Optical and electron energy loss experiments in ionic CrCl3crystals

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Abstract
The low-temperature reflectance of CrCl3 crystals has been measured in the ultraviolet region up to 31 eV with synchrotron radiation, and the high-frequency dielectric tensor elements epsilon xxT(E) have been deduced by Kramers-Kronig analysis. Energy losses of 50 keV electrons incident along the c axis of the crystal have also been measured as far as 80 eV. The results of the optical and electron loss experiments are described in terms of single-particle (charge transfer and interband transitions, excitons and inner core excitations) and collective excitations (valence-electron plasma oscillations). The energy gap Eg approximately=9.7 eV is assigned to direct allowed transitions Gamma -3 to Gamma 1+ at the zone centre; higher energy strong interband transitions occur at the symmetry points Gamma and Z, and along symmetry lines of the Brillouin zone ( Lambda line). The plasma resonance has been observed at 22.6+or-0.2 eV in the electron loss spectrum in excellent agreement with the maximum of the optical energy loss function. The high-energy losses observed beyond 40 eV have been attributed to Cr3+ 3p-shell excitations. Finally, the fractional ionic character (fiDT approximately=0.80-0.85) of CrCl3 has been evaluated in the framework of the Phillp's dielectric theory.