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

Abstract

Taking Cs₂NaBiCl₆, Cs₂AgInCl₆ and Cs₂AgBiCl₆ 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 Cs₂NaBiCl₆ than that of the Ag–Cl bond in Cs₂AgBiCl₆. Bader charge calculations confirm the nominal +2 valence state of Mn ions in both DPs. Mn²⁺ ions introduce two defect levels in the band gap of the Cs₂NaBiCl₆ host, accounting for the d–d transition (⁴T₁–⁶A₁ transition) of Mn²⁺ and thus the subsequent orange PL. The changes of the crystal field and their influences on the emission energy of Mn²⁺ 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 d⁵ in the Cs₂NaBiCl₆:Mn²⁺ and Cs₂AgInCl₆:Mn²⁺ systems. The results show that Dq in Cs₂AgInCl₆:Mn²⁺ is stronger than that in Cs₂NaBiCl₆:Mn²⁺. 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 Cs₂AgInCl₆:Mn²⁺ system, which results in the blue-shift of the Mn²⁺ introduced PL peak from Cs₂AgInCl₆ to Cs₂NaBiCl₆. Our results provide a new strategy to modulate the emission wavelengths, i.e., tuning the crystal field.