Synthesis and structure of the perovskite oxide carbonate Ba4ScCu2O7−x(CO3)y (x ≈ 0.3, y ≈ 0.9): a new example of ordering in the small cation sites

In this paper we report further work on the perovskite oxide carbonate, Ba4ScCu2O7−x(CO3)y (x≈0.3, y≈0.9). The X-ray diffraction pattern for this phase can be indexed on a 2ap×2ap×cp cell (where ap, cp are the primitive perovskite cell lengths). On heating this phase to higher temperature, partial loss of CO32− occurs and the pattern now can be indexed on a √2ap×√2ap×2cp cell, similar to that previously observed for the system Ba4YCu2+xO6+y(CO3)z. Structural studies using powder neutron diffraction show that Ba4ScCu2O7−x(CO3)y (x≈0.3, y≈0.9) exhibits a new example of ordering within the small cation sites, with an ordered arrangement of Sc, Cu and C (from the carbonate group). It appears, however, from the refined data that this ordering may not be complete, which can probably be explained by the presence of small regions of the low carbonate content phase. Discussion of the factors influencing the relative stabilities of the high and low carbonate content phases is also presented.