Structure and magnetic properties of the α-NaFeO₂-type honeycomb compound Na₃Ni₂BiO₆

We present the structure and magnetic properties of Na₃Ni ₂BiO₆, which is an ordered variant of the α-NaFeO₂ structure type. This layered compound has a 2:1 ordering of (Ni²⁺/Bi⁵⁺)O₆ octahedra within the a-b plane and sodium in octahedra between the layers. The structure is presented in the space group C2/m, determined through a combination of single crystal X-ray, powder neutron, and powder X-ray diffraction. Temperature dependent magnetic susceptibility measurements show Na₃Ni₂BiO ₆ to display long-range antiferromagnetic ordering below 11 K, despite the dominance of ferromagnetic interactions above T_N as indicated by a positive Weiss constant. Heat capacity measurements and low-temperature neutron diffraction support the magnetic ordering and are consistent with a T_N of 10.4 K. A magnetic phase can be refined with (010) antiferromagnetic ordering along the b-axis in the honeycomb layer and moments aligned parallel to c. The compounds Na₃Mg ₂BiO₆ and Na₃Zn₂BiO₆, synthesized as nonmagnetic analogues of Na₃Ni₂BiO ₆, are briefly described.