Defect Chemistry of Yttrium-Doped Barium Zirconate: A Thermodynamic Analysis of Water Uptake

TLDR

Yttrium‑doped BaZrO₃ shows temperature‑dependent water uptake, with high‑temperature nonlinearity attributed to electronic defects and differing from earlier studies that used higher temperatures and assumed negligible hole concentration. Thermogravimetric measurements were performed on 20–40 % Y‑doped BaZrO₃ from 50–1000 °C under 0.023 atm H₂O to determine equilibrium constants for water incorporation. The equilibrium constants are Arrhenius‑linear only below 500 °C, with hydration enthalpies of –22 to –26 kJ mol⁻¹ and entropies around –40 J K⁻¹ mol⁻¹, both smaller than prior reports, indicating significant entropy gain in the oxide; the constants are largely independent of yttrium content.

Abstract

Thermogravimetry has been used to evaluate the equilibrium constants of the water incorporation reaction in yttrium-doped BaZrO3 with 20−40% yttrium in the temperature range 50−1000 °C under a water partial pressure of 0.023 atm. The constants, calculated under the assumption of a negligible hole concentration, were found to be linear in the Arrhenius representation only at low temperatures (≤500 °C). Nonlinearity at high temperatures is attributed to the occurrence of electronic defects. The hydration enthalpies determined here range from −22 to −26 kJ mol−1 and are substantially smaller in magnitude than those reported previously. The difference is a direct result of the different temperature ranges employed, where previous studies have utilized higher temperature thermogravimetric measurements, despite the inapplicability of the assumption of a negligible hole concentration. The hydration entropies measured in this work, around −40 J K−1 mol−1, are similarly smaller in magnitude than those previously reported and are considerably smaller than what would be expected from the complete loss of entropy of vapor-phase H2O upon dissolution. This result suggests that substantial entropy is introduced into the oxide as a consequence of the hydration. The hydration reaction constants are largely independent of yttrium concentration, in agreement with earlier reports.