Preparation of Mg1+yAl2-xO4:xMn4+, yMg2+ deep red phosphor and their optical properties

Abstract

Exploration of efficient deep red phosphor based on non-rare-earth ion activated oxide is of great practical value in the field of phosphors converted white light-emitting diode lighting. A spinel Mg1+yAl2-xO4:xMn4+, yMg2+ phosphor with deep red emission is synthesized by a solid-state reaction route. The crystal structure and morphology are characterized by powder X-ray diffraction and scanning electron microscopy. The luminescent performance is characterized by fluorescence spectrophotometer and fluorescence decay curves. The results demonstrate that the synthesized phosphor shows that two excited spectrum bands centered at 290 nm and 438 nm cover a broad spectral region from 220 nm to 500 nm due to the Mn4+-O2- charge transfer band and the 4A2-4T1 and 4T2 transitions of Mn4+ ions. Upon excitation at 300 nm, a strong, narrow red emission band is observed between 600 and 700 nm peaked at 652 nm as a result of the spin-forbidden 2Eg-4A2g electron transition of Mn4+. The corresponding chromaticity coordinate is (0.7256, 0.2854). Additionally, the concentration quenching of Mn4+ in the MgAl2O4 host is evaluated in detail, which indicates that the optimum doping concentration of Mn4+ is experimentally determined to be 0.14 mol%. The critical distance is calculated to be 52.15 according to the Blasse equation, which elucidates that the concentration quenching mechanism is consequently very likely to be induced by the multipole-multipole interaction. The crystal field strength (Dq) and the Racah parameters (B and C) are estimated to evaluate the nephelauxetic effect of Mn4+ suffered in MgAl2O4:Mn4+ host lattice. Luminous mechanism is explained by Tanabe-Sugano energy level diagram of Mn4+ ion. The ratio of Dq/B equals 1.74, indicating that Mn4+ ions experience a weak crystal field in the MgAl2O4 host and emission peak energy of 2Eg-4A2g transition is dependent on the nephelauxetic effect. The red emission intensity of Mg1+yAl2-xO4:xMn4+, yMg2+ increases on account of excess Mg2+ which would compensate for the local charge balance surrounding Mn4+ ions, furthermore, lead the Mn4+-Mn4+ pairs connected with interstitial O2- to transform into isolated Mn4+ ions, and thus eliminating energy transfer and enhancing the luminescence efficiency effectively. The decay times of two time-dependent curves of Mg1+yAl2-xO4:xMn4+,yMg2+ are 0.672 ms and 0.604 ms, and each entire decay curve could be well-fitted to single-exponential, confirming that there is only a single Mn4+ ion luminescence center. The decay time of Mn4+ luminescence is prolonged with the increase of Mg2+ content, indicating that excitation energy transfer and non-radiative relaxation between Mn4+-Mn4+ pairs decrease, the reason is that photoexcitation energy can be temporarily stored in the trapping centers induced by excess positive charges. These results imply that Mn4+ doped Mg1+yAl2 -xO4:xMn4+, yMg2+ is a promising candidate of deep-red phosphors for near-UV and blue light emitting diodes. These findings in the paper would be beneficial not only to developing a low-cost and safe strategy to produce high-efficient Mn4+ activated luminescent materials for white light emitting diodes, but also to providing a new insight into improving the photoluminescence properties of Mn4+.