Effect of non-stoichiometry and synthesis temperature on the structure and conductivity of Ln2+xM2−xO7−x/2 (Ln=Sm–Gd; M=Zr, Hf; x=0–0.286)

The effect of non-stoichiometry on the crystal structure and total conductivity of Ln2+xZr2-xO7-x/2 (Ln=Sm-Gd; x=0-0.286) was investigated. The intensity of the pyrochlore superlattice reflections from Ln2+xZr2-xO7-x/2 is shown to decrease with increasing Ln concentration. Within the homogeneity range of the pyrochlore phase of Ln2+xZr2-xO7-x/2 (Ln=Sm-Gd), the activation energy of high-temperature conduction in samples prepared by 1600 deg C sintering of mechanically activated oxide mixtures (Ln2O3 and ZrO2) is 0.87-1.04eV. The highest conductivity among the Ln2+xZr2-xO7-x/2 (Ln=Sm-Gd) materials is offered by stoichiometric Ln2Zr2O7 samples with a pyrochlore structure, which contain 5-8.1% LnZr+ZrLn anti-structure pairs, except for Gd2Zr2O7 (22%). The crystal structure of Ln2+xHf2-xO7-x/2 (Ln=Sm-Gd) is investigated after sintering at 1000-1670 deg C. The compounds Ln2.096Hf1.904O6.9 (Ln=Eu, Gd) prepared by 1200 deg C sintering of mechanically activated oxides (Ln2O3 and HfO2) undergo a fluorite-type to pyrochlore phase transition above 1200 deg C. The conductivity of Gd2Hf2O7 and Sm2.096Hf1.904O6.952 sintered at 1600 deg C seems to be ionic above 780 deg C, with an activation energy of 0.77 and 0.82eV, respectively. In this work, using mechanical activation of starting mixtures, the conductivity of the Ln2+xHf2-xO7-x/2 (Ln=Sm-Gd) hafnates was raised close to the level of Ln2+xZr2-xO7-x/2 (Ln=Sm-Gd). The hafnates synthesized by the procedure in question are similar in structural disorder to Ln2+xZr2-xO7-x/2 (Ln=Sm-Gd), and the disorder ensures high oxygen ion mobility and, accordingly, significant high-temperature conductivity.