Origin of internal field and visualization of 180 degrees domains in congruent LiTaO3 crystals

We have observed a strong correlation between the internal fields and the infrared absorption spectra of OH- ions in congruent LiTaO3 crystals. After reversal of the direction of polarization (P-s) of the crystal by an electric field at room temperature, (a) the internal field is antiparallel to P-s, (b) the relative intensities of the infrared absorption bands of OH- ions (at similar to 3462, similar to 3476, and similar to 3490 cm(-1)) change, and (c) the 180 degrees domain walls are visible under a light microscope. Annealing the domain-reversed crystal at 200 degrees C or above and cooling back to 25 degrees C has the following effects: (a) The internal field realigns parallel to P-s. (b) The shape of the OH- spectrum returns to the shape observed before domain reversal. The time-temperature dependence of the shape of the infrared absorption spectrum of OH- ions also shows the same characteristics as that of the realignment process for the internal field. (c) The optical domain wall contrast becomes extremely weak. Infrared absorption studies suggest that domain inversion as well as the heating of LiTaO3 crystal induce structural changes that are similar in nature. It is proposed that the origin of internal fields lies in OH- ions and/or nonstoichiometric point defects in the LiTaO3 crystals. We also explain the optical 180 degrees domain wall contrast based on internal fields. (C) 1996 American Institute of Physics.