Dynamic and static contributions to the zero-field splitting term of divalent nickel in fluosilicate crystals

Previously published electron paramagnetic resonance (EPR) data for Ni2+(3d7) in hydrated and deuterated crystals of zinc and nickel fluosilicate have been re-analyzed to separate the static and dynamic contributions to the fine-structure (or axial zero-field-splitting) term D in the spin-Hamiltonian. While the Debye parameters of ZnSiF6·6H2O and ZnSiF6·6D2O were determined in an earlier low-temperature study1, those of the undiluted nickel crystals, for which good data between 4 and 77 K is lacking, have been estimated here by comparison of the data above 60 K with those for the corresponding zinc compounds. The numerical results indicate that the measured values of D are composed of a negative dynamical contribution produced by the rotational motions of the water ligands, together with both negative and positive static contributions produced by the water ligands, and the more distant [SiF6]2− and [Ni.6H2O]2+ complexes, respectively. A point-charge calculation shows the contribution of the latter to be roughly +2.5 cm−1 in NiSiF6·6H2O. The exceptionally large temperature variations of D in these crystals is attributed to the near-cancellation of the positive and negative static contributions, leaving the effect of the dynamical contribution dominant.