Lithium niobate: Crystal-lattice defects and surface acoustic waves

Ferroelectric poly-domain Czochralski-grwon LiNbO3 single crystals become single domain by applying a poling field. In YZ-LiNbO3, however, domains remain pinned at small angle grain boundaries after the crystal has been poled. The dislocations in these grain boundaries were imaged in the transmission electron microscope and identified by computer simulation. Thus knowing dislocation directions and Burgers-vectors, the electric fields around a dislocation in any piezoelectric medium can be calculated. Stroboscopic synchrotron topography is a new method developed for time resolved imaging of lattice distortions caused by the surface acoustical waves (SAW) or spurious bulk waves. The contrast formation in the Bragg case can be divided into two parts. One contribution is the surface reflected beam which shows intensity modulation due to the periodic curvature of the reflecting net planes. Throughs of the SAW focus the reflected beam and crests defocus it. The focal distance depends on the Bragg angle, the wave amplitude and the angle between the acoustic wave vector and the plane of incidence. The second contribution to the image is coming from wavefields which first enter the crystal and are then deviated back to the entrance surface by the strain fields of the SAW. Experimental evidence and theoretical explanation of the image formation are given in this work. (Author)