While double perovskites $A_{2}B{B}^{\prime }{\mathrm{O}}_6$, if ordered, usually form a rock-salt–type structure with a checkerboard $B\text{−}{B}^{\prime }$ ordering, it is surprising that ${\mathrm{Ca}}_2{\mathrm{FeMnO}}_6$ has alternate ${\mathrm{FeO}}_2$ and ${\mathrm{MnO}}_2$ layers in its perovskite structure. Here, by using density functional calculations, we demonstrate that this unusual layered ordering facilitates, and is largely helped by, the ${\mathrm{Fe}}^{3+}\text{−}{\mathrm{Fe}}^{5+}$ charge disproportionation (CD) of the formal ${\mathrm{Fe}}^{4+}$ ions, which would otherwise be frustrated in the common rock-salt structure. To further verify the important role of the CD for stabilization of this layered ordering, we carry out a comparative study for the isostructural ${\mathrm{Ca}}_2{\mathrm{TiMnO}}_6$ which has a simple ${\mathrm{Ti}}^{4+}\text{−}{\mathrm{Mn}}^{4+}$ state free of the CD. Our calculations indicate that ${\mathrm{Ca}}_2{\mathrm{TiMnO}}_6$ instead prefers the standard rock-salt structure to the layered one. Thus our study shows a nontrivial interplay between the CD and the type of ion ordering and proves that the CD is strongly involved in stabilizing the unusual layered order of ${\mathrm{Ca}}_2{\mathrm{FeMnO}}_6$.

• Received 26 April 2018
• Revised 4 July 2018

DOI:https://doi.org/10.1103/PhysRevB.98.085105