Synthesis, Structure, and Properties of the Layered Perovskite La 3Ni 2O 7-δ

Abstract

Single-phase samples of La 3Ni 2O 7-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 La 3Ni 2O 6.92 sample to La 3Ni 2O 7.00 shows little change in the unit-cell parameters. Reduction of La 3Ni 2O 6.92 to La 3Ni 2O 6.35 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 La 3Ni 2O 7-δ. The electrical transport properties change with decreasing oxygen content from metallic for La 3Ni 2O 7.00 to semiconducting for La 3Ni 2O 6.92 and La 3Ni 2O 6.35. The fully reduced sample, La 3Ni 2O 6.35, 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 La 3Ni 2O 7-δ are discussed in terms of the mixed-valent character of nickel and vacancy-induced localization effects.