Low-temperature crystallization of the K2TiF6:Mn4+ phosphor with near-unity quantum efficiency in dilute HF solution

Abstract

Mn⁴⁺-activated fluorides are commercial red phosphors for white LEDs, yet their conventional precipitation synthesis relies on highly concentrated (≥40 wt%) HF acid that hinders scale-up. Here we show a relatively green route for preparing K₂TiF₆:Mn⁴⁺ in dilute HF solution but with near-unity quantum efficiency. Cooling the solution prepared using 10–30 wt% HF from 20 °C to −40 °C kinetically suppresses [MnF₆]²⁻ hydrolysis and induces crystallization of K₂TiF₆:Mn⁴⁺ platelets exhibiting internal quantum efficiency >90%. The influences of HF concentration, cooling protocol and K₂MnF₆ amount on crystallization behavior and photoluminescence properties were investigated. The HF solution with decreasing concentration is favorable for a higher yield of K₂TiF₆:Mn⁴⁺. A cooling rate of 1 °C per 30 min results in the optimum crystal morphology and Mn⁴⁺ incorporation efficiency. K₂Ti_1–xF₆:xMn⁴⁺ with nominal x = 0.02 exhibit an internal quantum efficiency of 98.9%, an external quantum efficiency of 73.9% and a lifetime of 5.62 ms. The as-grown crystals possess superior resistance to water relative to the powder counterpart; oxalic-acid treatment can further boost water-resistance, and 76% photoluminescence was retained after 3 h water immersion, eightfold higher than powder analogues. Packaged white LEDs deliver 124.6 lm W⁻¹ luminous efficacy at 20 mA with R_a = 83.2 and R₉ = 70. This work provides a new “dilute HF + low temperature” strategy for the green synthesis of high-performance fluoride phosphors.