The effect of non-stoichiometry on the crystal structure and total conductivity of Ln(2+x)Zr(2-x)O(7-x/2) (Ln = Sm-Gd; x = 0-0.286) was investigated. The intensity of the pyrochlore superlattice reflections from Ln(2+x)Zr(2-x)O(7-x/2) is shown to decrease with increasing Ln concentration. Within the homogeneity range of the pyrochlore phase of Ln(2+x)Zr(2-x)O(7-x/2) (Ln = Sm-Gd), the activation energy of high -temperature conduction in samples prepared by 1600 degrees C sintering of mechanically activated oxide mixtures (Ln(2)O(3) and ZrO2) is similar to 0.87-1.04 eV. The highest conductivity among the Ln(2+x)Zr(2-x)O(7-x/2) (Ln = Sm-Gd) materials is offered by stoichiornetric Ln(2)Zr(2)O(7) samples with a pyrochlore structure, which contain 5-8.1% Ln(Zr)+ Zr-Ln anti-structure pairs, except for Gd2Zr2O7 (similar to 22%). The crystal structure of Ln(2+x)Hf(2-x)O(7-x/2) (Ln = Sm-Gd) is investigated after sintering at 1000-1670 degrees C. The compounds Ln(2.096)Hf(1.904)O(6.9) (Ln=Eu, Gd) prepared by 1200 degrees C sintering of mechanically activated oxides (Ln(2)O(3) and HfO2) undergo a fluorite-type to pyrochlore phase transition above 1200 degrees C. The conductivity of Gd2Hf2O7 and Sm2.096Hf1.904O6.952 sintered at 1600 degrees C seems to be ionic above 780 degrees C, with an activation energy of 0.77 and 0.82 eV, respectively. In this work, using mechanical activation of starting mixtures, the conductivity of the Ln(2+x)Hf(2-x)O(7-x/2) (Ln = Sm-Gd) haftiates was raised close to the level of Ln(2+x)Zr(2-x)O(7-x/2) (Ln = Sm-Gd). The haftiates synthesized by the procedure in question are similar in structural disorder to Ln(2+x)Zr(2-x)O(7-x/2) (Ln = Sm-Gd), and the disorder ensures high oxygen ion mobility and, accordingly, significant high-temperature conductivity. (c) 2006 Elsevier B.V All rights reserved.