CHEMICAL SYNTHESIS AND PROPERTIES OF LI-1-DELTA-XNI1+DELTA-O2 AND LI[NI2]O4

Magnetic and transport properties of the layered (R3-m) metastable system Li1-δ-xNi1+δO2 and the cubic spinel Li[Ni2]O4 are reported for samples prepared from (Li1-δNiδ)NiO2, δ=0.04 and 0.08, by chemical extraction of lithium with NO2PF6 at room temperature. Transformation of the layered phase to the cubic spinel was accomplished by heating (δ+x)≈0.5 samples to 250-300°C in an evacuated, sealed quartz tube. These mixed-valent compounds are small-polaron semiconductors, and the low-spin Ni3+ ions carry a nickel magnetic moment close to 1 μB; the low-spin Ni4+ ions are diamagnetic. The magnetic interactions are those predicted for partially filled crystal-field orbitals of e parentage, which are associated with σ bonding in octahedral sites. Nevertheless, there is no cooperative Jahn-Teller distortion of the type expected for strong Jahn-Teller ions with localized e-orbital configurations in octahedral sites. These findings cast doubt on any interpretation of these oxides that leaves unmodified the magnitude of the Hubbard U and the positions of the redox energies from their values in NiO. © 1992 Academic Press, Inc.