First-principles study of codoping in lanthanum bromide

Abstract

Co-doping of Ce-doped LaBr3 with Ba, Ca, or Sr improves the energy resolution that can be achieved by radiation detectors based on these materials. Here, we present a mechanism that rationalizes of this enhancement that on the basis of first principles electronic structure calculations and point defect thermodynamics. It is shown that incorporation of Sr creates neutral VBr − SrLa complexes that can temporarily trap electrons. As a result, Auger quenching of free carriers is reduced, allowing for a more linear, albeit slower, scintillation light yield response. Experimental Stokes shifts can be related to different CeLa − SrLa − VBr triple complex configurations. Co-doping with other alkaline as well as alkaline earth metals is considered as well. Alkaline elements are found to have extremely small solubilities on the order of 0.1 ppm and below at 1000 K. Among the alkaline earth metals the lighter dopant atoms prefer interstitial-like positions and create strong scattering centers, which has a detrimental impact on carrier mobilities. Only the heavier alkaline earth elements (Ca, Sr, Ba) combine matching ionic radii with sufficiently high solubilities. This provides a rationale for the experimental finding that improved scintillator performance is exclusively achieved using Sr, Ca, or Ba. The present mechanism demonstrates that co-doping of wide gap materials can provide an efficient means for managing charge carrier populations under out-of-equilibrium conditions. In the present case dopants are introduced that manipulate not only the concentrations but the electronic properties of intrinsic defects without introducing additional gap levels. This leads to the availability of shallow electron traps that can temporarily localize charge carriers, effectively deactivating carrier-carrier recombination channels. The principles of this mechanism are therefore not specific to the material considered here but can be adapted for controlling charge carrier populations and recombination in other wide gap materials.

First-principles study ofCe3+-dopedlanthanum silicate nitride phosphors: Neutral excitation, Stokes shift, and luminescent center identification

“…[18][19][20][21][22][23] Compared to the semi-empirical model, they might also have advantages in the design of phosphors: first, such theoretical simulations can provide a detailed understanding of interactions and effects involved in the optical processes; second, without relying on empirical information they have the potential to precisely simulate 4f → 5d neutral transition in new potential materials, not only for the absorption process but also for the emission process. Still, a routine use of first-principle methods might not yield the sought understanding.…”

Defect Engineering by Codoping in KCaI3:Eu2+ Single-Crystalline Scintillators

“…As described in Ref. 41, a relatively low concentration of codoping may have a large impact on the defect formation and hence the luminescent properties of the scintillators through the formation of complex defects. Because all the substitution and interstitial defects introduced by the codoping elements are electron donors, it is reasonable to limit our study of the complex defects to those intrinsic defects forming acceptors.…”

“…To compensate for the spurious interactions between the defects in neighboring cells, we used the finite-size correction scheme based on a multipole expansion [41,42] …”

First-principles search for efficient activators for LaI3

“…Consequently, the positions of the energy levels of the dopants calculated from first-principles methods can provide valuable guidance in the search for bright scintillators. Similar approaches can also be employed to investigate co-doping schemes that may lead to improved resolution [14][15][16]. In this work, this approach was applied to LaI 3 using density functional theory (DFT) with Hubbard corrections or hybrid exchange-correlation functionals, whereby the Hubbard on-site correction and the fraction of exact exchange were calibrated to experimental data on pure and Ce-doped LaI 3 and then employed to simulate a range of dopants previously untested.…”