Synthesis, Structure, and Properties of the Layered Perovskite La3Ni2O7-δ

Single-phase samples of La3Ni2O7-δ with δ = 0, 0.08, and 0.65 were prepared and characterized by powder X-ray diffraction, TGA, electrical resistivity, and magnetic susceptibility. The crystal symmetry of the as-prepared (δ = 0.08) and oxygenated (δ = 0.00) samples is orthorhombic (space group Fmmm). Oxidation of the as-prepared La3Ni2O6.92 sample to La3Ni2O7.00 shows little change in the unit-cell parameters. Reduction of La3Ni2O6.92 to La3Ni2O6.35 results in a structural phase transition from orthorhombic to tetragonal symmetry, a dramatic decrease in the c cell parameter, and significant increase in the a and b cell parameters. These data are consistent with loss of oxygen from the apical octahedral sites connecting two perovskite layers in La3Ni2O7-δ. The electrical transport properties change with decreasing oxygen content from metallic for La3Ni2O7.00 to semiconducting for La3Ni2O6.92 and La3Ni2O6.35. The fully reduced sample, La3Ni2O6.35, shows a linear dependence of In ρ versus T-1/4 in the temperature range 90-140 K, suggesting a variable-range hopping mechanism. A nearly temperature-independent magnetic susceptibility is observed in the temperature range 100-300 K for the δ = 0 and 0.08 samples. The relatively high values and the temperature dependence of the magnetic susceptibility indicate a Stoner enhanced Pauli paramagnetism for δ = 0 and evidence for a weak ferromagnetism in the δ = 0.65 sample. The electrical and magnetic properties of La3Ni2O7-δ are discussed in terms of the mixed-valent character of nickel and vacancy-induced localization effects.