Design and Development of a Bluish-Green Luminescent Material (K2HfSi3O9:Eu2+) with Robust Thermal Stability for White Light-Emitting Diodes

A dipotassium hafnium trisilicate K2HfSi3O9 with wadeite structure was synthesized by solid-state reaction for the first time. Its detailed crystal structure and electronic structure were determined using XRD Rietveld refinement and density functional theory (DFT), respectively. An expected asymmetric emission band was observed in K2HfSi3O9:Eu2+ due to Eu2+ ions occupying the two symmetrically inequivalent K+ (K1 and K2) ion sites in the host lattice. The K2HfSi3O9:Eu2+ has obvious heat-sensitive luminescent features (25 °C, blue emission; 250 °C, green emission), which can be rationalized by the energy transfer from Eu1 to Eu2 under thermal stimulation. Moreover, we developed promising phosphor K2HfSi3O9:Eu2+,Sc3+ by employing the crystal-site engineering method, and tuned the emission color from blue to green with the variation of Sc3+ ions. The target phosphor K2HfSi3O9:2%Eu2+, 6%Sc3+ shows strong absorption in the spectral range of ∼250–490 nm and a narrow green-emitting band (fwhm ∼59 nm) peaking at ∼507 nm with zero emission loss at ambient temperature up to 200 °C. Such robust thermal performance was explained by possible transfer energy from the defect levels to the 5d band of Eu2+, and certified by the thermoluminescence spectrum and the decay curves in the temperature range of 25 to 250 °C. Combing this excellent green phosphor with n-UV (395 nm) or blue chips (450 nm), pc-WLEDs with high luminance, adequate color rendering index, and correlated color temperature were obtained.