Abstract: In order to obtain large, high-quality crystals or crystal layers of a me orthophosphate, in particular, GaPO.sub.4 or AlPO.sub.4, from a nutrient solution with the use of a seed plate, the proposal is put forward that in the initial phase of the growth process a seed plate of alpha-quartz (.alpha.-SiO.sub.2) be introduced into the nutrient solution, and that fluoride ions (F.sup.-) be added to this solution, at least for formation of the primary crystal layer on the quartz seed plate.

Abstract: In order to prevent crystal seeds with retrograde solubility from dissolving in the hydrothermal growing solution at the beginning of the growing process, and to protect the finished crystal at the end of the growing process, and in order to reach the thermodynamic growth equilibrium as soon as possible, the proposal is put forward that the growing solution be prepared, pre-saturated and pre-heated to a temperature close to saturation temperature outside of the growing tank, and that, for the growing process proper, the hot growing solution be added to the pre-heated growing tank provided with crystal seeds, where it should be further heated to a temperature at which the crystals start growing, and, further, that the hot growing solution be quickly removed from the growing tank as soon as the growing process has been completed.

Abstract: In order to prevent crystal seeds with retrograde solubility from dissolving in the hydrothermal growing solution at the beginning of the growing process, and to protect the finished crystal at the end of the growing process, and in order to reach the thermodynamic growth equilibrium as soon as possible, the proposal is put forward that the growing solution be prepared, pre-saturated and pre-heated to a temperature close to saturation temperature outside of the growing tank, and that, for the growing process proper, the hot growing solution be added to the pre-heated growing tank provided with crystal seeds, where it should be further heated to a temperature at which the crystals start growing, and, further, that the hot growing solution be quickly removed from the growing tank as soon as the growing process has been completed.

Abstract: In order to reduce the water content in piezoelectric GaPO.sub.4 crystal elements the proposal is put forward that each GaPO.sub.4 crystal element should be cut in its entirety from one uniform growth zone of a GaPO.sub.4 raw crystal bar and should not contain any zone boundaries, and that the piezoelectric GaPO.sub.4 crystal elements should be treated with a dehydrating solution, until an infrared coefficient of .alpha.<1.5 cm.sup.-1 is obtained at a wavelength in the vicinity of 2.0 .mu.m.