Predicting from first principles the chemical evolution of crystalline compounds due to radioactive decay: The case of the transformation of CsCl to BaCl

In this Brief Report, we use density functional theory to predict the existence of a heretofore unobserved crystalline compound, BaCl, and additionally predict it to be isostructural with NaCl (rocksalt). Due to the chemistry of Ba, which strongly prefers a 2+ charge state, compounds where Ba nominally exhibits a +1 charge (e.g., BaCl) are unlikely to be synthesized via conventional solid-state approaches. However, in considering the chemical evolution of Cs-137 to Ba-137 via beta(-) radioactive decay in a model nuclear waste form CsCl, we find that BaCl may be indeed relevant. The mechanical stability of this surprising structure is confirmed through examination of its elastic constants and phonon-dispersion relations. We have also analyzed the chemical bonding of rocksalt BaCl and found it to exhibit a complex mixture of ionic, metallic, and covalent characters. From our results, we demonstrate that the chemical evolution of crystalline structures due to radioactive decay may be a viable synthesis route for unforeseen materials with interesting properties.