Single-crystal pulsed neutron diffraction from NiF₂and FeF₂: the effect of magnetic order on the fluorine positions

Extended single-crystal data sets have been collected by means of pulsed neutron diffraction from NiF2 at 295, 80 and 15 K, and from FeF2 at 295 and 15 K to examine the effect of magnetic order on the fluorine nuclear positions. A predicted internal displacement has been validated in NiF2, whereas in FeF2 it could not be established unambiguously. The positional parameters are compared with those obtained from gamma-ray diffraction data, recorded from the same samples. In view of the excellent agreement there is no evidence for any polarization effects in the antiferromagnetic phase.