Factors Influencing the Stabilization of Ni+ in Perovskite-Related Oxides

Abstract

With an objective to identify the factors influencing the stabilization of the unusual valence state Ni+ in perovskite-related oxides, the reduction of the Ruddlesden–Popper series precursors An+1NinO3n+1 (A=lanthanide and alkaline earth and n=1, 2, and 3) containing the Ni2+/3+ or Ni3+/4+ couple has been investigated systematically. La2−xSrxNiO3+δ, Ln3Ni2O6+δ (Ln=lanthanide), and Ln4Ni3O8 oxides containing the Ni+/2+ couple have been obtained by a reduction of, respectively, La2−xSrxNiO4, Ln3Ni2O7, and Ln4Ni3O10 with dilute hydrogen over a narrow range of temperature. During the reduction process, oxygen atoms are removed from the NiO2 planes of La2−xSrxNiO4 to give orthorhombic La2−xSrxNiO3+δ, but from the rock salt planes present in between the NiO2 planes of Ln3Ni2O7 and Ln4Ni3O10 to give tetragonal Ln3Ni2O6+δ and Ln4Ni3O8, respectively. Bond length matching between the stretched Ni+–O and A–O bonds to minimize the internal stresses and coordination preference and size of the A cations are found to play an important role in accessing Ni+. Although the Ni+/2+ couple is isoelectronic with the Cu2+/3+ couple in copper oxide superconductors, the Ni+/2+ oxides exhibit localized semiconducting properties due to a larger charge transfer gap.