Study of gallium phosphate and langasite crystals and resonators by X-ray topography

The gallium phosphate (GaPO₄) is, as berlinite, a close crystallographic analogue of quartz with the advantage that it has no α-β phase transition. It was shown that this material has very attractive properties for the applications to piezoelectric devices, due to its large coupling coefficients and to the existence of compensated cuts. Furthermore, it can be used up to extremely high temperatures (900°C). To obtain much larger crystals, epitaxial growth of GaPO₄ on berlinite plates with several orientations was performed. The langasite crystals (LGS: La₃Ga₅SiO₁₄) were obtained by the Czochralski method. This new material has probably one compensated cut with a large coupling coefficient and very reduced angular sensitivity. Using the synchrotron radiation delivered by the DCI storage ring at the LURE (Orsay, France), we have studied by the X-ray topography technique the crystalline perfection of new GaPO₄ samples and different langasite crystals of different sources. For this study we have used traverse and section topography