ABSTRACT Cerium‐based metal halides (Ce‐MHs) are ideal for real‐time x‐ray imaging for their nanosecond photoluminescence decay and emission spectra that match common photodetectors. However, moisture‐ and oxygen‐induced oxidation during fabrication and severe grain‐boundary light‐scattering in thick films with randomly oriented grains degrade both light yield and spatial resolution of Ce‐MH films. Here, we investigate how the intrinsic crystal symmetry influences the microstructure of Ce‐MH films during single‐source evaporation conducted under anhydrous and oxygen‐free conditions. Under kinetically constrained growth, comparison of orthorhombic Cs 3 CeBr 6 (CCB) and monoclinic Cs 3 CeCl 6 (CCC) reveals that the higher symmetry CCB reproducibly develops oriented pixelated grains. We attribute this behavior to the fact that CCB nuclei present fewer competing nucleation orientations, these nuclei therefore survive competitive growth and evolve into pixelated crystalline films. Furthermore, CCB exhibits enhanced ambient stability owing to a low water‐adsorption energy (0.62 eV), and reduced performance degradation because halide‐vacancy formation induces smaller local charge perturbations that hinder the formation of deep‐level traps. On these basics, the large‐area (10 × 12 cm 2 ) CCB films exhibit a near‐single‐crystal light yield of 28150 photons MeV −1 , a fast decay time (22.7 ns), a high spatial resolution (26.4 lp mm −1 ), and ∼3 min window for handling and transport prior to encapsulation.
Xie et al. (2026) studied this question.