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Phosphor-converted white light-emitting diodes (LEDs) are currently taking over the lighting market because of their high luminous efficiency, environmentally friendly nature and long lifetime. A new generation of saturated red fluoride phosphors, using Mn4+ as the activator, has gained interest in further enhancing the color rendering properties and efficiency of white LEDs for lighting and display applications. They can be described as A2MF6:Mn4+ (A = K, Na, Sc, NH4 and M = Si, Ge, Ti, Sn), KNaMF6:Mn4+, BaMF6:Mn4+ (M = Si, Ti) or ZnMF6·H2O (M = Si, Ge) compounds, in which Mn is a substitute for the M(iv) element of the fluoride host. A two-step co-precipitation synthesis method has recently been developed because of the increased control of the Mn valence state and the relatively low cost. In this method, K2MnF6 is first synthesized as a precursor which then serves as a source for the preparation of [MnF6]2− complexes in further phosphor synthesis. In-house production of K2MnF6 is required as it quickly degrades. Here, we investigate the structural properties after synthesis, as well as the main degradation routes of K2MnF6 when the material is subjected to heat and humidity or used in further synthesis reactions. It is found that impurities, such as KHF2, K2MnF5·H2O and Mn ions in an oxygen coordination, can be formed as a result of parasitic reactions during synthesis. Even in pure K2MnF6, degradation occurs due to heat and hydrolysis both of which induce reduction of the Mn4+ ion. Heating in air causes the material to form Mn2+ as KMnF3/KF·MnF2 starts to form at high temperatures due to hydrolysis. In dilute HF solutions the Mn4+ ion is partially reduced to Mn3+, often incorporated in hydrated structures as KMnF4·H2O and K2MnF5·H2O. The Mn3+ ion is found to affect the optical absorption properties. [ABSTRACT FROM AUTHOR]

Correction for ‘K2MnF6 as a precursor for saturated red fluoride phosphors: the struggle for structural stability’ by Reinert Verstraete et al., J. Mater. Chem. C, 2017, 5, 10761–10769. [ABSTRACT FROM AUTHOR]

K2SiF6:Mn4+ is a promising red phosphor for the use in LEDs for general lighting and displays. Its narrow red emission and excellent thermal stability make it suitable for improved color rendering. The long millisecond-range decay and saturation at high excitation power can be overcome by using a remote phosphor light emitting diode (LED) setup. [ABSTRACT FROM AUTHOR]

Traditional light sources, e.g., incandescent and fluorescent lamps, are currently being replaced by white light-emitting diodes (wLEDs) because of their improved efficiency, prolonged lifetime, and environmental friendliness. Much effort has recently been spent to the development of Mn 4+ -doped fluoride phosphors that can enhance the color gamut in displays and improve the color rendering index, luminous efficacy of the radiation, and correlated color temperature of wLEDs used for lighting. Purity, stability, and degradation of fluoride phosphors are, however, rarely discussed. Nevertheless, the typical wet chemical synthesis routes (involving hydrogen fluoride (HF)) and the large variety of possible Mn valence states often lead to impurities that drastically influence the performance and stability of these phosphors. In this article, the origins and consequences of impurities formed during synthesis and aging of K 2 SiF 6 :Mn 4+ are revealed. Both crystalline impurities such as KHF 2 and ionic impurities such as Mn 3+ are found to affect the phosphor performance. While Mn 3+ mainly influences the optical absorption behavior, KHF 2 can affect both the optical performance and chemical stability of the phosphor. Moisture leads to decomposition of KHF 2 , forming HF and amorphous hydrated potassium fluoride. As a consequence of hydrate formation, significant amounts of water can be absorbed in impure phosphor powders containing KHF 2 , facilitating the hydrolysis of [MnF 6 ] 2- complexes and affecting the optical absorption of the phosphors. Strategies are discussed to identify impurities and to achieve pure and stable phosphors with internal quantum efficiencies of more than 90%.