Crystal structure and thermodynamic properties of dinickel diphosphate dihydrate Ni₂(H₂O)₂[P₂O₇]

Single crystals of dinickel diphosphate dihydrate, Ni₂(H₂O)₂[P₂O₇], have been synthesized by a hydrothermal method. Its structure was refined in the monoclinic P2₁/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) ų, and Z = 4) based on low-temperature X-ray diffraction data until R- 0.016. Corrugated chains of NiO₅(H₂O) 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 Me₂(H₂O)₂[X₂O₇] 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 Ni₂(H₂O)₂[P₂O₇]. The magnetic susceptibility χ evidences a short range correlation maximum at T_max = 11.9 K accompanied by the onset of long-range magnetic order at T_N = 9.4 K. Below T_N, 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_p and magnetization M studies, the magnetic phase diagram of Ni₂(H₂O)₂[P₂O₇] has been established.