Trapping of Holes and Excitons in Scintillators: CsI and ${\rm LaX}_{3}$ ( ${\rm X}={\rm Cl}$, Br)

We present computational results for trapping of polarons and excitons in undoped CsI, LaCl
₃
and LaBr
₃
, using plane-wave-pseudopotential density functional theory with Hartree-Fock exact exchange. The optimized V
K
center and STE in CsI is a distortion of two iodine atoms from the lattice to form an interstitial bound I
₂
^-
molecule, consistent with previous theoretical and experimental results. In both LaCl
₃
and LaBr
₃
, the relaxed STE configuration involves only one displaced halide ion, and does not form an X
₂
^-
. The calculated luminescence energy for the STE in LaCl
₃
and LaBr
₃
is 4.1 and 3.7 eV, respectively.