Structure and electrical conductivity of Ln2+xHf2-xO7-x/2 (Ln = Sm-Tb; x=0, 0.096)

Data are presented on the evolution of the pyrochlore structure in the Ln(2+x)Hf(2-x)O(7-delta) (Ln = Sm, Eu: x = 0.096) solid solutions and Ln(2)Hf(2)O(7) (Ln = Gd, Tb) compounds prepared from mechanically activated oxide mixtures. Sm2.096Hf1.904O6.952 is shown to undergo pyrochlore-disordered pyrochlore-pyrochlore (P-PI-P) phase transformations in the temperature range 1200-1670 degrees C. The former transformation leads to a rise in 840 degrees C conductivity from 10(-4) to 3 x 10(-3) S/cm in the samples synthesized at 1600 degrees C, and the latter leads to a drop in 840 degrees C conductivity to 6 x 10(-4) S/cm in the samples synthesized at 1670 degrees C. The reduction in the conductivity of Sm2.096Hf1.904O6.952 is accompanied by the disappearance of the assumed superstructure. In the range 1300-1670 degrees C, Eu2+xHf2-xO7-delta (x = 0.096) and Ln(2)Hf(2)O(7) (Ln = Gd, Tb) have a disordered pyrochlore structure. The highest 840 degrees C conductivity is offered by Eu2.096Hf1.904O6.952, Gd2Hf2O7, and Tb2Hf2O7 synthesized at 1670 degrees C: 7.5 x 10(-3), 5 x 10(-3), and 2.5 x 10(-2) S/cm, respectively.