Crystal structure and oxygen content of the double perovskites GdBaCo2−xFexO6−δ

The iron solubility limit, x, in GdBaCo2−xFexO6−δ determined by means of X-ray diffraction was found to be close to 0.65 in air. The crystal structure changes of the double perovskites GdBaCo2-xFexO6−δ (x=0–0.6) were studied by means of “in situ” X-ray diffraction in temperature range from 25 to 900 °C in air. The oxygen content, 6−δ, was determined for these double perovskites in air as a function of temperature by means of thermogravimetric technique in range 25≤T, °C≤ 1100. The Pmmm-P4/mmm structure transition was found to occur in GdBaCo2-xFexO6-δ (0≤x≤0.4) with increasing temperature. This transition is observed at the same temperature for the compositions with 0≤x≤0.1 while the transition temperature reaches maximum for x=0.2 and that decreases linearly with further iron increase. The double perovskite GdBaCo1.4Fe0.6O6−δ was shown to have the tetragonal P4/mmm structure at room temperature. The P4/mmm-Pmmm structure transition occurs at temperature as low as 170 °C for this double perovskite while reverse one is already observed at 290 °C in air. The Pmmm-P4/mmm structure transition was found to be strongly related to the oxygen content for the undoped and slightly doped (x≤0.2) double perovskites while there is no such relation for the double perovskites enriched by iron (x≥0.2).