Optical and Electronic Properties of Undoped $La_2Be_2O_5$ Single Crystals in the Far Ultraviolet Energy Range

The optical and electronic properties of undoped $La_{2}Be_{2}O_{5}$ single crystals were determined using low-temperature ($\mathit{T}$=10 K) far ultraviolet (4.9–33 eV) synchrotron radiation spectroscopy, optical absorption spectroscopy (=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 $\mathit{E_{e}}$=5.49 eV at 80 K for the low-energy tail of the host absorption, the bandgap $\mathit{E_{g}}$=6.78 eV at 10 K, the energy threshold for the creation of unrelaxed excitons $\mathit{E_{ex}}$=6.28 eV, and other electronic properties