Ab-initio cluster calculations of hole formation and trapping in PbF2 and PbF4

We have used ab-initio quantum chemistry computer codes to model the formation of holes and the energy barriers for their diffusion in two lead fluoride hosts of potential interest for scintillation-PbF₂ and PbF₄. The crystals were modeled by Pb₂₄F₄₈ and Pb₁₄F₅₆ atomic clusters embedded in arrays of several thousand point charges to reproduce the Madelung potential to an accuracy of several mV throughout the cluster. Cubic PbF₂ has the same crystal structure as CaF₂, however their electronic structures are different. It is known experimentally that in CaF₂ holes travel easily along rows of F atoms which accounts for the high luminous efficiency of the scintillator CaF₂:Eu. In contrast, these calculations show that in PbF₂ holes are trapped on the Pb atoms by an energy barrier of ~1 eV. This result is consistent with the failure of PbF₂ as an activated scintillator. Similar calculations on the experimentally unexplored crystal PbF₄ predict that the holes are trapped on F atoms with an energy barrier of ~1 eV and is therefore not a promising host for an activated scintillator. These computational techniques can be applied to other crystals to find those with mobile holes for new heavy-atom scintillators and solid-state detectors