Thermal properties of a ${\mathrm{Bi}}_2$${\mathrm{O}}_3$:${\mathrm{Y}}_2$${\mathrm{O}}_3$ oxygen conductor at low temperatures

Specific-heat and thermal-conductivity measurements (1.7-40 K) are reported for the first time on a ${\mathrm{Bi}}_2$ ${\mathrm{O}}_3$:${\mathrm{Y}}_2$ ${\mathrm{O}}_3$ oxygen conductor having a stabilized, defect-fluorite structure. Glasslike behavior is found in the properties measured, and the magnitudes of the coefficients involved are very similar to those found in amorphous materials. The thermal conductivity has a plateau region ∼20-30 K and a $T^2$ temperature dependence below ∼3 K. The specific heat has a maximum in $\frac{C}{T^3}$ at 12 K describable by a single Einstein term ($\omega =33.1\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$) and a linear tunneling term resolvable below 3.5 K. The fitted Debye temperature is 248.9 K (±0.4%). This oxygen conductor has a large concentration of oxygen vacancies at elevated temperatures, but the Einstein term indicates that the majority of these vacancies is ordered at low temperatures. Using the vacancy concentration from the Einstein term, we find excellent agreement with the theory of McWhan et al. for the coefficient of the tunneling term. Comparisons are made with the low-temperature properties of stabilized zirconias and the alkali-metal $\beta$-aluminas.