In this study, we determined the crystallographic nature and electrical transport properties of Nd-9.20(SiO4)(6)O-1.8 and (La0.46Nd0.54)(9.33)(SiO4)(6)O-2, which are defect-containing oxyapatites. The intensity data measured by synchrotron powder X-ray diffraction were analyzed by a Rietveld method. From the total conductivity data, the oxygen partial pressure region where the oxygen ionic conductivity (sigma(O2-)) predominates was determined to narrow down owing to the substitution of neodymium ions. A comparison of various solid solutions under similar temperature conditions ranging from 873 K to 1273 K showed that the sigma(O2-) values were lowest for (La0.46Nd0.54)(9.33)(SiO4)(6)O-2 samples. The activation energy of the oxygen ionic conductivity increased with an increasing neodymium content. (C) 2012 Elsevier B.V. All rights reserved.