Luminescent, optical and electronic properties of La₃Ta_0.5Ga_5.5O₁₄ single crystals grown in different atmospheres

Luminescent, optical and electronic properties of La₃Ta_0.5Ga_5.5O₁₄ single crystals grown in different atmospheres are presented. The absorption bands at 255, 290, 350 and 480 nm were detected; the intensity of bands increases with the concentration of oxygen in the growth atmosphere. It is shown that the shift of the fundamental absorption edge with the temperature obeys Urbach rule. The corresponding fitting allowed to estimate the slope coefficient σ=0.35, which implies self-trapping of excitons in La₃Ta_0.5Ga_5.5O₁₄. Calculations of the band structure, partial densities of states and reflectivity spectra were performed. The bandgap of La₃Ta_0.5Ga_5.5O₁₄ was determined as E_g=5.6 eV. The luminescence properties under UV, VUV and X-ray excitation were studied. Intrinsic emission band at 440-450 nm is attributed to the excitons self-trapped at TaO₆ molecular complexes. Extrinsic emission bands at 410, 440 and 550 nm are attributed to the emission of excitons trapped by antisite defects, F-centers and oxygen deficient oxyanionic complexes.