Isomorphism of Sr[Li₃AlO₄] and Sr[Li₃GaO₄] – Syntheses, Crystal Structure, and Europium(II) Luminescence

Abstract

While highly efficient red-emitting inorganic phosphors have been discovered in the substance class of alkaline earth oxo(nitrido)lithoaluminates, new narrow-band green-and yellow-emitting components are being sought to improve the performance of phosphor-converted light-emitting diodes (pc-LEDs). Various solid-state reactions were carried out under protective gas atmosphere in nickel crucibles and sealed tantalum ampules to synthesize Sr[Li 3 AlO 4 ], Sr[Li 3 GaO 4 ], and five substitutional derivates of Sr[Li 3 (Al 1−x Ga x )O 4 ] at moderate temperatures. The observation of a linear increase in the unit cell parameters as a function of the increasing gallium mole fraction x in Sr-[Li 3 (Al 1−x Ga x )O 4 ] revealed Vegard behavior in the solid-solution series, which was derived from powder X-ray diffraction data. The isomorphic crystallization of the new oxolithogallate Sr[Li 3 GaO 4 ] and the known oxolithoaluminate Sr[Li 3 AlO 4 ] in an ordered variant of the U[Cr 4 C 4 ] aristotype was verified on the basis of powder and single-crystal X-ray diffraction data. Photoluminescence spectroscopy was used to investigate the narrow-band emissions in the substitution series of Eu 2+ -activated Sr[Li 3 (Al 1−x Ga x )O 4 ] under blue-light excitation. The emission maximum was shifted to higher energies as the gallium mole fraction increased. Peak wavelengths were observed at λ em = 572 nm (fwhm equals 47 nm, 1446 cm −1 , 0.18 eV) for yellow-emitting Sr[Li 3 AlO 4 ]:Eu 2+ and at λ em = 554 nm (fwhm equals 49 nm, 1589 cm −1 , 0.20 eV) for green-emitting Sr[Li 3 GaO 4 ]:Eu 2+ . Sr[Li 3 AlO 4 ]:Eu 2+ has excellent thermal quenching resistance with a photoluminescence emission intensity of >93% at T = 423 K relative to the room temperature value, making this inorganic phosphor a potential candidate for solid-state lighting applications.