Unveiling Temperature-Induced Structural Phase Transition and Luminescence in Mn2+-Doped Cs2NaBiCl6 Double Perovskite

Halide double perovskites like Cs2NaBiCl6 are good host materials for luminescent dopants like Mn2+. The nature of photoluminescence (PL) depends on the local structure around the dopant ion, and doping may sometimes influence the global structure of the host. Here, we unveil the correlation between the temperature-induced (global) structural phase transition of Mn2+-doped Cs2NaBiCl6 with the local structure and PL of the Mn2+ dopant. X-ray diffraction analysis shows Mn2+-doped Cs2NaBiCl6 is in a cubic (Fm3m) phase between 300 and 110 K, below which the phase changes to tetragonal (I4/mmm), which persists at least until 15 K. The small (similar to 1%) doping amount does not alter the phase transition behavior of Cs2NaBiCl6. Importantly, the phase transition does not influence the Mn2+ d-electron PL. The PL peak energy, intensity, spectral width, and lifetime do not show any signature of the phase transition between 300-6 K. The hyperfine splitting in temperature-dependent electron paramagnetic spectra of Mn2+ ions also remain unchanged across the phase transition. These results suggest that the global structural phase transition of the host does not influence the local structure and emission property of the dopant Mn2+ ion. This structure-property insight might be explored for other transition-metal- and lanthanide-doped halide double perovskites as well. The stability of dopant emission regardless of the structural phase transition bodes well for their potential applications in phosphor-converted light emitting diodes.