Electronic shell structures, self-trapped excitons, and defect-bound excitons in Li2B12H12

Abstract

First-principles studies of electronic structures, excitons, and defects in Li₂B₁₂H₁₂ reveal unique photophysical properties of Li₂B₁₂H₁₂ and its potential as a neutron scintillator material. We identify the electronic shell structure in (B₁₂H₁₂)²⁻, which gives rise to strong chemical stability of (B₁₂H₁₂)²⁻ and a large band gap of Li₂B₁₂H₁₂. Based on hybrid functional calculations, we show that excitons in Li₂B₁₂H₁₂ are strongly self-trapped and predict that the exciton excitation and emission are both in the UV range and are not related to the bright blue emission observed in the experiment. The experimentally observed blue emission in Li₂B₁₂H₁₂ is likely due to the emission of excitons bound to hydrogen vacancies. The calculated strong localization and large exciton trapping energy at hydrogen vacancies indicate enhanced radiative recombination, while the calculated large Stokes shift suggests that the self-absorption and the resonance transfer of the excitation energy may be suppressed. These properties should promote efficient defect emission at room temperature. The efficient defect emission, combined with the large thermal neutron capture cross-section by Li and B and the efficient energy transfer between fast neutrons and H, suggests potential applications of Li₂B₁₂H₁₂ as a thermal or fast neutron scintillator. Optical characterization of V_H may also be used to monitor H and Li concentrations in Li₂B₁₂H₁₂-based hydrogen storage materials and solid-state electrolytes in Li batteries.