Composition-controlled metal-nonmetal transition in La2-xSrxNiO4-δ

Materials with the general composition La2-xSrxNiO4-δ retain the perovskite-related structure of space group I4/mmm up to x = 1.50. They undergo a composition -dependent metal-nonmetal transition in electrical conductivity. The composition parameters x and δ determine the conductivity type. For x < 1 the materials are nonmetals. For x > 1.1 metallic conductivity is observed that persists to a limit of at least x = 1.50 if the oxygen deficiency δ is low. For compositions near the transition, the conductivity type is also dependent on δ. Metallic samples become nonmetals upon heating in flowing Ar at 1,000C, which revert to metals upon oxidation in O2 also at 1,000C. All nonmetal-to-metal transitions are accompanied by a change in color from black to reddish brown. The Ni average valence v was determined from the metals composition and iodometric titration data. The δ value was calculated from υ by using the equation δ = 0.5x + (1 - 0.5υ), which was derived by using a formal valence convention. Both υ and δ increase monotonically with x in the range 0 ≤ x ≤ 1.5. A phase diagram is proposed for the metal-nonmetal transition in which the boundary is given by δ = 0.5x - 0.51, which corresponds to υ ∼ 3.02. No stoichiometric 214 nickelate (δ = 0) could be prepared in the metallic region, and no superconductive transition was observed in any material down to ∼ 3 K