Chemical synthesis and properties of Li 1−δ− xNi 1+δO 2 and Li[Ni 2]O 4

Abstract

Magnetic and transport properties of the layered ( R 3¯ m ) metastable system Li 1−δ− x Ni 1+δ O 2 and the cubic spinel Li[Ni 2 ]O 4 are reported for samples prepared from (Li 1−δ Ni δ )NiO 2 , δ=0.04 and 0.08, by chemical extraction of lithium with NO 2 PF 6 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 Ni 3+ ions carry a nickel magnetic moment close to 1 μ B ; the low-spin Ni 4+ 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.