Beyond Lattice Sharing: Unusual Isolation of Dense Sensitizer Lattice as a New Route to Multicolor Upconversion

ABSTRACT Efficient upconversion benefits from high sensitizer densities; however, dense sensitizer networks can promote migration‐assisted quenching. Disrupting the sensitizer connectivity offers a route to mitigate these losses. However, such isolation is intrinsically difficult in oxide hosts because the rigid high‐phonon oxygen framework favors defect association and local distortion. Herein, we show that lattice topology governs sensitizer network isolation in the highly non‐centrosymmetric oxide Na 3 YbGe 3 O 9 , where isolated YbO 6 polyhedra interrupt direct Yb 3+ –Yb 3+ connectivity and may limit long‐range energy migration. This topology enables full Yb 3+ occupancy while retaining pronounced upconversion luminescence. Under 980 nm excitation, Er 3+ , Tm 3+ , and Ho 3+ doping produced multicolor upconversion, with dominant blue emission in Na 3 YbGe 3 O 9 :Tm 3+ despite complete Yb 3+ occupancy. The partial substitution of Yb 3+ with Gd 3+ in the Er 3+ ‐doped system further enhanced the upconversion luminescence intensity by a factor of 4.92. Compared with Y 2 O 3 :Yb 3+ /Er 3+ , the Na 3 YbGe 3 O 9 ‐based system retained pronounced upconversion emission at high Yb 3+ concentrations. Furthermore, the 75 mol% Gd 3+ ‐substituted sample retained 67.42% of its initial emission intensity after 480 min of water immersion, demonstrating sustained upconversion luminescence under aqueous conditions. These results highlight how sensitizer concentration and lattice topology govern upconversion, identifying fragmented sensitizer networks as a promising design strategy for advanced materials.