A-site ordered quadruple perovskites, AA3B4O12, show many interesting physical and chemical properties, for example, inter-site charge transfer and disproportionation, giant dielectric constant, multiferroic properties, and high catalytic activity. Recently, we discovered other fascinating properties of quadruple perovskites when we introduced charge degree of freedom into the B site of BiMn7O12 through BiCuxMn7-xO12 solid solutions, which show very rich phase diagram and exhibit reentrant structural transitions and collapse of charge and orbital orders for a very wide composition range. Moreover, we found very complicated modulated structures. We propose neutron diffraction experiments to determine modulated crystal structures and magnetic structures of BiCuxMn7-xO12 solid solutions for different x values.
