Red emitting K2NbF7:Mn4+ and K2TaF7:Mn4+ for warm-white LED applications

Abstract

Mn4+ activated luminescent materials have attracted much attention recently. In particular, alkali metal hexafluorometallates, such as K2SiF6:Mn4+ or K2TiF6:Mn4+, emit light in the red spectral range upon blue excitation and are thus applied onto phosphor converted LEDs (pcLEDs). K2NbF7:Mn4+ and K2TaF7:Mn4+ exhibit efficient red photoluminescence peaking at 627nm, which can be assigned to the 2Eg → 4A2g intraconfigurational transition of Mn4+ ([Ar]3d3 configuration) as part of the [MnF6]2- octahedra onto the niobium/tantalum site in the respective host structures. Photoluminescence properties, such as temperature dependence of the PL intensity and luminescence decay are presented. Additionally, the band structure of both undoped host materials has been treated with Density Functional Theory (DFT). The theoretical results have been evaluated experimentally with diffuse UV reflectance spectroscopy. Finally, luminous efficacy and color rendering values of simulated warm white emitting pcLEDs comprising a dichromatic phosphor blend employing K2NbF7:Mn4+ or K2TaF7:Mn4+ as the red emitting component are calculated and compared to the performance of warm white emitting pcLEDs comprising different commercial phosphors as red emitting component.