Guanidinium-Based Hybrid Antimony Halide Scintillators for High-Resolution X‑ray Imaging and 3D Reconstruction

Herein, we systematically investigate how the coordination environments of Sb³⁺ influences self-trapped exciton (STE)-mediated scintillation in a series of zero-dimensional guanidinium-based hybrid antimony halides. Four compounds, (DPG)₃SbCl₆, (DPG)₃SbBr₆, (TPG)₂SbCl₅, and (TPG)₂SbBr₅, featuring either symmetric octahedral [SbX₆]^3– or distorted square-pyramidal [SbX₅]^2– units, are investigated.

Through combined structural analyses, spectroscopic studies, and theoretical calculations, we reveal that octahedral coordination effectively suppresses the stereochemical activity of the Sb³⁺ 5s² lone pair, reduces lattice relaxation, and minimizes nonradiative losses. In contrast, the distorted square-pyramidal geometry induces enhanced structural relaxation and electron–phonon coupling, resulting in pronounced thermal quenching.