ИСТИНА

The crystals of La3Ta0.5Ga5.5O14 (langatate) are applied as the piezoelectric sensors and also considered for application as the nonlinear crystal for tunable lasers and the host for rare-earth doped phosphors [1]. In spite of the extensive studies of optical properties the origin of the defects in langatate is not clear. Study of luminescent properties can provide data for the identification of the origin of defect states in langatate, however this method has not been applied for such purpose so far. The complex investigation of luminescent, optical and electronic properties of the langatates grown in different atmospheres has been performed for the determination of the origin of defects. The crystals were grown by the Czochralski method in the atmospheres of pure argon and argon with admixture of 0.5% or 2% of O2. The absorption and reflection spectra as well as luminescence characteristics were studied in the temperature range 4.2 – 500 K. The first - principles calculations of the structural, electronic, and optical properties were performed using the CASTEP module of Materials Studio package. The results of band structure calculations were verified using experimental reflection spectra. The absorption bands at 255, 290, 350 and 490 nm were detected, which intensity increases when growth atmosphere contains oxygen. The shift of the fundamental absorption edge with the temperature was fitted using Urbach formula. The fit allowed to estimate the bandgap of langagate, Eg = 5.68 eV and slope coefficient σ = 0.35, which indicates strong self-trapping of excitons. Several luminescence bands were detected. The band at 440 nm is attributed to the intrinsic luminescence of excitons self-trapped at TaO6, which electronic states form the vicinity of bandgap according to the calculations. Defect-related emission bands were detected at 420, 440 and 550 nm. The bands are attributed to the emission from F and F+ centers. The involvement of antisite defects into luminescence process is discussed. 1. S. Georgescu, et al., Phys. B: Cond. Mat. 407 (2012) 1124; E. Boursier, et al., Opt. Let. 39 (2014) 4033.