Phase equilibria, structure, oxygen nonstoichiometry, and thermal expansion of oxides in the 1/2Y₂O₃–SrO–1/2Fe₂O₃ system

Abstract

Phase equilibria in the 1/2Y 2 O 3 -SrO-1/2Fe 2 O 3 system were systematically studied at 1373 K in air. The homogeneity ranges and crystal structure of the solid solutions Sr 1−x Y x FeO 3−δ with 0.875 ≤ x ≤ 1 (sp. gr. Pnma) and 0.05 ≤ x ≤ 0.25 (sp. gr. Pm3 m) and Sr 3−z Y z Fe 2 O 7−δ with 0 ≤ z ≤ 0.25 (sp. gr. I4/mmm) were determined by X-ray diffraction analysis of quenched samples. The structural parameters of the single-phase oxides were refined by the Rietveld profile method. The isothermal-isobaric phase diagram for the 1/2Y 2 O 3 -SrO-1/2Fe 2 O 3 system at 1373 K in air is presented. The changes of oxygen content in the solid solutions Sr 1−x Y x FeO 3−δ (0.05 ≤ x ≤ 0.25) and Sr 3−z Y z Fe 2 O 7−δ with 0 ≤ z ≤ 0.25 vs temperature in air were determined by the thermogravimetric analysis. Oxygen content was also calculated from the iodometric titration results. The gradual substitution of strontium by yttrium in Sr 1−x Y x FeO 3−δ (0.05 ≤ x ≤ 0.25) leads to a decrease in the oxygen content and the mean oxidation state of iron. The average thermal expansion coefficients for Sr 1−x Y x FeO 3−δ (0.875 ≤ x ≤ 1 and 0.05 ≤ x ≤ 0.25) and Sr 3−z Y z Fe 2 O 7−δ (0 ≤ z ≤ 0.25) were calculated within the temperature range 298-1373 K in air. K E Y W O R D S crystal structure, oxygen content, phase diagram, solid solution, thermal expansion, total electric conductivity, X-ray diffraction

Conductivity and stability of ceramic Sr1-xYxFeO3-δ solid solutions

“…XRD results confirmed the formation of single-phase Sr 1-x Y x FeO 3-δ (x = 0.1, 0.2) with cubic structure (SG Pm3m) in accordance with the results reported earlier [13]. The XRD pattern for single-phase Sr 0.8 Y 0.2 FeO 3-δ refined by the Rietveld analysis is shown in Fig.…”

“…In contrast to the brownmillerite phase, the latter as a rule exhibits enhanced mobility of oxygen and higher conductivity values. This was shown more recently in a detailed study of phase equilibria in the ½Y 2 O 3 -SrO -½Fe 2 O 3 system where the introduction of even a small amount of yttrium into strontium ferrite was found to stabilize the cubic perovskite structure of Sr 1-x Y x FeO 3-δ solid solutions (0.05≤x≤0.25) [13].…”

“…The maximum in the value of the conductivity at 600 K (Fig. 2) correlates well with the beginning of oxygen release from the crystal lattice of Sr 1-x Y x FeO 3-δ (x = 0.1, 0.2) [13]. The increasing concentration of oxygen vacancies causes a decrease in the concentration of holes, which serve as the majority of mobile charge carriers [17][18][19][20][21][22].…”

“…The conductivity value σ = 9.3 S•cm -1 at 373 K for Sr 0.8 Y 0.2 FeO 3-δ (the lowest temperature in our study) practically coincides with that reported in [22] (for the highest in their temperature range). Since the oxygen content in Sr 1-x Y x FeO 3-δ (x = 0.1, 0.2) remains constant while heating from the room temperature up to 600 K [13], it can be inferred that the concentration of charge carriers has not changed either. Thus, the increase of the low-temperature conductivity (up to 600 K) for both compounds appears due to the increase in charge-carrier mobility, since it is thermally activated (see inset in Fig.…”

Phase equilibria and stability of intermediate phases in the Sm2O3–BaO–Fe2O3 system

“…It means that these phases have not lain in a single plane inside the tetrahedron (Figure ). In order to demonstrate phase equilibria of such system in a plane view, a projection of the diagram to a base triangle corresponding to a metallic components’ composition is often used 22,60‐61 . In this type of presentation, the oxygen content in the condensed phases could not be calculated.…”