Mn⁴⁺-doping-induced structural transformation and luminescence enhancement in bulk (NH₄)₂SiF₆: Mn⁴⁺ crystal grown via cooling crystallization

Mn⁴⁺-activated fluoride bulk crystals have recently emerged as promising phosphor materials due to their superior water resistance, thermal stability, and quantum efficiency compared to conventional powder forms. Herein, we report the successful growth of millimeter-sized (NH₄)₂SiF₆:Mn⁴⁺ crystals with intense red emission under 365 nm UV excitation via a facile cooling crystallization approach. The as-grown crystals exhibit a distinctive yellow body color and attain dimensions of up to ∼3 mm. Intriguingly, Mn⁴⁺-doping triggers a structural phase transition in the host matrix from cubic symmetry (space group, Fm- 3 m ) to hexagonal symmetry (space group, P 6 3 mc ), accompanied by progressive lattice distortions. This structural evolution manifests spectroscopically through the emergence of a zero-phonon line (ZPL) in photoluminescence spectra, with the ZPL-to- v ₆ intensity ratio escalating from 0 % to 34 % as Mn⁴⁺ concentration increases. Concomitantly, all seven characteristic emission peaks display systematic blueshifts. Furthermore, the microscopic morphology, thermal quenching behavior, and decay property of the crystal are significantly influenced by the doping-induced structural modifications. This study establishes a structural engineering strategy for optimizing luminescent performance in (NH₄)₂SiF₆:Mn⁴⁺ crystal phosphors.