Interplay among the Ligand Field, Covalency, and Spin Localization in Tailoring the Optical Properties of Mo3+-Doped Halide Perovskites

Abstract

Doping Cr3+ with 3d3 electrons gave rise to the famous Ruby laser, owing to intraconfigurational spin-flip (ICSF) d–d electronic transitions. Mo is directly below Cr in the periodic table. But there are both chemical and spectroscopic differences between the 4d and 3d electrons. For example, Cr3+ is highly stable, but Mo3+ oxidizes in ambient conditions, until a very recent report showing ambient-stable ICSF near-infrared (NIR) emission from Mo3+-doped Cs2NaInCl6 double perovskite. Here, we elucidate how chemical bonding, structure, and spin localization govern the optical properties of Mo3+ ions by preparing a series of ambient-stable Mo3+-doped Cs2MM′X6 (M: Na, Ag; M′: In, Bi; X: Cl, Br) double perovskites. The ligand field splitting (Δo) of Mo3+ 4d3 electrons could be varied over ∼3480 cm–1, but the ICSF emission energies remain independent of Δo, varying only by 172 cm–1 depending upon the bond covalency (Racah parameters B and C). Hyperfine sublevel correlation (HYSCORE) spectroscopy shows that the 4d3 electron spin remains localized around the dopant center, and therefore, the spectroscopic characteristics of d–d transitions remain unchanged even after a temperature-dependent structural phase transition of the hosts. The obtained bonding–structure–function link is important to design Mo3+-doped samples for applications from advanced NIR emitters to optically active spin states.