Hole and Electron Doping of Topochemically Reduced Ni(I)/Ru(II) Insulating Ferromagnetic Oxides

LaSr_2-NiRuO₆, LaSr_4-NiRuO₈, and LaSr_3-NiRuO₇ are, respectively, the n = ∞, 1, and 2 members of the (LaSr_1-()Sr(Ni_0.5Ru_0.5)O₃ compositional series. Reaction with CaH₂, in the case of the LaSr_2-NiRuO₆ perovskite phases, or Zr oxygen getters in the case of the LaSr_4-NiRuO₈ and LaSr_3-NiRuO₇ Ruddlesden-Popper phases, yields the corresponding topochemically reduced (LaSr_1-()Sr(Ni_0.5Ru_0.5)O₃ compounds (LaSr_2-NiRuO₄, LaSr_4-NiRuO₆, and LaSr_3-NiRuO₅), which contain Ni and Ru cations in square-planar coordination sites. The x = 1 members of each series (LaSrNiRuO₄, LaSr₃NiRuO₆, and LaSr₂NiRuO₅) exhibit insulating ferromagnetic behavior at low temperature, attributable to exchange couplings between the Ni¹⁺ and Ru²⁺ centers they contain. Increasing the La³⁺ concentration (x > 1) leads to a reduction of some of the Ru²⁺ centers to Ru¹⁺ centers and a suppression of the ferromagnetic state (lower T_c, reduced saturated ferromagnet moment). In contrast, increasing the Sr²⁺ concentration (x < 1) oxidizes some of the Ru²⁺ centers to Ru³⁺ centers and enhances the ferromagnetic coupling (increased T_c, increased saturated ferromagnet moment) for the n = ∞ and n = 2 samples but appears to have no influence on the magnetic ordering temperature of the n = 1 samples. The magnetic couplings and influence of doping are discussed on the basis of superexchange and direct exchange couplings between the square-planar Ni and Ru centers.