Disordered langasites La₃Ga₅MO₁₄ : Eu³⁺ (M = Si, Ge, Ti) as red-emitting LED phosphors

Abstract

Polycrystalline powders of La3Ga5SiO14 (LGSi), La3Ga5GeO14 (LGGe), and La3Ga5TiO14 (LGTi) doped with Eu3+ were studied with respect to their use as luminescent materials in solid state lighting based on light-emitting diodes. The langasites were synthesized with up to 35% of trivalent europium to achieve the highest possible doping amount. According to diffuse reflection measurements the undoped compounds have band gap energies of 4.51 eV (LGSi), 4.54 eV (LGGe) and 4.07 eV (LGTi). The luminescence behavior between 77 K and 500 K was investigated, including excitation, emission and lifetime measurements to analyze the impact of the structural differences between the three langasites on the spectroscopic properties of the materials. Depending on the excitation wavelengths, 300 nm (charge transfer) and 394 nm (4f levels, i.e.7F0 → 5L6), different quenching temperatures were achieved for LGSi : Eu3+ 20% (TQ,300 = 438 K, TQ,394 = 422 K), LGGe : Eu3+ 20% (TQ,300 = 325 K, TQ,394 = 441 K) and LGTi : Eu3+ 20% (TQ,300 = 500 K, TQ,394 = 467 K). The quenching observed can be explained by three semi-quantitative configurational coordinate diagrams. Independent from the excitation wavelength and the temperature (77-300 K) decay times of 1.1 ms were measured. At room temperature and with an excitation wavelength of 394 nm maximum quantum efficiencies of 40% for LGSi : Eu, of 80% for LGGe : Eu, and of 81% for LGTi : Eu were reached. Finally, to prove the applicability as red LED phosphors, the langasites were built into LEDs with (In,Ga)N chips emitting at 394 nm.

Why Do Relative Intensities of Charge Transfer and Intra-4f Transitions of Eu3+ Ion Invert in Yttrium Germanate Hosts? Unravelling the Underlying Intricacies from Experimental and Theoretical Investigations

“…However, in the present case Eu 3+ is substituting the yttrium site having the same oxidation state (i.e., 3+ (isovalent doping)); thus, doping of europium­(III) cannot induce an additional effective charge on the host site. Reinhardt and co-workers proposed that the reason for decreased intensities of the CTB in La 3 Ga 5 GeO 14 :Eu 3+ and La 3 Ga 5 SiO 14 :Eu 3+ is a resonant crossover from the potential energy curve of the CT state to the potential energy curves of lower 4f states. Struck and Fonger , also reported direct feeding of the excited 5 D­(4f 6 ) levels of Eu 3+ by the CT state and also 5 D quenching via these CT states.…”

“…However, there have been numerous investigations where excitation intensity of CTB is surprisingly weaker than that of intra-4f transitions, which differs greatly from that of common perception of selection rules. Abnormally weak CTB has been frequently reported in germanates − categories of hosts; and occasionally in few other hosts as well. For instance, BaZrGe 3 O 9 :Eu 3+ , YGe 0.5 M 0.5 O 4 :Eu 3+ (M = Mo and W), Zn 2 GeO 4 , K 2 SrGe 8 O 18 :Eu 3+ , La 3 Ga 5 GeO 14 :Eu 3+ , YInGe 2 O 7 :Eu 3+ , and Ca 2 Ge 7 O 16 :Eu 3+ systems have been reported to possess either weak or competitive CTB with respect to intra- 4f transition; however, no specific reasoning was provided to explain this abnormal experimental observation.…”

“…Xin and co-workers attributed this abnormality to induced effective positive charge on europium in YBa 3 (PO 4 ) 3 lattice. Reinhardt and co-workers proposed for La 3 Ga 5 GeO 14 : Eu a possible crossover between low-lying CT states and 5 D J levels as the reason for suppressed CTB. However, both these suggestions (induced effective charge on Eu 3+ and crossover at Eu 3+ state and CT state) do not provide a comprehensive/consistent explanation for this experimental observation and such an explanation is still lacking.…”

Revealing Eu3+-doped yttrium pyrogermanate as a soft UV excitable phosphor: retaining the pros of the commercial phosphor and compensating for the cons

“…Germanates in recent times have become a potential host for rare earth ions due to their particular advantages of single-phase synthesis, high thermal stability, and low phonon energy. 52–59 Most importantly, many germanate hosts with Eu-dopant show dominant excitation efficiency for intra-4f transition suitable for n-UV-LEDs. 59 Motivated by these facts, in the present work we have carried out a systematic investigation on the yttrium pyrogermanate, i.e.…”