Crystal structure and thermodynamic properties of dinickel diphosphate dihydrate Ni 2 (H 2 O) 2 [P 2 O 7 ]

Single crystals of dinickel diphosphate dihydrate, Ni (H O) [P O ], have been synthesized by a hydrothermal method. Its structure was refined in the monoclinic P2 /n space group (unit cell parameters a = 6.2517(1) Å, b = 13.7892(3) Å, c = 7.2894(2) Å, β = 94.507(2)°, V = 626.45(2) Å , and Z = 4) based on low-temperature X-ray diffraction data until R- 0.016. Corrugated chains of NiO (H O) octahedra sharing edges are aligned in the [101[combining macron]] direction. They are linked into a three-dimensional framework through diphosphate groups and hydrogen bonds. A detailed crystal chemical analysis of the family Me (H O) [X O ] revealed correlations between the unit-cell parameters of the isotypic transition metal phosphates and arsenates, their structural features and the sizes of structure forming cations. Despite the isolation of the cis and trans edge-sharing infinite zigzag chains of Ni-centered octahedra from each other no pronounced low dimensionality is seen in the magnetic response of Ni (H O) [P O ]. The magnetic susceptibility χ evidences a short range correlation maximum at T = 11.9 K accompanied by the onset of long-range magnetic order at T = 9.4 K. Below T , the title compound exhibits the features of an archetype three-dimensional easy-axis antiferromagnet which experiences a sequence of spin-flop and spin-flip phase transitions. Basing on specific heat C and magnetization M studies, the magnetic phase diagram of Ni (H O) [P O ] has been established.