Extrinsic electronic states to tune the luminescence and bonding nature of Cs2NaInCl6 double perovskite

Halide double perovskites have been extensively investigated in recent years as more stable and environmentally friendly materials with significant optoelectronic properties. Herein, we introduce Mn2+ ions in the Cs2NaInCl6 lattice to impart new electronic pathways to the otherwise weak optically active double perovskite for tuning its luminescent behaviour. X-ray diffraction, Raman, UV-visible, Photoluminescence (PL), and time-resolved PL (TRPL) spectroscopy are used to investigate the effect of Mn 2+ feed ratio on structural, vibrational, and optical properties. The chemical environment and surface morphology of the Mn2+ ions doped Cs2NaInCl6 double perovskite were investigated using X-ray photoelectron (XPS), energy dispersive X-ray (EDS) spectroscopies, and scanning electron microscopy. Results of the Rietveld refinement and Raman spectra divulge a decrease in In-Cl and Na-Cl bond length upon Mn2+ incorporation. The microstructure of the Cs2NaInCl6 double perovskite system was also studied using HRTEM analysis. UV-visible studies demonstrated a tremendous increase in absorption and a slight increase in band gap upon Mn2+ doping. PL and TRPL measurements of Mn2+: Cs2NaInCl6 discloses its red luminescence at 614 nm corresponding to the d-d atomic transition of Mn2+ with a long lifetime of 2.1 ms. Electron density investigations using maximum entropy method (MEM) demonstrate clear evolution of In-Cl and Na-Cl bonds from a highly ionic nature in pure Cs2NaInCl6 to strong covalent nature in Mn2+: Cs2NaInCl6 double perovskites. This affirms the simultaneous replacement of In, Na ions by Mn2+ to maintain charge neutrality in the compound and tune the electronic states of the Cs2NaInCl6 system.