New crystal-structure phase in oxygen-nonstoichiometric double-layered perovskite NdBaCo$_2$O$_{5.72}$

The crystal structure of an NdBaCo$_2$O$_{5+\delta}$ ($\delta=0.72$) solid solution with a layered perovskite structure is studied by neutron diffraction. It is found that a new crystal-structure phase with the cation-anion ratio $1$:$1$:$2$:$5.75$ forms in this solid solution. At the same time, oxygen vacancy ordering occurs in the plane of the rare-earth ion in the crystallographic position $1c\left(0\ 0\ \frac{1}{2}\right)$ of the $Pmmm$ space group. Oxygen vacancy diffusion along the crystallographic axes in the pane of the rare-earth element is also proposed as a mechanism for the formation of other crystal-structure phases found in double-layered cobaltites.