First-principles study of electronic, elastic, and optical properties of Yb:CaF₂ crystals

The electronic structure, elastic, and optical properties of CaF₂ and Yb-doped CaF₂ are investigated by the first-principles calculations. The obtained lattice parameters, band structure, density of state, elastic constants, and imaginary part of dielectric function ε₂ for CaF₂ agree well with the experimental and other theoretical results. A good comparison of the properties is made between CaF₂ and Yb:CaF₂, and it is found that: firstly, Yb doping leads to the crystal structure distortion of CaF₂ and reduction of the elastic constants. Secondly, an impurity band is found at the top of valence band in the band structure of Yb:CaF₂ and contributes to the bandgap narrowing. Thirdly, the peaks at about 26 eV in the optical curves disappear after Yb doping, and the remaining two peaks shift towards the longer-wavelength side, with the peak intensities drop down, indicating that Yb doping makes the ultraviolet transmittance of the system improve and the range of light in the UV region widen. Furthermore, Yb doping induces new peaks in the optical curves, which may be caused by 4f electron transitions of Yb atom. Finally, the transmittance of the doped system increases when the Yb-doped proportion decreases.