Factors influencing the stabilization of Ni+ in perovskite-related oxides

With an objective to identify the factors influencing the stabilization of the unusual valence state Ni+ in perovskite-related oxides, the reduction o f the Ruddlesden-Popper series precursors A(n+1)Ni(n)O(3n+1) (A = lanthanid e and alkaline earth and n = 1, 2, and 3) containing the Ni2+/3+ Or Ni3+/4 couple has been investigated systematically. La2-xSrxNiO3+delta, Ln(3)Ni(2 )O(6+delta) (Ln = lanthanide), and Ln(4)Ni(3)O(8) oxides containing the Ni/2+ couple have been obtained by a reduction of, respectively, La2-xSrxNiO4 , Ln(3)Ni(2)O(7), and Ln(4)Ni(3)O(10) with dilute hydrogen over a narrow ra nge of temperature. During the reduction process, oxygen atoms are removed from the NiO2 planes of La2-xSrxNiO4 to give orthorhombic La2-xSrxNiO3+delt a, but from the rock salt planes present in between the NiO2 planes of Ln(3 )Ni(2)O(7) and Ln(4)Ni(3)O(10) to give tetragonal Ln(3)Ni(2)O(6+delta) and Ln(4)Ni(3)O(8), respectively. Bond length matching between the stretched Ni +-O and A-O bonds to minimize the internal stresses and coordination prefer ence and size of the A cations are found to play an important role in acces sing Ni+. Although the Ni+/2+ couple is isoelectronic with the Cu2+/3+ coup le in copper oxide superconductors, the Ni+/2+ oxides exhibit localized sem iconducting properties due to a larger charge transfer gap. (C) 1999 Academ ic Press.