Revisiting the reported emission spectrum of Cs2NaBiCl6

Abstract

In this study, we revisit Cs₂NaBiCl₆ and focus upon several assignments previously given for emission bands. A broad emission band spanning from 1200 nm to 1550 nm (centered at ∼1350 nm) is observed for Fe³⁺ when excited at 450 nm or shorter wavelengths into its electron transfer bands in chloride double perovskites. This band was previously assigned to a color center (A. Huang, M. Liu, C.-K. Duan, K.-L. Wong and P. A. Tanner, Understanding the Ultraviolet, Green, Red, Near Infrared and Infrared Emission Properties of Bismuth Halide Double Perovskite, Inorg. Chem. Front., 2022, 9, 6379–6390) because it was observed as an unintended impurity in pristine chloride double perovskites. The undoped material also exhibits Cr³⁺ emission which overlaps the 1 μm Bi³⁺ emission. We are unaware of previous reports of Fe³⁺ emission in chloride hosts at an octahedral site. The ms lifetime is typical of 3dⁿ–3dⁿ transitions. This work demonstrates the requirement to check for unintended impurity emission bands from cations at ppm or lower levels.