The first layer potassium–bismuth-nickel oxophosphate KBi₄Ni₂(PO₄)₃O₄: Synthesis, crystal structure, and expected magnetic properties

The new potassium–bismuth–nickel oxophosphate obtained by hydrothermal synthesis in the Bi(OH)₃–NiCO₃–K₂CO₃–K₃PO₄ system is studied by X-ray diffraction, IR spectroscopy, and Raman spectroscopy. Parameters of the orthorhombic cell are as follows: a = 13.632(1) Å, b = 19.610(2) Å, and c = 5.4377(3) Å; V = 1452.64(2) ų; and space group Pnma. The structure is solved and refined to the final discrepancy factor R₁ = 5.76% in the anisotropic approximation of atomic displacements using 3606 reflections with I > 2σ(I). The crystal-chemical formula (Z = 4) is KBi₄{Ni₂O₄(PO₄)₃}, where the composition of the layer nickel–phosphate polyanion is enclosed in braces. Theoretical calculations show that all exchange spin interactions between Ni²⁺ ions are antiferromagnetic and very weak (J < 0.1 cm^–1) because of the polyatomic character of bridging Ni–O–P–O–Ni and Ni–O–Bi–O–Ni groups. Thus, this compound is expected to be paramagnetic with very weak antiferromagnetic exchange interactions and appreciable energy of zero-field splitting of the spin levels of Ni²⁺ ions.