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:

$$\begin{gathered} SrO + {\raise0.5ex\hbox{$\scriptstyle 1$}\kern-0.1em/\kern-0.15em\lower0.25ex\hbox{$\scriptstyle 2$}}Cr_2 O_3 + {\raise0.5ex\hbox{$\scriptstyle 3$}\kern-0.1em/\kern-0.15em\lower0.25ex\hbox{$\scriptstyle 4$}}O_2 \to SrCrO_4 \hfill \\ \Delta G^ \circ /J mol^{ - 1} = 273,825 + 131.57 T/K( \pm 200) \hfill \\ 3SrO + Cr_2 O_3 + O_2 \to Sr_3 Cr_2 O_8 \hfill \\ \Delta G^ \circ /J mol^{ - 1} = - 472,345 + 184.20 T/K( \pm 635) \hfill \\ \end{gathered} $$