Mixed layered oxide phases NaxLi1−xNiO2: a detailed description of their preparation and structural and magnetic identification

The mixed layered oxide phases NaxLi1−xNiO2 were prepared and characterised by X-ray diffraction, chemical analysis and a thorough magnetic study. Single phase compounds were obtained for the nominal compositions 0⩽x⩽0.3 and 0.8⩽x⩽1.0. The chemical analysis reveals that only up to 6% of sodium can actually be substituted for lithium in the lithium-rich samples, and that heavy nickel substitution for lithium occurs simultaneously (up to 7%). The magnetic susceptibility data confirm this and clearly show the ferromagnetic coupling between the nickel layers, which is caused by the disordered Ni2+ ions. On the sodium-rich side the perfect layering of NaNiO2 is maintained upon the substitution of lithium for sodium, the chemical analysis shows that up to 30% of lithium can be substituted for sodium. The lithium-rich compounds crystallise in the α-NaFeO2-structure (space group R-3m), the most sodium-rich compounds adopt the structure type of NaNiO2 (space group C2/m), whereas, around the composition Na0.7Li0.3NiO2 (nominal composition “Na0.8Li0.2NiO2”), a second (R-3m)-type structure is formed with increased lattice parameters, when compared to LiNiO2. We show that when the lithium substitution for sodium is small, the antiferromagnetism and the Jahn-Teller transition of NaNiO2 are only marginally affected, but when the nominal Na0.8-composition is reached, the Jahn-Teller transition completely disappears and both the saturation and spin-flop fields are changed.