A Study on T-phase Phosphor in Binary Ba2SiO4-Ca2SiO4 Systems for White-Light-Emitting Diode

Abstract

T-phase phosphors were developed in binary Ba2SiO4-Ca2SiO4 systems activated by trivalent cerium, divalent europium, and divalent manganese and their luminescence characteristics were studied for phosphor-converted white-light-emitting diodes application. For the binary Ba2SiO4-Ca2SiO4 systems, the (α'L, α'H, α, β, γ, T, X) phase transition occurs depending on the firing temperature and the composition variation of Ba/Ca, and they have diverse crystal structure orthorhombic, monoclinic, and hexagonal. In the region of Ba1.2Ca0.8SiO4 – Ba0.8Ca1.2SiO4, a T-phase structure belonged to the space group P3 ̅m1 with hexagonal unit cell (a = 0.573 nm and c = 0.729 nm for the Ba1.20Ca0.80SiO4) is confirmed. The following phosphors have developed for high-quality white-light generation; (i) Ba1.2Ca0.8-2xSiO4:xCe3+/Li+ as a novel blue phosphor, (ii) deep-blue and red-emissive Ba1.2Ca0.8-2x-ySiO4:xCe3+/Li+, yMn2+ phosphor, and (iii) The mixtures of the blue/red-emissive Ba1.2Ca0.74-2xSiO4:xCe3+/Li+, 0.06Mn2+ and the green-emissive Ba1.2Ca0.7SiO4:0.1Eu2+. Ba1.2Ca0.8-2xSiO4:xCe3+/Li+ phosphor exhibits two absorption bands at 280 nm and 325 nm, and an intense blue emission peaking at 400 nm. With increasing Ce/Li concentration, the lattice expands and the emission peak is blueshifted. This correlation is explained in terms of the crystal field effect and the configurational coordinate diagram. This phosphor shows much high thermal quenching temperature (225 °C) due to a weak electron-phonon interaction. Ce3+/Mn2+-codoped T-phase (Ba, Ca)2SiO4 phosphor shows two emission bands peaking at around 600 nm (red) and 400 nm (deep-blue) from the forbidden 4T¬1-6A1 transition of Mn2+ ions and the allowed 4f-5d transition of Ce3+ ions, respectively. The strong interaction between Ce3+ and Mn2+ ions is investigated in terms of energy transfer, crystal field effect, and microstructure by changing their concentration. The correlation between decay times and electron paramagnetic resonance signals of Mn2+ ions demonstrates that local distortion around Mn2+ sites are influenced by the doped Ce3+ ions which violate their selection rule. They show much higher quenching temperature (250 °C) than junction temperature (125 °C) of light-emitting diode. Finally, high-quality white-light generation is achieved through both Ce3+/Mn2+-doubly doped and Eu2+-singly doped (Ba, Ca)2SiO4 phosphors. The mixtures, excited by the near-ultraviolet of 365 nm, show the various qualities of white lights depending on the mixture ratio; the correlated color-temperatures from 3500 to 7000 K, and the color-rendering indices up to 95 %.