Heleen Sijbom

K 2 SiF 6 :Mn 4+ as a red phosphor for displays and warm-white LEDs: a review of properties and perspectives

Phosphor-converted white light-emitting diodes (pc-WLEDs) are energy efficient and environmentally friendly light sources with a long lifetime, applicable in both display backlights and general lighting. Adding red-emitting phosphors improves the color quality of white LEDs compared to the prototype combination of a blue LED and the yellow Y3Al5O12:Ce3+. Efficient narrow-band red-emitting phosphors like K2SiF6:Mn4+ can meet the market’s needs. This review recapitulates research since 2008 on K2SiF6:Mn4+ as the first and most discussed fluoride phosphor. The limited nephelauxetic effect, typical for fluorides, allows for the tuning of the Mn4+ emission in the red part of the spectrum below 650 nm. This is reflected in the spectroscopic parameters of the crystal field theory. Synthesis methods are described, showing the evolution from etching Si wafers to solution synthesis resulting in consistent luminescent and thermal properties. Though important for applications, long-term stability is often neglected, although (in)organic coatings improving stability emerge. This leads not only to warm-white LEDs with high efficacies and good color rendering, but also to efficient displays with a large color gamut.

K2MnF6 as a precursor for saturated red fluoride phosphors: the struggle for structural stability

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.

Red Mn4+-Doped Fluoride Phosphors: Why Purity Matters

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 Mn4+-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 K2SiF6:Mn4+ are revealed. Both crystalline impurities such as KHF2 and ionic impurities such as Mn3+ are found to affect the phosphor performance. While Mn3+ mainly influences the optical absorption behavior, KHF2 can affect both the optical performance and chemical stability of the phosphor. Moisture leads to decomposition of KHF2, 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 KHF2, facilitating the hydrolysis of [MnF6]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%.

Correction: K2MnF6 as a precursor for saturated red fluoride phosphors: the struggle for structural stability

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.