Mixed layered oxide phases Na xLi 1− xNiO 2: a detailed description of their preparation and structural and magnetic identification

Abstract

The mixed layered oxide phases Na x Li 1− x NiO 2 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 Ni 2+ ions. On the sodium-rich side the perfect layering of NaNiO 2 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-NaFeO 2-structure (space group R-3m), the most sodium-rich compounds adopt the structure type of NaNiO 2 (space group C2/m), whereas, around the composition Na 0.7Li 0.3NiO 2 (nominal composition “Na 0.8Li 0.2NiO 2”), a second (R-3m)-type structure is formed with increased lattice parameters, when compared to LiNiO 2. We show that when the lithium substitution for sodium is small, the antiferromagnetism and the Jahn–Teller transition of NaNiO 2 are only marginally affected, but when the nominal Na 0.8-composition is reached, the Jahn–Teller transition completely disappears and both the saturation and spin-flop fields are changed.