Synthesis and Optical Properties of Li_3Ba_2La_3(MoO_4)_8:Sm~(3+) Powders for pcLEDs

摘要

A series of Sm~(3+)-activated molybdates Li_3Ba_2(La_(1-x)Sm_x)_3(MoO_4)_8 with 0 ≤ x ≤ 1 (0% to 100% Sm~(3+)) have been prepared by the conventional solid-state synthesis method, and their optical properties were investigated. Reflection, excitation and emission spectra were recorded and put in relation to the various [Xe]4f~5 → [Xe]4f~5 transitions of Sm~(3+). The positions of the charge transfer bands of Sm~(3+) and Mo~(6+) were resolved by Gaussian peak fitting. Emission spectra recorded at 100 K revealed the Stark sublevels of the Sm~(3+) energy levels. Time-dependent emission measurements of the ~4G_(5/2) → ~6H_(9/2) transition were performed to disentangle the influence of temperature and activator concentration on the decay constants. The results are discussed in the context of the structure of the host material. Sm~(3+) occupies two different crystallographic sites at higher activator concentrations, which results in a bi-exponential decay curve. Temperature-dependent emission spectra were recorded to determine the thermal quenching behavior of the material. Internal and external quantum efficiencies (IQE and EQE) have been calculated. The IQE is independent of temperature, while the emission intensity strongly decreases at temperatures higher than 400 K. It is concluded that the photon escape efficiency in Li_3Ba_2La_3(MoO_4)_8 correlates with temperature. An EQE of 44% was achieved for the 2% Sm~(3+) sample, which is comparatively high for Sm~(3+). Color points and luminous efficacies were calculated. The color point is independent of the Sm~(3+) concentration, but a blue-shift was observed with increasing temperature. This shift may be caused by lattice expansion and a subsequent decrease of spin-orbit coupling.