Nominal ${\mathrm{La}}_2{\mathrm{NiMnO}}_6$ prepared by solid-state reaction in air in accordance with earlier reports is shown to contain excess oxygen as well as the coexistence of two ferromagnetic phases of comparable Curie temperatures, one monoclinic and the other rhombohedral. As originally predicted, ordering of ${\mathrm{Ni}}^{2+}$ and ${\mathrm{Mn}}^{4+}$ ions gives ferromagnetic ${\mathrm{Ni}}^{2+}-\mathrm{O}-{\mathrm{Mn}}^{4+}$ interactions and transforms the orthorhombic Pbnm space group to monoclinic ${P2\mathrm{}}_1/n$ with $\beta \approx 90°$ and the rhombohedral $R3\mathrm{}¯c$ space group to $R3\mathrm{}¯m$ or $R3\mathrm{}¯.$ Synthesis by the Pechini method in Ar, air, and ${\mathrm{O}}_2$ atmospheres under different thermal treatments also consistently gave ${\mathrm{O}}_{6+\delta };$ the lowest $\delta =0.05\mathrm{}\left(1\right)\mathrm{}$ was attained for a single ${P2\mathrm{}}_1/n$ phase reacted under Ar at 1350 °C. Lowering the mean A-site atomic radius in ${\mathrm{La}}_{2-x}{\square }_x{\mathrm{NiMnO}}_6$ and ${\mathrm{Nd}}_2{\mathrm{NiMnO}}_6$ also stabilizes the monoclinic phase, and near oxygen stoichiometry was attained in ${\mathrm{La}}_{2-x}{\square }_x{\mathrm{NiMnO}}_6$ for $x\approx \mathrm{0.09.}$ Excess oxygen is accommodated in the perovskite structure by the creation of cation vacancies, and it is shown that lanthanum vacancies create deep three-hole acceptor traps. Comparison with the double perovskite ${\mathrm{La}}_2{\mathrm{CoMnO}}_6$ and ${\mathrm{La}}_2{\mathrm{NiRuO}}_{6+\delta }$ versus ${\mathrm{La}}_2{\mathrm{CoRuO}}_6$ signals that stabilization of lanthanum vacancies is associated with a ${\mathrm{Ni}}^{3+}/{\mathrm{Ni}}^{2+}$ redox couple that is stabilized by a counter octahedral-site cation M having a strong covalent component to its M-O bonding. It is therefore proposed that in the presence of ${\mathrm{Ni}}^{2+},$ but not ${\mathrm{Co}}^{2+},$ a lanthanum vacancy is stabilized by the formation of two holes trapped deeply in molecular orbitals of an ${\mathrm{O}}_{12}$ cluster of the oxygen atoms that neighbor a lanthanum vacancy. Transport data also indicate a lowering of the separation of the ${\mathrm{Mn}}^{4+}/{\mathrm{Mn}}^{3+}$ and ${\mathrm{Ni}}^{3+}/{\mathrm{Ni}}^{2+}$ redox couples from $E_g>~0.8\mathrm{eV}$ to $E_g\approx 0.3\mathrm{eV}$ in the ordered ${\mathrm{Ni}}^{2+},$ ${\mathrm{Mn}}^{4+}$ array. This lowering and a motional enthalpy $\Delta H_{\mathrm{mn}}\approx 0.1\mathrm{eV}$ of electrons is attributed to locally cooperative Jahn-Teller deformations of low-spin ${\mathrm{Ni}}^{3+}$ and high-spin ${\mathrm{Mn}}^{3+}$ octahedral sites. The magnetization $M(5\mathrm{}\mathrm{K},50\mathrm{kOe})$ is lowered by both local atomic disorder and the formation of antiphase boundaries. It is shown that a prolonged anneal at 800 °C reduces the local atomic disorder, but it does not remove the antiphase boundaries. Synthetic strategies to increase the magnetization must be designed to reduce the concentration of antiphase boundaries and cation vacancies as well as the atomic disorder.

• Received 9 April 2003

DOI:https://doi.org/10.1103/PhysRevB.68.064415