39K NMR and EPR study of multiferroic K3Fe5F15

39K NMR spectra and relaxation times of polycrystalline K3Fe5F15 have been used as a microscopic detector of the local magnetic fields at the magnetic transition at TN = 123 K. The NMR lineshape widens abruptly upon crossing TN due to the onset of internal magnetic fields, while we find no significant lineshift. The paraelectric to ferroelectric transition at Tc = 490 K and the magnetic transition at TN have also been studied using X-band EPR (electron paramagnetic resonance). An increase and subsequent decrease in the EPR susceptibilities is observed on approaching TN from above. There is also a significant increase in the linewidth. At the same time the g-factor first decreases and then increases with decreasing temperature. The local magnetic field is different at different K sites and is much smaller than the magnetic field around the Fe sites. This seems to be consistent with the behaviour of a weak ferrimagnet. The ferrimagnetism does not seem to be due to spin canting as the lattice is disordered, but may arise from thermal blocking of superparamagnetic percolation clusters. The ferroelectric transition at Tc shows no electronic anomaly, demonstrating that we are dealing with a classical phonon anomaly as found in conventional oxides rather than an electronic transition.