Cooperative Upconversion Luminescence Properties of Yb3+ and Tb3+ Heavily Codoped Silicate Garnet Obtained by Multiple Chemical Unit Cosubstitution

As is well-known, the aliovalent substitution level is usually very limited due to the charge mismatch. Particularly, the single phase can hardly be obtained by solid-state reaction for the famous silicate garnet Ca3Sc2Si3O12 (CSS), even when the doping level of trivalent rare earth ion (RE3+) for Ca2+ in CSS is lower than 2 mol %, which largely restricts CSS to be an ideal host for RE3+-activated luminescence materials especially where high doping concentration is required. Herein, by using the strategy of multiple chemical unit cosubstitution, we obtained RE3+ heavily doped single-phase CSS via the sol-gel method followed by high-temperature sintering. Multiple chemical unit substitutions of [REO8], [AlO6], and [AlO4], respectively, for [CaO8], [ScO6], and [SiO4] polyhedra can act as charge compensators for each other to promote the doping level of RE3+ up to 20 mol %, which is high enough for most of the RE3+-doped luminescence materials. Moreover, intense cooperative upconversion (UC) luminescence (UCL) was observed in Yb3+ and Tb3+ codoped CSS, whose intensity is 37 times higher than that of the reported Y3Al5O12 with garnet structure as well, making it a potential candidate for optical applications like a tunable UC laser. The results show that the preferred formation of the Yb3+-Yb3+ pair in CSS can largely enhance the efficiency of the cooperative UC process. Besides, the UCL properties were investigated in detail to understand the UC processes and the underlying energy transfer mechanisms. It is confirmed that the multiple chemical unit cosubstitution is an effective strategy to promote the aliovalent substitution level or design solid solution materials to enhance or tune the luminescence properties where relatively high doping concentration is required.