Theoretical and experimental study on the electronic and optical properties of K 0.5 Rb 0.5 Pb 2 Br 5 : a promising laser host material.

The data on the electronic structure and optical properties of bromide K 0.5 Rb 0.5 Pb 2 Br 5 achieved by first-principle calculations and verified by X-ray spectroscopy measurements are reported. The kinetic energy, the Coulomb potential induced by the exchange hole, spin-orbital effects, and Coulomb repulsion were taken into account by applying the Tran and Blaha modified Becke-Johnson function (TB-mBJ), Hubbard U parameter, and spin-orbital coupling effect (SOC) in the TB-mBJ + U + SOC technique. The band gap was for the first time defined to be 3.23 eV. The partial density of state (PDOS) curves of K 0.5 Rb 0.5 Pb 2 Br 5 agree well with XES K Ll and Br Kβ 2 , and XPS spectra. The valence band (VB) is characterized by the Pb-5d 3/2 and Pb-5d 5/2 sub-states locating in the vicinities of -20 eV and -18 eV, respectively. The VB middle part is mainly formed by K-3p, Rb-4p and Br-4s states, in which the separation of Rb-4p 3/2 and Rb-4p 1/2 was also observed. The strong hybridization of Br-p and Pb-s/p states near -6.5 eV reveals a major covalent part in the Br-Pb bonding. With a large band gap of 3.23 eV, and the remarkably high possibility of inter-band transition in energy ranges of 4-7 eV, and 10-12 eV, the bromide K 0.5 Rb 0.5 Pb 2 Br 5 is expected to be a very promising active host material for core valence luminescence and mid-infrared rare-earth doped laser materials. The anisotropy of optical properties in K 0.5 Rb 0.5 Pb 2 Br 5 is not significant, and it occurs at the extrema in the optical spectra. The absorption coefficient α ( ω ) is in the order of magnitude of 10 6 cm -1 for an energy range of 5-25 eV.