Point defects in NiO observed via perturbed angular correlation spectroscopy

The perturbed angular correlation method has been applied to study the trapping and release of defects in polycrystalline NiO in the temperature range 300-1400 K. Radioactive 111In probes were implanted into NiO powder samples and oxidized Ni-foils at 400 keV. During annealing sequences at 600-1100 K, a total of eight non-vanishing electric field gradients (EFGs) was observed. Their annealing behaviour allowed a classification of six of them as radiation-induced defects. Two further EFGs found after annealing of the NiO-lattice were attributed to intrinsic defects trapped at the 111In tracers. Below the Neel temperature the PAC spectra are governed by combined (antiferro)magnetic and electric hyperfine interactions. Some of the defect complexes show smaller supertransferred fields which can be explained by missing next-neighbour ions. Point charge calculations were performed which allowed a modelling of these EFGs and suggest defect configurations. (author)