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

We have used nb-initio quantum chemistry computer codes to model the format ion of holes and the energy barriers for their diffusion in two lead fluori de hosts of potential interest for scintillation-PbF2 and PbF4. The crystal s were modeled by Pb24F48 and Pb14F56 atomic clusters embedded in arrays of several thousand point charges to reproduce the Madelung potential to an a ccuracy of several mV throughout the cluster. Cubic PbF2 has the same cryst al structure as CaF2, however their electronic structures are different. It is known experimentally that in CaF2 holes travel easily along rows of F a toms which accounts for the high luminous efficiency of the scintillator Ca F2:Eu. In contrast, these calculations show that in PbF2 holes are trapped on the Pb atoms by an energy barrier of similar to 1 eV. This result is con sistent with the failure of PbF2 as an activated scintillator. Similar calc ulations on the experimentally unexplored crystal PbF4 predict that the hol es are trapped on F atoms with an energy barrier of similar to 1 eV and is therefore not a promising host for an activated scintillator. These computa tional techniques can be applied to other crystals to find those with mobil e holes for new heavy-atom scintillators and solid-state detectors.