A Novel and Highly Efficient Rare-Earth Element-Free K <sub>2</sub>Ge <sub>1-X</sub>Ti <sub>x</sub>F <sub>6</sub>: Mn <sup>4+</sup> Red Phosphor for Warm White LEDs

Red light-emitting materials are usually obtained by doping rare-earth ions and play a vital role in the phosphor-converted white light-emitting diodes. However, the high cost, non-renewable energy, and environmental pollution problem promote the development of rare-earth element-free phosphors. Here, a series of novel red light-emitting K2Ge1-xTixF6: Mn4+ solid solution phosphors with high internal quantum efficiency of 96.28% are first obtained by controlling the concentration of Ti4+. The phosphors can be prepared by using a simple room-temperature co-precipitation method. The results show that the luminescent properties of these red phosphors can be effectively improved by optimally tuning the crystal field environment and increasing the radiation transition probability via Ti4+-doping. The emission intensity at 423 K is as high as 83.46% of the initial value at 323 K and the activation energy is 0.5053 eV in the optimal K2Ge0.8Ti0.2F6: Mn4+ phosphor. A warm white light-emitting diode with record-high color rendering index of 92.4, low correlated color temperatures of 3113 K, and high luminous efficiency of 123.1 lm/W is simultaneously obtained by encapsulating the as-prepared K2Ge0.8Ti0.2F6: Mn4+ red phosphor with Y3Al5O12:Ce3+ yellow phosphor on blue chips. This work proves the prospects of K2Ge1-xTixF6: Mn4+ as low-cost and rare-earth element-free red light-emitting phosphors for potentially viable application in warm white light-emitting diodes.