It was noted that the typical output of 1-frequency methods was an image with a space resolution of the order of the wavelength used. Such images contained direct evidence about the space structure of a lattice or macroscopic defects. However, they were not very helpful for the study of the internal structure of atomic-size defects. Measurements which used sweeping of electromagnetic wave frequency or sweeping of an external parameter furnished spectra or dependences of the measured characteristics upon the swept quantity. The spectra contained indispensable information about the energetic characteristics of interactions of intrinsic and extrinsic defects with the lattice ions. Different frequencies in a very wide region had to be used for the study of these interactions (multi-frequency spectroscopy). An application of this new approach to defects in lithium niobate permitted the determination of the characteristics of various interactions with magnitudes from 104 to 1016Hz, and to correlate them with definite models for the defects.

Multifrequency Spectroscopy of Defects in Complex Oxides. G.Malovichko, V.Grachev, O.F.Schirmer: Physica Status Solidi A, 2005, 202[2], 207-12