Origin of tunable emission wavelengths for (Sr 1-xCax) 0.95Ga2S4:0.05Eu and Sr1-yGa2S4:yEu phosphors

(Sr1-xCax)0.95Ga2S4:0.05Eu (x = 0, 0.25, 0.50, 0.75, 1) and Sr1-yGa2S4:yEu (y = 0.05, 0.25, 0.50, 0.75, 1) phosphors were synthesized from CaS, SrS, EuS, and Ga2S3 by carrying out solid-state reactions. X-ray diffraction (XRD) results indicated that the (Sr1-xCax)0.95Ga2S4:0.05Eu and Sr1-yGa2S4:yEu phosphors formed solid solutions between SrGa2S4 and CaGa2S4, and between SrGa2S4 and EuGa2S4, respectively. The photoluminescence excitation (PLE) and photoluminescence (PL) spectra of these phosphors were examined. The wavelength at maximum intensity (λmax) of the PL spectra changed gradually from 537 to 559 nm as the calcium ion mole fraction (x) increased in the (Sr1-xCax)0.95Ga2S4:0.05Eu phosphors. λmax changed from 537 to 545 nm as the europium mole fraction (y) in the Sr1-yGa2S4:yEu phosphors increased. The luminescence parameters, such as the zero phonon line energy (E0), Stokes shift (ΔS), and red shift (D), were calculated from the PLE and PL spectra of these phosphors. The origins of the wavelength changes were deduced from the luminescence parameters of these phosphors. The three-band white light emitting diode (WLED) was fabricated by coating the (Sr0.5Ca0.5)0.95Ga2S4:0.05Eu as a greenish yellow-emitting phosphor and K2SiF6:Mn4+ as a red-emitting phosphor onto a commercial blue LED. The photoluminescence property of the three-band WLED was investigated.