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

K-39 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 K-39 resonance to lower frequency is observed with decreasing temperature which is due to an isotropic hyperfine interaction with A(iso) = -(0.30 +/- 0.05) x 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 K-39 shifts parallels that of the magnetic susceptibilities over the whole temperature range measured. (C) 1999 Elsevier Science B.V. All rights reserved.