CHARGE-DENSITY ANALYSES OF THE ANTIFERROMAGNETS NIF2 AND FEF2 BY MEANS OF GAMMA-RAY DIFFRACTION

Accurate single-crystal diffraction data have been collected from NiF2 at room temperature and in the antiferromagnetic state at 15 K, and from antiferromagnetic FeF2 at 11 K. The gamma-ray wavelength used was lambda=0.0392 angstrom, and the maximum value of sin theta/lambda was 1.40 angstrom-1. Different wavelengths have been applied to correct the strongest reflections of NiF2 for extinction. On magnetic ordering in NiF2 a shift of the fluorine ions of 3 X 10(-3) angstrom is found; this confirms a prediction deduced from optical birefringence. In all cases, the centroids of the atomic charge density agree excellently with the nuclear positions determined by neutron diffraction. The static charge density is described by a rigid pseudoatom model; the resulting occupancies of the metal 3d orbitals agree with expectations from crystal-field theory; magnetic ordering hardly affects the density deformations in NiF2. The critical points of the chemical bonds have been determined to obtain information about possible covalency.