Studies on the photoelectronic properties of a manganese (Mn)-doped lead-free double perovskite.

Taking Cs 2 NaBiCl 6 , Cs 2 AgInCl 6 and Cs 2 AgBiCl 6 as examples of lead-free double perovskites (DPs), we study the photoluminescence (PL) properties of Mn-doped DPs. The electron localization function (ELF) reveals the more ionic nature of the Na-Cl bond in Cs 2 NaBiCl 6 than that of the Ag-Cl bond in Cs 2 AgBiCl 6 . Bader charge calculations confirm the nominal +2 valence state of Mn ions in both DPs. Mn 2+ ions introduce two defect levels in the band gap of the Cs 2 NaBiCl 6 host, accounting for the d-d transition ( 4 T 1 - 6 A 1 transition) of Mn 2+ and thus the subsequent orange PL. The changes of the crystal field and their influences on the emission energy of Mn 2+ ions in different DPs are evaluated by calculating the Racah parameters ( B and C ) and the crystal field strength (Dq) obtained from energies of the terms of d 5 in the Cs 2 NaBiCl 6 :Mn 2+ and Cs 2 AgInCl 6 :Mn 2+ systems. The results show that Dq in Cs 2 AgInCl 6 :Mn 2+ is stronger than that in Cs 2 NaBiCl 6 :Mn 2+ . The analyses on bonding interactions of the Mn-Cl bond via ELF and the integrated projected pCOHP also confirm the stronger ionic bonding interactions and thus the boost of the crystal field strength in the Cs 2 AgInCl 6 :Mn 2+ system, which results in the blue-shift of the Mn 2+ introduced PL peak from Cs 2 AgInCl 6 to Cs 2 NaBiCl 6 . Our results provide a new strategy to modulate the emission wavelengths, i.e. , tuning the crystal field.