Theoretical and experimental study on the electronic and optical properties of K0.5Rb0.5Pb2Br5: a promising laser host material

Tuning Phonon Energies in Lanthanide-doped Potassium Lead Halide Nanocrystals for Enhanced Nonlinearity and Upconversion

Optical applications of lanthanide-doped nanoparticles require materials with low phonon energies to minimize nonradiative relaxation and promote nonlinear processes like upconversion. Heavy halide hosts offer low phonon energies but are challenging to synthesize as nanocrystals. Here, we demonstrate the size-controlled synthesis of low-phonon-energy KPb₂ X₅ (X=Cl, Br) nanoparticles and the ability to tune nanocrystal phonon energies as low as 128 cm^-1 . KPb₂ Cl₅ nanoparticles are moisture resistant and can be efficiently doped with lighter lanthanides. The low phonon energies of KPb₂ X₅ nanoparticles promote upconversion luminescence from higher lanthanide excited states and enable highly nonlinear, avalanche-like emission from KPb₂ Cl₅  : Nd³⁺ nanoparticles. The realization of nanoparticles with tunable, ultra-low phonon energies facilitates the discovery of nanomaterials with phonon-dependent properties, precisely engineered for applications in nanoscale imaging, sensing, luminescence thermometry and energy conversion.

Theoretical and experimental study on the electronic and optical properties of K0.5Rb0.5Pb2Br5: a promising laser host material

The data on the electronic structure and optical properties of bromide K_0.5Rb_0.5Pb₂Br₅ 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_0.5Rb_0.5Pb₂Br₅ agree well with XES K Ll and Br Kβ₂, 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_0.5Rb_0.5Pb₂Br₅ 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_0.5Rb_0.5Pb₂Br₅ 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 10⁶ cm⁻¹ for an energy range of 5–25 eV.

The data on the electronic structure and optical properties of bromide K_0.5Rb_0.5Pb₂Br₅ achieved by first-principle calculations and verified by X-ray spectroscopy measurements are reported.