COMPOSITION-CONTROLLED METAL-NONMETAL TRANSITION IN LA2-XSRXNIO4-DELTA

Materials with the general composition La(2-x)Sr(x)NiO4-delta 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 delta-determine the conductivity type. For x < 1 the material 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 delta-is low. For compositions near the transition the conductivity type is also dependent on delta. Metallic samples become nonmetals upon heating in flowing Ar at 1000-degrees-C, which revert to metals upon oxidation in O2 also at 1000-degrees-C. All nonmetal-to-metal transitions are accompanied by a change in color from black to reddish brown. The Ni average valence upsilon-was determined from the metals composition and iodometric titration data. The delta value was calculated from upsilon-by using the equation delta = 0.5x + (1 - 0.5-upsilon), which was derived by using a formal valence convention. Both upsilon and delta increase monotonically with x in the range 0 less-than-or-equal-to x less-than-or-equal-to 1.5. A phase diagram is proposed for the metal-nonmetal transition in which the boundary is given by delta = 0.5x - 0.51, which corresponds to upsilon almost-equal-to 3.02. No stoichiometric 214 nickelate (delta = 0) could be prepared in the metallic region, and no superconductive transition was observed in any material down to approximately 3 K.