We propose a universal
approach to model intervalence
charge transfer
(IVCT) and metal-to-metal charge transfer (MMCT) transitions between
ions in solids. The approach relies on already well-known and reliable
ab initio RASSCF/CASPT2/RASSI-SO calculations for a series of emission
center coordination geometries (restricted active space self-consistent
field, complete active space second-order perturbation theory, and
restricted active space state interaction with spin-orbit coupling).
Embedding with ab initio model potentials (AIMPs) is used to represent
the crystal lattice. We propose a way to construct the geometries
via interpolation of the coordinates obtained using solid-state density
functional theory (DFT) calculations for the structures where the
activator metal is at specific oxidation (charge) states of interest.
The approach thus takes the best of two worlds: the precision of the
embedded cluster calculations (including localized excited states)
and the geometries from DFT, where the effects of ionic radii mismatch
(and eventual nearby defects) can be modeled explicitly. The method
is applied to the Pr activator and Ti, Zr, Hf codopants in cubic Lu2O3, in which the said ions are used to obtain energy
storage and thermoluminescence properties. Electron trap charging
and discharging mechanisms (not involving a conduction band) are discussed
in the context of the IVCT and MMCT role in them. Trap depths and
trap quenching pathways are analyzed.