The strong dependence of the properties of ferroelectric lithium niobate crystals on the concentration of intrinsic (non‐stoichiometric) and extrinsic (impurity) defects is analyzed. Spectra of optical absorption and magnetic resonance spectroscopy are compared for crystals with different compositions. The results show clearly that the possibility to vary the concentration of intrinsic defects offers extraordinary informative opportunities for the investigation of the fundamental physics of the material. © 2003 American Institute of Physics