High-Efficiency Mn4+-Doped Rb2GeF6 Single-Crystal Phosphors: Enhanced Moisture Resistance and Performance in Warm White LEDs

Mn4+-doped fluorides are renowned for their highly efficient narrow-band red emission, making them promising candidates for white light-emitting diodes and wide-gamut displays. However, performance degradation at high power levels and insufficient moisture resistance limit their application. Herein, a millimeter-sized hexagonal Rb2GeF6:Mn4+ single-crystal phosphor with superior photoluminescence quantum efficiency (QE ≈ 97%) was synthesized via a one-step saturated crystallization method. The single crystal demonstrates high stability in aqueous environments compared to its powdered counterpart, attributed to reduced hydrolysis of the [MnF6]2– group. The calculated band structure of Rb2GeF6 reveals it to be a nonmagnetic, direct band gap semiconductor with a 5.89 eV band gap. A white LED device incorporating the red-emitting crystal and yellow-emitting Y3Al5O12:Ce3+ achieves a high color-rendering index of 84.5 at a low correlated color temperature of 2296 K, demonstrating the potential for advanced warm white LEDs. This research provides a foundation for the development of high-efficiency Mn4+-doped fluoride single-crystals for lighting applications.