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 NaNiO 2 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 H x NiO 2 · n H 2 O ( x < 1, n ∼1) with an enlarged basal spacing of 9.6 Å. The materials had a monoclinic structure ( C 2/ m ); unit cell parameters for a typical composition of H 0.66 NiO 2 ·0.9H 2 O were a = 4.8993(8) Å, b = 2.8256(4) Å, c = 9.7598(9) Å, and β = 98.88(2)°. Rietveld refinement revealed that the structure was composed of pseudohexagonal NiO 2 sheets accommodating partially occupied three layers of H 2 O molecules and H 3 O + ions in the galleries. The highly expanded layered structure is analogous to Na 0.3 NiO 2 ·1.3H 2 O and other layer oxides such as buserite-type manganese oxide and superconducting Na x CoO 2 · y H 2 O but differs in alkali-free composition. The 9.6 Å phase underwent partial dehydration to give a basal spacing of 7 Å upon exposure to atmosphere with a relative humidity of <30%.