Nonstoichiometric Ellenbergerite-Type Phosphates: Hydrothermal Synthesis, Crystal Chemistry, and Magnetic Behavior

We synthesized single crystals of Na_0.55Ni₆(OH)₃(H_0.61PO₄)₄ (I) and polycrystals of (Na, Ni)_0.64Ni_5.68(OH)₃(H_0.67PO₄)₄ (II) with ellenbergerite-like structures using the hydrothermal method. The phases crystallize in the hexagonal space group P6₃mc with the following unit cell parameters: a = 12.5342(1) Å, c = 4.9470(1) Å, and V = 673.08(2) ų for I; a = 12.4708(2) Å, c = 4.9435(2) Å, and V = 665.82(2) ų for II; and Z = 2. Their crystal structures are based on a 3D framework built from NiO₆ octahedra and PO₄ tetrahedra. The difference between I and II lies in the way the structural channels are filled along the [001] direction. These channels accommodate segments of Na- and (Na, Ni)-centered chains of face-sharing octahedra in the structures I and II, respectively. The magnetic susceptibility χ and the specific heat C_p evidence pronounced low-dimensional magnetic behavior at elevated temperatures and the formation of the weakly ferromagnetic long-range order at T_N = 61 K and T_N = 63 K. Analysis of the χ(T) data within both chain and dimer spin models allows the estimation of the leading exchange interaction parameters in the compounds under study.