Highly swollen layered nickel oxide with a trilayer hydrate structure

This paper reports the synthesis of alkali-free layered nickel oxides with a highly swollen hydrate structure, derived from NaNiO2 through soft-chemical processes involving oxidation with bromine and subsequent acid treatment. Complete removal of interlayer Na+ ions and subsequent hydration yielded a single phase of HxNiO2 center dot nH(2)O (x < 1, n similar to 1) with an enlarged basal spacing of 9.6 angstrom. The materials had a monoclinic structure (C2/m); unit cell parameters for a typical composition of H0.66NiO2 center dot 0.9H(2)O were a=4.8993(8) angstrom, b=2.8256(4) angstrom, c=9.7598(9) angstrom, and beta=98.88(2)degrees. Rietveld refinement revealed that the structure was composed of pseudohexagonal NiO2 sheets accommodating partially occupied three layers of H2O molecules and H3O+ ions in the galleries. The highly expanded layered structure is analogous to Na0.3NiO2 center dot 1.3H(2)O and other layer oxides such as buserite-type manganese oxide and superconducting NaxCoO2 center dot yH(2)O but differs in alkali-free composition. The 9.6 angstrom phase underwent partial dehydration to give a basal spacing of 7 angstrom upon exposure to atmosphere with a relative humidity of < 30%.