Specific features of Yb3+ ions in electronic band energy structure and optical functions of RbNd(WO4)2 crystals: Synchrotron ellipsometry measurements and DFT simulations

The influence of 5% and 10% (in molar units) of ytterbium doping of RbNd(WO4)2 single crystals on the dielectric permittivity ε in the spectral range of electronic excitations was studied. The corresponding differences of the pseudo-dielectric functions 〈ε(E)〉 for pure and Yb-doped RbNd(WO4)2 were studied by spectroscopic ellipsometry method including VUV spectral range using synchrotron radiation light source. This method opens an opportunity for spectroscopic diagnostics of the rare earth dopants in crystals using information concerning the electronic inter-band optical transitions. To better understand the experimental results, for the first time the ab initio calculations of band structure, related properties and optical spectra of the centrosymmetric RbNd(WO4)2 single crystal were performed using the VASP code. The calculated real and imaginary parts of dielectric function dispersions ε1(E) and ε2(E) agree satisfactorily with our experimental results. It was established that differences of optical functions for pure and doped crystals may serve as a powerful tool for spectroscopic diagnostic of localized rare earth ions with respect to the strong inter-band transitions in the wide band dielectrics.