Stability and oxide ion conductivity in rare-earth aluminium cuspidines

RE4(Al2−xGexO7+x/2□1−x/2)O2 (RE=Gd3+ and Nd3+) oxy-cuspidine series have been prepared by ceramic method (RE=Gd3+) and freeze-dried precursor method (RE=Nd3+). The compositional ranges and the high temperature stability have been determined for both series. Gadolinium aluminium cuspidines are stable at very high temperatures but the analogous neodymium compounds are only stable below 1273 K. Joint Rietveld analyses of neutron powder diffraction (NPD) and laboratory X-ray powder diffraction (LXRPD) have been carried out for Nd4(Al2O7□1)O2 and Nd4(Al1.5Ge0.5O7.25□0.75)O2 compositions. Furthermore, Rietveld refinement of synchrotron X-ray powder diffraction (SXRPD) data were carried out for Gd4(Al1.0Ge1.0O7.5□0.5)O2 composition. The refinements have confirmed the known structural features of the cuspidine framework. These cuspidines series are oxide ion conductors with negligible electronic contribution as determined from impedance spectroscopy at variable oxygen partial pressures. The enhancement in the overall oxide conductivity along the two oxy-cuspidine series is two orders of magnitude. Typical ionic conductivity values for doped samples are around 4×10−5 S cm−1 at 973 K.