EPR spectrometer comprising a ferroelectric single crystal resonator, a rectangular channel with holes in its narrow sides for inserting a sample and wires, a permanent magnet with planar poles, and a coil within the channel connected through the holes by the wires with a modulation source and a NMR spectrometer comprising a magnet, a probe with means for transmitting a radio frequency magnetic pulse and detecting NMR signal, and a ferroelectric single crystal resonator. A ferroelectric single crystal consisting essentially of potassium tantalate and the alkali metal substituting from about 1% to 3% of potassium or a group (V) metal substituting in part for tantalum, having cubic form of perovskite crystalline structure, and essentially free of impurities and defects is also disclosed.