Assessment of material perfection and observation of vibration modes in lithium tantalate by X-ray topography

The extended defects of lithium tantalate (LiTaO₃) have been studied by reflection and transmission topography using the white radiation delivered by the DCI Synchrotron at LURE (Orsay, France). Some low-angle subgrains were observed in slices cut perpendicularly to the Y, Y - 45° and Z directions. The typical size of the subgrains is a few cm³. Dislocations were imaged in thin plates; their density was found to be in the region of several hundred per cm². The high intensity of the synchrotron source has permitted an experimental study of the thickness vibration modes in this material, which is highly absorbent for X-rays. The two thickness shear modes existing in X-cut plates were observed using conventional and stroboscopic topography. The fundamental fast shear mode and its anharmonics display strong trapping properties. However, the slow shear mode is not strongly trapped and extends far from the electroded region. In most cases, the fast shear mode and its anharmonics are coupled to plate modes. The time structure of the radiation of the DCI Synchrotron has been used to show that, in most instances, these coupled modes include progressive components with a large standing-wave ratio. The existence of these progressive waves, characterized by a large amplitude, was observed in all the samples examined. They carry a fraction of the acoustic energy toward the mounting structure of the resonators and are the origin of the limited Q factors (a few thousands) observed in conventional X-cut resonators.