Structure and magnetic properties of the spin-1/2-based honeycomb NaNi₂BiO_6-δand its hydrate NaNi₂BiO_6-δ·1.7H₂O

We present the structure and magnetic properties of the honeycomb anhydrate NaNi₂BiO_6-δ and its monolayer hydrate NaNi₂BiO_6-δ·1.7H2O, synthesized by deintercalation of the layered α-NaFeO₂-type honeycomb compound Na₃Ni₂BiO₆. The anhydrate adopts ABAB-type oxygen packing and a one-layer hexagonal unit cell, whereas the hydrate adopts an oxygen packing sequence based on a three-layer rhombohedral subcell. The metal-oxide layer separations are 5.7 A˚ in the anhydrate and 7.1 A˚ in the hydrate, making the hydrate a quasi 2-D honeycomb system. The compounds were characterized through single crystal diffraction, powder X-ray diffraction, thermogravimetric analysis, and elemental analysis. Temperature-dependent magnetic susceptibility measurements show both to have negative Weiss temperatures (-18.5 and -14.6 K, respectively) and similar magnetic moments (2.21 and 2.26 μ_B/Ni, respectively), though the field-dependent magnetization and heat capacity data suggest subtle differences in their magnetic behavior. The magnetic moments per Ni are relatively high, which we suggest is due to the presence of a mixture of Ni²⁺ and Ni³⁺ caused by oxygen vacancies.