The high temperature drift mobility (ad) of charge carriers in nonstoichiometric cerium dioxide (CeOz_,X) has been calculated by combining the electrical conductivity and nonstoichiomet~ data on the basis of the oxygen vacancy model with correct ionization state. The electrical conductivity was measured by a four-probe d.c. technique and the nonstoichiometry by thermogravimetric analysis. The dilute solution model of the point defects is valid up to x = 0.03. From the magnitude of pd and its temperature dependence, the charge carriers in CeO,_, are proposed to be small-poiarons formed by localization of electrons at cerium sites and the charge transport process is proposed to occur by a hopping mechanism. The observed temperature dependence of s,+ is in accord with that derived by Holstein and Friedman for small-poiaron transport by the hopping mechanism. The activation energy of mobility is found to increase with increasing x as expected for the hopping model.

In the ZrO2–CaO system cubic solid solutions of the fluorite-type structure are formed between 12 and 22 mole % CaO for specimens fired at 2000°C and subsequently quenched from 1400°C. The lattice parameter increased linearly from 5.125 to 5.144 Å with increasing CaO content. Results of x-ray diffraction intensity studies favored the oxygen-ion vacancy model over the cation interstitial model for these solid solutions. As the CaO content was increased from 13 to 20 mole %, the electrical conductivity at 1000°C decreased from 5×10—2 to 6×10—3 Ω—1 cm—1 and the activation energy increased from 1.11 to 1.35 eV. Specimens heated at 1000°C for one week showed extra x-ray diffraction lines. These data coupled with accompanying changes in electrical conductivity are discussed in terms of an order—disorder transition.

Powders of silicon nitride, aluminum nitride, and rare-earth oxides were weighed and mixed in an agate jar under absolute ethyl alcohol on a rotating mill for 2 h. Dried mixtures were hotmessed at 1700°C