Tunable Multicolor Luminescence in Double Perovskite [(CH₃)₄N]₂KEu_1–<i>x</i>Tb<i>_x</i>(NO₃)₆ Single Crystals

Tunable multicolor luminescence materials can flexibly meet the needs of smart lighting, enabling efficient light energy use and minimizing waste. Lead-free hybrid double perovskites A₂M^IM^III(NO₃)₆ hold great potential in luminescence, benefiting from their tunable composition, high light absorption, low synthesis cost, and environmental friendliness. However, achieving tunable multicolor emission within a single matrix of these materials has yet to be realized. In this study, a series of [(CH₃)₄N]₂KEu_1-<i>x</i>Tb<i>_x</i>(NO₃)₆ single crystals have been synthesized using an environmentally friendly and mild aqueous solution evaporation method. The two emitter centers, Tb³⁺ and Eu³⁺, display energy transfer from Tb³⁺ to Eu³⁺. The emission color of the as-synthesized crystals gradually changes from red to orange, then to yellow, and finally to green with an increase of Tb³⁺ concentration, achieving yellow and green light emission in the three-dimensional rare-earth hybrid double perovskites for the first time. Green emission from [(CH₃)₄N]₂KTb(NO₃)₆ displays the highest quantum yield at 87%. The millisecond-level emission decay time and high decomposition temperatures (365 °C) of [(CH₃)₄N]₂KEu_1-<i>x</i>Tb<i>_x</i>(NO₃)₆ single crystals highlight their potential for use in luminescent devices and phosphors, among other fields.