ELECTRICAL AND MAGNETIC PROPERTIES OF La_2−xSr_xNiO₄: A TENTATIVE PHASE DIAGRAM

La_2−xSr_xNiO₄ shows a maximum in the c/a ratio around x = 0.6 up to which composition the concentration of holes is equal to that expected theoretically. The material becomes a degenerate semiconductor above a certain temperature up to x = 0.8, but is metallic at room temperature when x ≥ 1.0. All the compositions with x ≥ 0.05 are paramagnetic in the 15 - 300K range. Based on these measurements, an electronic phase diagram is tentatively proposed. It is noteworthy that the antiferromagnetic order disappears at low doping levels in both La_2−xSr_xNiO₄ and La_2−xSr_xCuO₄, but metallicity is seen at 300K only at a much higher x value in the former system.