Oxygen stoichiometry, ferromagnetism, and transport properties of La2-xNiMnO6+δ -: art. no. 064415

Nominal La2NiMnO6 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 Ni2+ and Mn4+ ions gives ferromagnetic Ni2+-O-Mn4+ interactions and transforms the orthorhombic Pbnm space group to monoclinic P2(1)/n with betaapproximate to90degrees and the rhombohedral R (3) over barc space group to R (3) over barm or R (3) over bar. Synthesis by the Pechini method in Ar, air, and O-2 atmospheres under different thermal treatments also consistently gave O6+delta; the lowest delta=0.05(1) was attained for a single P2(1)/n phase reacted under Ar at 1350 degreesC. Lowering the mean A-site atomic radius in La(2-x)square(x)NiMnO(6) and Nd2NiMnO6 also stabilizes the monoclinic phase, and near oxygen stoichiometry was attained in La(2-x)square(x)NiMnO(6) for xapproximate to0.09. 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 La2CoMnO6 and La2NiRuO6+delta versus La2CoRuO6 signals that stabilization of lanthanum vacancies is associated with a Ni3+/Ni2+ 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 Ni2+, but not Co2+, a lanthanum vacancy is stabilized by the formation of two holes trapped deeply in molecular orbitals of an O-12 cluster of the oxygen atoms that neighbor a lanthanum vacancy. Transport data also indicate a lowering of the separation of the Mn4+/Mn3+ and Ni3+/Ni2+ redox couples from E(g)greater than or equal to0.8 eV to E(g)approximate to0.3 eV in the ordered Ni2+, Mn4+ array. This lowering and a motional enthalpy DeltaH(mn)approximate to0.1 eV of electrons is attributed to locally cooperative Jahn-Teller deformations of low-spin Ni3+ and high-spin Mn3+ octahedral sites. The magnetization M(5 K, 50 kOe) is lowered by both local atomic disorder and the formation of antiphase boundaries. It is shown that a prolonged anneal at 800 degreesC 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.