Preparation, crystal structure and properties of HoBaCo2−xFexO5+δ

A series of samples of overall composition HoBaCo2−xFexO5+δ with 0.0 ≤ x ≤ 0.8 were prepared by glycerin nitrate technique in air and in pure oxygen. Irrespective of atmosphere used the homogeneity range of single phase iron substituted holmium barium cobaltate was proved to exist within the range 0.0 ≤ x ≤ 0.4. All samples prepared in air and samples within the compositional range 0.2 ≤ x ≤ 0.4 obtained in oxygen possess tetragonal ap × ap × 2ap (sp. gr. Р4/mmm) structure. HoBaCo2O5+δ and HoBaCo1.9Fe0.1O5+δ synthesized in oxygen crystallized in orthorhombic ap × 2ap × 2ap (sp. gr. Рmmm) structure. The structural parameters were refined by the Rietveld analysis. It was shown that parameter a remains practically constant while parameter c gradually increases with the increase of iron content. The changes of oxygen content in HoBaCo2−xFexO5+δ (x = 0, 0.2, 0.4) were measured by TGA within the temperature range 25–1100 °C in air. The absolute value of oxygen content was determined by the reduction of the samples in hydrogen flow. The influence of oxygen and iron content on the crystal structure has been discussed. The chemical stability of HoBaCo2−xFexO5+δ in contact with the solid electrolyte materials Ce0.8Sm0.2O2 and Zr0.85Y0.15O2 was examined.