Covalency in NiF2from magnetisation density data and from ab initio theory

Abstract

The polarised neutron diffraction data for NiF₂ have been reanalysed in terms of a covalent valence orbital population model in which the canting angles that the directions of the magnetic moment on the nickel atom, on the fluorine atom, and on the mid-bond overlap population, make with the magnetic field are all allowed to be different. A much better fit to the full data is obtained than for an earlier crystal field model, chi ²=3.8 versus 70. The Ni-F bonding is deduced to be surprisingly covalent; 28 % of the spin appears to be delocalised onto the fluorine atoms, with much of it cancelled by the antiferromagnetism, and with large overlap populations. The canting of the moment is found to be critically dependent on the details of the Ni-F bonding, and is very different on the Ni and on the F sites. An ab initio calculation on a (NiF₆)^4- unit to simulate the crystal environment, using the DV-X_alpha local density method, confirms the high covalence in the Ni-F bonding and gives insight into the relative sizes of the ferro- and antiferromagnetic components of the magnetic moments.