Optical and electron energy loss experiments in ionic CrBr3 crystals

Abstract

The fundamental reflectance of CrBr3 crystals has been measured over the energy range 2-31 eV from 300 to 30 K using synchrotron radiation. The high-frequency dielectric tensor elements epsilon xxT(E) have been determined by Kramers-Kronig analysis. Energy losses of 50 keV electrons incident along the c axis of the crystal have also been measured up to 30 eV. The results have been described in terms of single-particle excitations (charge transfer, inter-band excitons and transitions) and collective valence electron plasma oscillations. The assignments of direct inter-band transitions both at the critical points Gamma and Z and along symmetry lines of the Brillouin zone ( Lambda line) have been made by considering the intersecting-sphere model calculation of the band structure of CrBr3. The fundamental energy gap, assigned to Gamma 3- to Gamma 1+ transitions, at the zone centre is observed around 8.00 eV. Plasma resonance has been observed at 19.8 eV in the electron energy loss spectrum in fine agreement with the maximum of the optical energy loss function. Finally, the fractional ionic character fiDT=0.79 has been evaluated in the framework of the Phillips dielectric theory.