Potentiometric determination of the gibbs energies of formation of SrZrO3 and BaZrO3 |
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Authors: | K T Jacob Y Waseda |
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Affiliation: | (1) Department of Metallurgy, Indian Institute of Science, 560012 Bangalore, India;(2) Institute for Advanced Materials Processing, Tohoku University, 980 Sendai, Japan |
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Abstract: | The Gibbs free energies of formation of strontium and barium zirconates have been determined in the temperature range 960
to 1210 K using electrochemical cells incorporating the respective alkaline-earth fluoride single crystals as solid electrolytes.
Pure strontium and barium monoxides were used in the reference electrodes. During measurements on barium zirconate, the oxygen
partial pressure in the gas phase over the electrodes was maintained at a low value of 18.7 Pa to minimize the solubility
of barium peroxide in the monoxide phase. Strontium zirconate was found to undergo a phase transition from orthorhombic perovskite
(o) with space groupCmcm; D
2h
17
to tetragonal perovskite (t) having the space group 14/mcm;D
4h
18
at 1123 (/+- 10) K. Barium zirconate does not appear to undergo a phase transition in the temperature range of measurement.
It has the cubic perovskite (c) structure. The standard free energies of formation of the zirconates from their component
binary oxidesAO (A = Sr, Ba) with rock salt (rs) and ZrO2 with monoclinic (m) structures can be expressed by the following relations: SrO (rs) + ZrO2 (m) → SrZrO3 (o) ΔG° = -74,880 - 14.2T (/+-200) J mol-1 SrO (rs) + ZrO2 (m) → SrZrO3 (t) ΔG° = -73,645 - 15.37T (/+-200) J mol-1 BaO (rs) + ZrO2 (m) → BaZrO4 (c) ΔG° = -127,760-1.79T (/+-250) J mol-1 The results of this study are in reasonable agreement with calorimetric measurements reported in the literature. Systematic
trends in the stability of alkaline-earth zirconates having the stoichiometry AZrO3 are discussed. |
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