Layered (Cu,Fe) oxides of double perovskite structure. II. Extension of solid solubility of copper in(Ba,La)Y(Cu0.5+xFe0.5−x)2O5+δvia high-pressure heat treatment

In this paper, we report successful extension of the solid-solubility limit of copper at the transition element site in the ${\mathrm{}(\mathrm{B}\mathrm{a},\mathrm{L}\mathrm{a})\mathrm{Y}(\mathrm{C}\mathrm{u},\mathrm{F}\mathrm{e})\mathrm{}}_2{\mathrm{O}}_{5+\mathrm{\delta }}$ phase of the double-perovskite “0112” structure. Upon the high-pressure heat treatment at 5 GPa and 1200 °C of the ambient-pressure synthesized samples that contained impurity phases, essentially single-phase samples were obtained for x up to 0.2 in the $\left({\mathrm{Ba}}_{1-2x}{\mathrm{La}}_{2x}\right)\mathrm{Y}({\mathrm{Cu}}_{0.5+x}{\mathrm{Fe}}_{0.5-x}{)}_2{\mathrm{O}}_{5+\mathrm{\delta }}$ system. TEM observations and TEM energy dispersive x-ray analyses for the high-pressure synthesized ${(\mathrm{B}\mathrm{a}}_{0.6}{\mathrm{La}}_{0.4}{\left)\mathrm{Y}\right(\mathrm{C}\mathrm{u}}_{0.7}{\mathrm{Fe}}_{0.3}{)}_2{\mathrm{O}}_{5+\mathrm{\delta }}$ material showed that the ratio of copper to iron varies in the sample with an average value of $\mathrm{C}\mathrm{u}:\mathrm{F}\mathrm{e}=0.70:0.30,$ as measured for nine grains. It could thus be concluded that a 0112 phase in which the amount of copper is larger than that of iron in the oxygen pyramid was realized. In order to obtain the copper-rich 0112 phase, simultaneous substitution of the divalent barium site by trivalent lanthanum was found necessary. From ${}^{57}\mathrm{Fe}$ Mössbauer spectroscopy data, the relative intensities of five- and six-coordinated iron atoms were evaluated and the amount of excess oxygen in the structure could be estimated at $\delta \approx \mathrm{0.23.}$ Furthermore, both Mössbauer and magnetic-susceptibility measurements of the $x=0.2\mathrm{}$ sample showed that the Néel temperature of the copper-rich material is lower than the corresponding transition temperature observed for the material with the stoichiometric $x=0\mathrm{}$ composition. Finally, conditions for the successful extension of the copper solubility are discussed and related to the tolerance factor of the 0112 structure.