropic thermal displacement parameters are also observed for the oxygens associated to the isolated [SiO 4 ], suggesting a rotation of this tetrahedron around the Si site. Lastly, vacancies were also systematically observed in the lanthanum nine-coordinated sites.