Ordered langasites La3Ga5MO14:Eu3+ (M = Zr, Hf, Sn) as red-emitting LED phosphors

Abstract

The temperature-dependent luminescence behavior of three ordered langasites La3Ga5ZrO14 (LGZr), La3Ga5HfO14 (LGHf) and La3Ga5SnO14 (LGSn) doped with up to 20% Eu3+ were investigated. Band gap energies of 4.32 eV (LGZr), 4.43 eV (LGHf), and 4.46 eV (LGSn) were determined by diffuse reflection measurements of the undoped compounds. The influence of the structural differences on the luminescence properties of the three langasites were investigated by means of excitation, emission (77–500 K) and luminescence lifetime measurements (77–300 K). The quenching temperatures of the three langasites do vary. They were attained using different excitation wavelengths in the charge transfer band (300 nm) and in the 4f levels (394 nm). LGSn:Eu 20% Eu3+ showed the best temperature performance with intensities at 500 K of 85% (300 nm) and 67% of the intensity measured at 77 K. At room temperature and with an excitation wavelength of 394 nm, maximum quantum efficiencies of 41% for LGZr:Eu 20% Eu3+, 58% for LGHf:Eu 20% Eu3+ and 68% for LGSn:Eu 20% Eu3+ were reached. The quenching behavior observed is explained by semi-quantitative configurational coordinate diagrams. Based on these results and the high red color purity, the langasites were built into LEDs with (In,Ga)N chips emitting at 394 nm, to test the applicability of the materials as red LED phosphors.