Changes in defects under external influence in langasite crystals

In the growing process of multicomponent piezocrystalline crystals (La₃Ga₅SiO₁₄), even minor deflections from optimal growth conditions lead to changes in defect concentration and, consequently, to changes in the properties of the whole crystal. A comprehensive study of the crystalline lattice using X-ray structural analysis and optical spectroscopy shows the formation of point defects - vacancies for oxygen and lanthanum, substitution of gallium by silicon and vice versa - that correspond with quasichemical reactions. In the spectrum of optical transmission, the band 35000 cm^-1 is connected with a lanthanum vacancy (V_LA´´´), and bands 28500, 26000, 25000 cm^-1 with oxygen vacancies in three positions. Under γ-radiation to a dose of 200 C/kg, optical spectra are observed to increase absorption in the region 40000-30000 cm^-1, but which disappear at the annealing temperature of 300°C. The value of absorption at frequency 40000 cm^-1 can be used for the primary crystal quality assessment.