Microwave-hydrothermal synthesis and investigation of Mn-doped K2SiF6 microsize powder as a red phosphor for warm white LEDs

Abstract

Microsize powder of Mn-doped potassium hexaflourosilicate (PFS, K2SiF6) phosphor has been synthesized using the microwave-hydrothermal (MWHT) method, a new approach for obtaining this material. Special attention has been paid to the identification of the valence states for Mn doping ions using EPR and XANES techniques. Although photoluminescence results reveal luminescence properties typical for Mn4+ ions, the XANES spectra show the presence of Mn ions in various valence states (+2 to +4). The analysis of the temperature dependence for the Mn4+ integrated luminescence intensity and luminescence decay time has shown that the MWHT synthesis provides better performance of the PFS phosphor compared to that of commercially available PFS phosphors in respect of higher thermal stability because of improved crystallinity and lower concentration of defects. It has been revealed that above 500 K, Mn4+ luminescence decay becomes non-exponential due to the energy transfer to defect quenching centers. Repeated thermal treatment of microparticles up to 700 K results in considerably changed thermal quenching parameters due to the transformation of K2SiF6 into the K3SiF7 phase. The latter compound doped with Mn4+ shows spectral properties very similar to those of K2SiF6:Mn4+ but demonstrates better thermal stability and shorter decay time of Mn4+ luminescence at normal conditions which can make the red-emitting K3SiF7:Mn4+ phosphor a promising candidate for application in wLEDs.