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

After more than a century of studies on the optical properties of Bi³⁺ ion, 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, 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 PL emissions with the temperature dependence of the exciton energy of the systems has led to an unprecedent evidence of the charge transfer character of the emitting states in Bi³⁺-activated phosphors. Low temperature VUV spectroscopy together with the design of the vacuum referred binding energy diagram of the luminescent center are exploited to shed light on the origin of the absorption bands. In addition, the X-ray absorption near edge structure, unambiguously confirmed the stabilization of Bi³⁺ in Ca²⁺ site in both CaSnO₃ and CaZrO₃ perovskites. This breakthrough into the understanding of the excited state origin of Bi3+ could pave the way towards the design of a new generation of effective Bi³⁺-activated phosphors.