The crystalline quality of GaPO4 samples grown in different conditions, often selected to reduce the OH concentration, have been studied using the X-ray topographic technique with the synchrotron radiation delivered at the L.U.R.E. (Orsay, France). During this study, the high level of crystalline quality observed for some samples has indicated that gallium phosphate crystals could reach a perfection comparable to quartz. The Q factors and the piezoelectric properties of plates cut in the same crystals were measured. The results were compared with others obtained with older crystals and with results computed using the most recent constants available. This comparison indicates that the very reduced OH concentrations now obtained leads to an increase of the shear velocities and to very reduced variations of the insertions losses with temperature.
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