New solid electrolytes of the pyrochlore family

The class of oxygen-ion-conducting rare-earth pyrochlores has been considerably extended. New solid electrolytes, Ln2Ti2O7 (Ln = Dy-Lu) and Ln2Hf2O7 (Ln = Eu, Gd) pyrochlores, are intrinsic ionic conductors at elevated temperatures, as are the well known Ln2Zr2O7 (Ln = Sm-Gd) zirconates, which suggests that oxygen ion conduction in the rare-earth pyrochlore family has a general character. The thermodynamic order-disorder transitions that yield a PII cation- and anion-disordered pyrochlore phase possessing high oxygen ion conductivity occur throughout the rare-earth pyrochlore family: Ln2M2O7 (Ln = Sm-Lu; M = Ti, Zr, Hf). The composition-structure-oxygen-ionic conductivity relationship is analyzed for Ln2(M2 − x Ln x )O7 − δ (Ln = Sm-Lu; M = Ti, Zr, Hf) with x from 0 to 0.81.