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 °C sintering of mechanically activated oxide mixtures (Ln2O3 and ZrO2) is ∼ 0.87–1.04 eV. 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 °C. The compounds Ln2.096Hf1.904O6.9 (Ln = Eu, Gd) prepared by 1200 °C sintering of mechanically activated oxides (Ln2O3 and HfO2) undergo a fluorite-type to pyrochlore phase transition above 1200 °C. The conductivity of Gd2Hf2O7 and Sm2.096Hf1.904O6.952 sintered at 1600 °C seems to be ionic above 780 °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 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.