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

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, 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.