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 * < 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 1000 °C, which revert to metals upon oxidation in O2 also at 1000 °C. All nonmetal-to-metal transitions are accompanied by a change in color from black to reddish brown. The Ni average valence υ 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 v ≈ 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.
All Science Journal Classification (ASJC) codes
- Chemical Engineering(all)
- Materials Chemistry