Development of atomistic models to aid the design of new scintillator materials. (2010)

The development of more reliable scintillator materials can significantly advance the gamma-ray detection technology. Scintillator materials such as lanthanum halides (e.g., LaBr{sub 3}, CsBr{sub 3}), elpasolites (e.g., Cs{sub 2}LiLaBr{sub 6}, Cs{sub 2}NaLaBr{sub 6}, and Cs{sub 2}LiLaI{sub 6}), and alkali halides (e.g., CsI, NaI) are extremely brittle. The fracture of the materials is often a problem causing the failure of the devices. Lanthanum halides typically have a hexagonal crystal structure. These materials have highly anisotropic thermal and mechanical properties, and therefore they are likely to fracture under cyclic thermal and mechanical loading conditions. For example, fracture of lanthanum halides is known to occur in the field. Fracture during synthesis also complicates the growth of large lanthanum halide single crystals needed for sensitive radiation detection, and accounts for the high production cost of these materials. Elpasolites can have both cubic and non-cubic crystal structures depending on the constituent elements and composition of the compounds. This provides an opportunity to design cubic elpasolites with more isotropic properties and therefore improved mechanical performances. However, the design of an optimized cubic elpasolite crystal remains elusive because there is a tremendous number of possible elpasolites and the design criterion for cubic crystals is not clear. Alkalimore » halides have cubic crystal structures. Consequently, large CsI and NaI crystals have been grown and used in devices. However, these materials suffer from an aging problem, i.e., the properties decay rapidly over time especially under harsh environment. Unfortunately, the fundamental mechanisms of this aging have not been understood and the path to improve the alkali halide-based scintillators is not developed. Clearly, improved scintillator materials can be achieved via strengthened/toughened lanthanum halides, optimized cubic elpasolites, or new alkali halide-based crystals that are more resistant to aging. Without a fundamental understanding of the atomic origins of the mechanical and the thermodynamic properties of materials, past experimental efforts to develop improved scintillator materials have been prolonged. Here we report our recent progress on the development of atomistic models that can be used to accelerate the discovery of new scintillator materials with improved properties. First, we have developed a novel embedded-ion method interatomic potential approach that analytically addresses the variable charge interactions between atoms in ionic compound material systems. Based on this potential, molecular dynamics simulations have been used to study the mechanical properties of LaBr3 including slip systems, dislocation core structures, and material strength. We have also developed an atomistic model that can already be used to predict crystal structures and to derive crystal stability rules for alkali halides. This model is under further development for prediction of crystal structures of elpasolites. These efforts will facilitate the design of better scintillator materials.« less