Cations size mismatch versus bonding characteristics: synthesis, structure and oxygen ion conducting properties of pyrochlore-type lanthanide hafnates

This work describes the synthesis, structural characterization and electrical properties of solid solutions with the general formula Gd2Hf2-x B (x) O-7, where B = Ti4+, Sn4+ and Zr4+. All samples were successfully prepared in similar to 30 h, via a mechanochemical reaction in a planetary ball mill, using the corresponding elemental oxides as starting chemicals. The XRD and Raman spectroscopy analysis of the title samples revealed that on firing at 1500 degrees C Hf4+ substitution by Sn4+ and Ti4+ produces better ordered pyrochlore structures and decreases the electrical conductivity of Gd2Hf2O7 by more than two orders of magnitude (from 2.7 x 10(-4) at 700 degrees C to 8.71 x 10(-7) and 1.12 x 10(-6) Sm cm(-1), for Gd2Sn2O7 and Gd2Ti2O7, respectively). By contrast, the Gd2Hf2-x Zr (x) O-7 system remains disordered with conductivity increasing by almost an order of magnitude and reaching a value for Gd2Zr2O7 of 1.55 x 10(-3) Sm cm(-1) at 700 degrees C, whereas the activation energy for oxygen migration decreases in both, the Sn- and Ti-containing systems, and increases slightly in the Zr-containing solid solution. These changes cannot be only explained when taking into account the cations size ratio criteria; the covalency of the metal bond plays also a key role in determining the structural characteristics and electrical properties of the title three systems.