Solid solutions of NdxCe1-xO2-delta(0.05 <= x <= 055) are prepared via solid-state reactions and the grain ionic conductivity is measured using a two-probe ac impedance spectroscopy technique. The lattice parameters increased linearly with Nd content for 0.05 <= x <= 0.45. Both XRD and neutron diffraction analyses indicate at x = 0.50 the highly defective fluorite stabilizes into a C-type rare earth oxide structure with an associated doubling of the unit cell. The conductivity of the compounds follows an Arrhenius-type behavior showing a maximum conductivity of 0.054(1)S cm(-1) for Nd0.15Ce0.85O1.925 at 700 degrees C with an associated activation energy of 0.726(5) eV. The activation energy increases significantly during the phase transition from 35% to 40% Nd due to the stabilization of oxygen vacancies. The pre-exponential factor increased as well, resulting in an effective gradual decrease of conductivity across the phase transition. (C) 2012 Elsevier B.V. All rights reserved.