Nickel Oxidation State and Magnetic Properties of Hole-Doped and Reduced Nd2−xSrxNiOyCompounds

The NiK-edge XANES of reduced Nd_2−xSr_xNiO₄samples shows an increase of Ni³⁺content with Sr³⁺concentration. The appearance of mixed valence Ni²⁺/Ni³⁺produces a change in the magnetic properties of the system. The Nd³⁺sublattice orders antiferromagnetically atT_N∼13–20 K forx=0.4, 0.5, and 0.6, due to the interactions of Ni–Nd sublattices. Forx=0.8 this order begins to disappear because of the effect of the low spin state Ni³⁺created at the NiO₄planes. Additionally, the reduced family Nd_2−xSr_xNiO_y(y<4) shows the disappearance of Ni³⁺, the decrease of Ni²⁺with respect to the nonreduced form, and the appearance of Ni⁺. All the compounds of the reduced family show AF order at low temperature.