Many frequently used or promising piezoelectric materials belong to crystal classes 32 or 3m. Among them are ?? quartz and its crystallographic analogs (AlPO4, GaPO4, ;1;-GeO2, etc.), the numerous materials of the langasite (La-3Ga5SiO,sub>14) family and also lithium tantalate (LiTaO,sub>3) and lithium niobate (LiNbO3). In this paper we study the present state of the art for these materials, indicate their principal point and extended defects, and present methods to reduce the dislocation density. Large concentrations of intrinsic point defects often exist in crystal grown at very high temperatures. The point defects (intrinsic or related to impurities) modify the constants and can increase the acoustic losses. This is the case for the alkali ions and the OH that induce severe losses in different temperature intervals. The extended defects also affect the performances of the piezoelectric devices. Some, such as twins, ferroelectric domains, or large solid or liquid inclusions, have very detrimental effects. Dislocations, growth bands, and planar defects are more difficult to avoid and affect the devices in a more subtle manner. In quartz and its analogs, dislocations seem to increase the nonlinear elastic effects and have a collective effect on the vibration modes, particularly in energy trapping resonators. Growth bands and stacking faults also produce similar effects.
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