Phase relations in the system Sr-Cr-O and thermodynamic properties of SrCrO₄ and Sr₃Cr₂O₈

Abstract

The phase relations involving various oxides in the system Sr-Cr-O at 1250 K have been determined by equilibrating samples representing 17 compositions inside the ternary and subsequent phase identification in quenched samples. Optical and scanning electron microscopes were used in conjunction with X-ray diffraction (XRD) to identify phases; the composition of each phase was verified using energy dispersive analysis of X-rays (EDX). Four stable ternary oxides, SrCrO₄, Sr₃Cr₂O₈, Sr₂CrO₄, and SrCr₂O₄, were identified. All the compounds except Sr₃Cr₂O₈ were found stoichiometric within the accuracy of EDX. X-ray absorption near-edge structure (XANES) analysis showed that the average valence of Cr in Sr₃Cr₂O₈ is +5.2, suggesting the presence of mixed valence states in this compound. Only one of the ternary oxides, SrCr₂O₄, was found to coexist with Cr metal. The compounds SrCrO₄ and Sr₃Cr₂O₈ were stable in pure oxygen. The standard Gibbs energy of formation of SrCrO₄ was determined as a function of temperature in the range from 950 to 1280 K using a solidstate cell incorporating single-crystal SrF₂ as the electrolyte. The oxygen potential corresponding to the three-phase equilibria involving SrCrO₄, Sr₃Cr₂O₈, and Cr₂O₃ was measured from 1080 to 1380 K using a cell with yttria-stabilized zirconia as the solid electrolyte and pure oxygen gas at a pressure of 0.1 MPa as the reference electrode. The standard Gibbs energy of formation of Sr₃Cr₂O₈ was derived from the oxygen potential. The results can be represented by the following equations: SrO+½Cr₂O₃+¾O₂→SrCrO₄; ΔG°/Jmol^-1=273,825+131.57T/K(±200); 3SrO+Cr₂O₃+O₂→Sr₃Cr₂O₈; ΔG°/Jmol^-1=-472,345+184.20T/K(±635).