Optical and electronic properties of undoped La sub(2)Be sub(2)O sub(5) single crystals in the far ultraviolet energy range

The optical and electronic properties of undoped La sub(2) Be sub(2) O sub(5) single crystals were determined using low-temperature (T = 10 K) far ultraviolet (4.9-33 eV) synchrotron radiation spectroscopy, optical absorption spectroscopy (T = 80 and 320 K), and calculations for the dispersions of the optical functions. On the basis of the obtained data we determined the Urbach's formula parameters and the cut-off energy E sub(c) = 5.49 eV at 80 K for the low-energy tail of the host absorption, the bandgap E sub(g) = 6.78 eV at 10 K, the energy threshold for the creation of unrelaxed excitons E sub(ex) = 6.28 eV, and other electronic properties.