  Scintillators attract wide research interest for their distinct applications
in radiation detection. Elpasolite halides are among the most promising
scintillators due to their high structural symmetry and good scintillation
performance. Better understanding of their underlying scintillation mechanism
opens up new possibilities in scintillator development. In this work, we employ
a variety of experimental techniques to study the two mixed-anion elpasolites
Cs2NaRBr3I3 (R = La, Y). The emission of intrinsic Cs2NaRBr3I3 with a light
yield ranging from 20,000 to 40,000 ph/MeV is dominant by self-trapped exciton
emission. Partial substitution of R with Ce introduces a competing emission,
the Ce3+ 5d to 4f radiative transition. Ab initio calculations were performed
to investigate the electronic structures as well as the binding energies of
polarons in Cs2NaRBr6. The calculated large self-trapped exciton binding
energies are consistent with the observed high light yield due to self-trapped
exciton emission. The unique electronic structure of halide elpasolites as
calculated enhances the STE stability and the STE emission. The highly tunable
scintillation properties of mixed-anion elpasolites underscore the role of
their complex scintillation mechanism. Our study provides guidance for the
design of new elpasolites scintillators with exceptional energy resolution and
light yield desirable for applications.
