Further evidence for the coexistence of localized and itinerant 3d electrons in La₂NiO₄

Abstract

Measurement of a continuous increase in unit cell c/a ratio of La₂NiO₄ with temperature- but without change of symmetry from 77 to 773 K, refinement of the local distortion of the Ni²⁺-ion octahedral site to tetragonal (c/a>1) symmetry (c/a=1.027 at room temperature) as against a c/a<1 in K₂NiF₄, and determination of a constant oxygen content in air for all temperatures below 100 K provide evidence that the nickel d_z2 spin is localized whereas the d_x²−y² electrons occupy a narrow σ^∗_x²−y² band of itinerant-electron states containing strong electron correlations that increase with decreasing temperature.