Structure and photoluminescence of (Ca,Eu)(2)SiS(4) powders

The photoluminescence of Ca(2)SiS(4):Eu powders was investigated in detail as a function of europium concentration (from 0.1% Ca substitution to the fully substituted Eu(2)SiS(4)). At low europium dopant concentration (<10%) the powders crystallize in an orthorhombic structure and the emission spectrum is dominated by two broad emission bands, at 564 and 660 nm. The emission can be tuned from yellow (CIE x = 0.46,y = 0.53) to red (CIE x = 0.65,y = 0.35) by variation of the Eu concentration. An energetic coupling exists between both bands, leading to a broad excitation wavelength range. Powders with high europium concentration (>40%) crystallize in a monoclinic structure, details of which were determined by Rietveld refinement of x-ray diffraction data. For the composition CaEuSiS(4) (i.e. 50% substitution), the luminescence peaks at 614 nm, shifting to shorter wavelengths upon further substitution of Ca by Eu. Although considerable thermal quenching is present at room temperature in the fully Eu-substituted compound, Eu(2)SiS(4) is still photoluminescent, with a peak emission wavelength of 577 nm. A strong correlation is found between the crystallographic and luminescent properties of the (Ca,Eu)(2)SiS(4) powders. The broad emission and excitation bands make this phosphor a good candidate for use in phosphor-converted light-emitting diodes (pcLEDs).