39K MAS NMR of KNiF3: unexpected spin density at the potassium and first MAS NMR experiments in the antiferromagnetic state

39K MAS NMR spectra of KNiF3 were measured at variable temperature between 180 and 450 K. In the paramagnetic state (450–270 K), a shift of the 39K resonance to lower frequency is observed with decreasing temperature which is due to an isotropic hyperfine interaction with Aiso=−(0.30±0.05)×10−6 cm−1. This is in contrast to theory which predicts zero unpaired electron density at the potassium. In the antiferromagnetic state (below 250 K), a reduction of the shift contribution from the unpaired electrons is observed. The temperature dependence of the 39K shifts parallels that of the magnetic susceptibilities over the whole temperature range measured.