Effect of cation doping on ionic conductivity and crystal structure of oxyapatite-type lanthanum silicates

• Change in structure by Si-site substitution for lanthanum silicate was studied.
• Ionic movement was promoted with slight Si-site substitution with Al or Mg.
• Activated ion path perpendicular to c axis by doping may enhance ion conduction.

Crystal structures of oxyapatite-type lanthanum silicate (LSO), La9.667Si6O26.5, and Al- and Mg-doped LSOs; La9.667(Si5.8Al0.2)O26.4 and La9.667(Si5.5Mg0.5)O26.0, were refined using synchrotron powder X-ray diffraction (SXRD) data to investigate the origin of enhanced oxide ion (O2 −) conducting behavior with Si-site doping. It was found that temperature factors of oxygen atoms largely increased with slight Al- or Mg-doping into the Si-site of LSO. Especially, that for the 2a-site oxygen located on the c axis (O4) was enhanced by doping. In addition, that for the 12i-site oxygen, O3, the constituent of an SiO4 tetrahedron in the LSO lattice increased with doping anisotropically toward O4. It was suggested that the ionic movement perpendicular to the c axis via SiO4 tetrahedron was activated by cation (M) doping, and that contributed to the ion conduction in addition to the fast ion channel along c axis.