We study the formation, mobility, and stability of self-trapped excitons (STE) and self-trapped holes and electrons in NaI and NaI(Tl) using embedded cluster hybrid density functional theory calculations. This method employs an array of classical charges to provide an environment simulating the interior of an ionic solid in which the electronic structure of a modestly sized quantum-mechanical cluster is computed including nonlocal exchange effects which are necessary to describe localized excitations in NaI. In contrast with previous models, we find that both carriers in pure NaI have similar mobilities, with an activation energy of $\sim 0.2$ eV. We propose an alternate interpretation including a new migration mechanism for the STE. In Tl-doped material excitons preferentially trap at dopants, inducing off-center distortions that have a structure unlike an STE and provide a mechanism for light emission at multiple wavelengths.

• Received 9 August 2012

DOI:https://doi.org/10.1103/PhysRevB.87.115101