Layered (Cu,Fe) oxides of double perovskite structure: Correlation between structural and magnetic-property changes in ${\mathrm{B}\mathrm{a}\mathrm{Y}(\mathrm{C}\mathrm{u}}_{0.5}{\mathrm{Fe}}_{0.5}{)}_2{\mathrm{O}}_{5+\mathrm{\delta }}$ upon high-pressure heat treatment

In this paper, we report the correlation between the magnetic property and the local structure in ${\mathrm{B}\mathrm{a}\mathrm{Y}(\mathrm{C}\mathrm{u}}_{0.5}{\mathrm{Fe}}_{0.5}{)}_2{\mathrm{O}}_{5+\mathrm{\delta }}$ of the “0112” phase containing an unusually large amount of excess oxygen, δ of $\sim 0.2,$ when heat treated under a high pressure of 5 GPa. Magnetic susceptibility measurements showed that upon the high-pressure heat treatment, the lower-temperature magnetic transition which had been observed in normal-pressure synthesized, antiferromagnetic ${\mathrm{B}\mathrm{a}\mathrm{Y}(\mathrm{C}\mathrm{u}}_{0.5}{\mathrm{Fe}}_{0.5}{)}_2{\mathrm{O}}_5$ disappeared and the higher-temperature transition, i.e., the Néel temperature, was lowered. According to a Rietveld analysis based on x-ray-diffraction data and space group $P4/mmm,$ the distinction between Cu and Fe positions along the $c$ axis inside the oxygen pyramid disappeared. Coulometric titrations and Mössbauer spectroscopy measurements strongly suggested the existence of excess oxygen in the yttrium plane.