Influence Of Cation-doping On Luminescent Properties Of Magnesium Fluorogermanate Red-emitting Phosphors

With the high energy-conversion efficiency, low consumption, long lifetime and environment-friendly characteristics, white light-emitting diodes(WLEDs) are considered as the new light source in display and lighting applications. This thesis mainly focuses on the Mn4+-activated 3.5Mg O·0.5Mg F2·Ge O2 magnesium fluorogermanate red-emitting phosphor.The crystal structure, particle morphology, luminescent properties and thermal quenching properties of phosphors, influenced by foreign cations are studied and the main results are as follows:(1) The Mn4+-activated 3.5Mg O·0.5Mg F2·(1-2x)Ge O2·x Al2O3(0 ≤ x ≤ 0.15 mol)red-emitting phosphors were synthesized by high temperature solid-state reaction. A strong red emission corresponding to the 2E→4A2 transition of Mn4+ under ultraviolet light excitation is observed. With the increase of Al2O3 concentration, the excitation band at 300-380 nm,which is one of two broad wavelength range corresponding to the spin-allowed 4A2→4T1 and4A2→4T2 transition of Mn4+ ions, has a blue shift. In consideration of the crystal field theory and the Tanabe-Sugano energy diagram of 3d3 coordination, this shift was owed to the raise of4T1 energy level from the contraction and distortion of the octahedral lattice structure which is influenced by the decreased Mn4+-O2- distance. The temperature-dependent PL spectra under different content of Al2O3 revealed the improved thermal quenching characteristic. The optimal activation energy ΔE are obtained to be 0.41 e V for the Al2O3 concentration at 0.1mol.(2) A series of Si O2 doped 3.5Mg O·0.5Mg F2·(1-x)Ge O2·x Si O2:0.01Mn4+ in which x is from 0 to 0.2 mol were synthesized by high temperature solid-state reaction. The sample with0.1 mol Si O2 doping showed the optimal luminous efficiency and the long fluorescence decay time. The explanation was owed to partially relaxed 2E→4A2 transition of Mn4+ due to the lowered crystal structure symmetry. As a result, the radiative transition probability of 2E level was increased. The temperature-dependent PL spectra and fluorescence decay curves of3.5Mg O·0.5Mg F2·Ge O2·0.1Si O2:0.01Mn4+ phosphor were also measured. With temperature increase, the emission intensity for Stokes vibronic bands and anti-Stokes vibronic bands of2E→4A2 transition exhibit inverse characteristics. The phenomenon was related to the multi-phonon absorption transition process or the increased nonradiative transition probability.The crystal structure and luminescence characteristics for Si4+ and Al3+ doped3.5Mg O·0.5Mg F2·Ge O2:0.01Mn4+ phosphors were comparatively discussed. The conclusion is described as the influence of the valence state and electronegativity of doping cations on the 4A2-4T1 and 4A2-4T2 transitions, whereas the spin- and parity-forbidden 2E→4A2 transition is impacted by the ionic radius of doping cations in the host.(3) Rare-earth ions, including Eu3+, Tb3+ and Tm3+, doped3.5Mg O·0.5Mg F2·Ge O2:0.01Mn4+ red-emitting phosphors were synthesized by high temperature solid-state reaction. In consideration of the shortened decay time of Tm3+ from3.59 ms to 3.43 ms by Mn4+ co-doped, we confirmed that the energy transfer efficiency from Tm3+ to Mn4+ is 18.47%.