"Stable structures are locally electroneutral. For example, in the structure of the double
perovskite Sr2FeMoO6, MO6 (M = Fe2+, Mo6+) octahedra share all their vertices, and Sr2+ ions fill
the cubooctahedral cavities that are flanked by eight MO6 octahedra.[2] Each O2- ion is
coordinated to one Fe2+ and one Mo6+ ion in order to achieve local electroneutrality, and thus the
FeO6 and MoO6 octahedra alternate in the structure."
I don't know how to visualize Sr2FeMoO6 to see this description.