Journal of Materials Chemistry C (2017)

K2MnF6 as a precursor for saturated red fluoride phosphors: the struggle for structural stability
Reinert Verstraete, Heleen F. Sijbom, Katleen Korthout, Dirk Poelman, Christophe Detavernier and Philippe F. Smet
Journal of Materials Chemistry C 5 (2017) 10761-10769

Abstract
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, K2MnF6is 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 K2MnF6is required as it quickly degrades. Here, we investigate the structural properties after synthesis, as well as the main degradation routes of K2MnF6when 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.

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