Uncovering the origin of the emitting states in Bi3+-activated CaMO3 (M = Zr, Sn, Ti) perovskites: metal-to-metal charge transfer versus s-p transitions

TL;DR

This study combines experimental and theoretical methods to clarify the origin of emission states in Bi3+-activated CaMO3 perovskites, revealing the charge transfer nature of their luminescence and aiding phosphor design.

Contribution

It provides the first comprehensive evidence of charge transfer emissions in Bi3+-activated perovskites using temperature-dependent photoluminescence and spectroscopy.

Findings

Confirmed Bi3+ stabilization in Ca2+ sites via X-ray absorption

Identified charge transfer as the origin of emission states

Provided a detailed energy diagram of the luminescent centers

Abstract

After more than a century of studies on the optical properties of Bi3+ ions, the assignment of the nature of the emissions and the bands of the absorption spectra remain ambiguous. Here, we report an insight into the spectroscopy of Bi3+-activated CaMO3 perovskites (M = Zr, Sn, and Ti), discussing the factors driving the metal-to-metal charge transfer and sp -> s2 transitions. With the aim to figure out the whole scenario, a combined experimental and theoretical approach is employed. The comparison between the temperature dependence of the photoluminescence emissions with the temperature dependence of the exciton energy of the systems has led to an unprecedented evidence of the chargetransfer character of the emitting states in Bi3+-activated phosphors. Low-temperature vacuum ultraviolet spectroscopy together with the design of the vacuum-referred binding energy diagram of the luminescent center is exploited to shed light on the origin of the absorption bands. In addition, the X-ray absorption near the edge structure unambiguously confirmed the stabilization of Bi3+ in the Ca2+ site in both CaSnO3 and CaZrO3 perovskites. This breakthrough into the understanding of the excited-state origin of Bi3+ could pave the way toward the design of a new generation of effective Bi3+-activated phosphors.