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+ ions,\nthe assignment of the nature of the emissions and the bands of the absorption\nspectra remain ambiguous. Here, we report an insight into the spectroscopy of\nBi3+-activated CaMO3 perovskites (M = Zr, Sn, and Ti), discussing the factors\ndriving the metal-to-metal charge transfer and sp -> s2 transitions. With the\naim to figure out the whole scenario, a combined experimental and theoretical\napproach is employed. The comparison between the temperature dependence of the\nphotoluminescence emissions with the temperature dependence of the exciton\nenergy of the systems has led to an unprecedented evidence of the\nchargetransfer character of the emitting states in Bi3+-activated phosphors.\nLow-temperature vacuum ultraviolet spectroscopy together with the design of the\nvacuum-referred binding energy diagram of the luminescent center is exploited\nto shed light on the origin of the absorption bands. In addition, the X-ray\nabsorption near the edge structure unambiguously confirmed the stabilization of\nBi3+ in the Ca2+ site in both CaSnO3 and CaZrO3 perovskites. This breakthrough\ninto the understanding of the excited-state origin of Bi3+ could pave the way\ntoward the design of a new generation of effective Bi3+-activated phosphors.\n