Crystal structure and high-temperature thermodynamic properties of Pr-doped barium zirconates, BaZr1–xPrxO3 (x = 0.1, 0.5)

Abstract

The crystal structure of BaZr1–xPrxO3 (x = 0.1, 0.5) perovskites was studied by means of X-ray powder diffraction (XRD). It was found that, at room temperature, BaZr0.9Pr0.1O3 possesses cubic (Pm3‾m) crystal structure, whereas BaZr0.5Pr0.5O3 is rhombohedral (R3‾c). High-temperature XRD study revealed that rhombohedral distortions in the BaZr0.5Pr0.5O3 lattice gradually decrease upon heating. As a result, BaZr0.5Pr0.5O3 becomes ideally cubic (Pm3‾m) at temperatures higher than 723 K. The enthalpy increments, Δ298.15TH○, of the perovskites BaZr1–xPrxO3 (x = 0.1, 0.5) were measured by high-temperature drop calorimetry in the temperature range (373–1273) K in air. The Δ298.15TH○ data for BaZr1–xPrxO3 (x = 0.1, 0.5) were approximated using one Einstein term with, for BaZr0.5Pr0.5O3, an addition of the exponential terms describing the influence of the R3‾c→Pm3‾m transition. The high- and low-temperature thermodynamic properties of BaZrO3 reported in the literature were reevaluated and its heat capacity, Cp(T), was approximated with one Einstein term as well. Thus, analytical Cp(T) dependences are reported for BaZr1–xPrxO3 (x = 0, 0.1, 0.5). While the Cp(T) of BaZrO3 and BaZr0.9Pr0.1O3 are very close to each other, the values of Cp(T) of BaZr0.5Pr0.5O3 are higher, mostly because of the phase transition and the differences in the crystal structure. Using the thermodynamic data obtained in this work along with those reported earlier, it was shown that addition of praseodymium leads to decreasing relative stability of BaZr1–xPrxO3 towards interaction with CO2 and gaseous H2O.