Role of Multiple Charge States of Ce in the Scintillation of ABO3 Perovskites

Ce-activated A(2+)B(4+)O(3) perovskites represent a class of compounds currently under active exploration for their potential as scintillators. Depending on the chemistry and synthesis conditions, perovskites can crystallize in multiple crystal structures, and a Ce substitutional dopant in an ABO(3) perovskite can adopt different charge states (i.e., Ce3+ or Ce4+) as well as different substitutional sites (namely, the 12-fold-coordinated A site or the octahedrally coordinated B site). Here, we use first-principles density-functional-theory-and hybrid-functional-based computations to study relative trends in the structure, energetics, and electronic structure of bulk ABO(3) perovskites, where A = Ca, Sr, or Ba and B = Hf or Zr. Subsequently, we consider the relative energetics of preferential solution sites for Ce as a function of charge states, chemical potential, and defect configurations. Our results reveal that while Ce3+ or Ce4+ defects can be thermodynamically stable, depending on the choice of the substitutional site and synthesis conditions (i.e., prevailing chemical potential), only Ce3+ dopant at the A site leads to an electronic structure that can exhibit scintillation. Our comparative analysis shows that while the positions of the (5)d(1) and 4f levels of Ce3+ as a dopant at the A site are favorably placed in the band structure, these levels are consistently higher for the Ce4+ charge state and are unlikely to manifest any luminescence. The findings of this study are also discussed in relation to previously reported results and display excellent agreement with past experimental observations. In general, it is demonstrated that control of the Ce charge state and local chemical environment can be used-in addition to band-gap and band-edge engineering-to manipulate the relative position of scintillating states with respect to the valence-band maximum and conduction-band minimum. While this study specifically focuses on perovskites, the results (in particular, the relative alignment of the positions of the 5d(1) and 4f levels of Ce dopant as a function of the activator's charge state) are expected to be general and thus transferable to other chemistries.