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 exciting at 450 nm or shorter wavelengths into its electron transfer bands in chloride double perovskites. This band was previously assigned to a color center (Inorg. Chem. Front., 2022, 9, 6379) 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.