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

We synthesized single crystals of Na 0.55 Ni 6 (OH) 3 (H 0.61 PO 4 ) 4 ( I ) and polycrystals of (Na, Ni) 0.64 Ni 5.68 (OH) 3 (H 0.67 PO 4 ) 4 ( II ) with ellenbergerite-like structures using the hydrothermal method. The phases crystallize in the hexagonal space group P 6 3 mc with the following unit cell parameters: a = 12.5342(1) Å, c = 4.9470(1) Å, and V = 673.08(2) Å 3 for I ; a = 12.4708(2) Å, c = 4.9435(2) Å, and V = 665.82(2) Å 3 for II ; and Z = 2. Their crystal structures are based on a 3D framework built from NiO 6 octahedra and PO 4 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 I = 61 K and T N II = 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.