Large micro-sized K2TiF6:Mn4+ red phosphors synthesised by a simple reduction reaction for high colour-rendering white light-emitting diodes

A simple chemical method for synthesising large micro-sized K2TiF6:Mn4+ phosphors (50-100 mu m) is presented, based on a direct reduction reaction with K2TiF6 particles immersed in KMnO4/HF solution. The Mn4+ concentration in K2TiF6 primarily increases as the initial KMnO4 concentration is increased, subject to the diffusion of Mn4+ ions. However, the relative emission intensity of our synthesised K2TiF6:Mn4+ is first enhanced and then declines with increasing KMnO4 concentration, with an optimum KMnO4 concentration of 0.016 g ml(-1), which most likely depends on the concentration quenching effect. The synthesised K2TiF6:Mn4+ (0.35-4.4 at%) phosphors show good thermal stability and can be adopted to fabricate high CRI (R-a > 85) white LEDs for indoor lighting.