Synthesis, Luminescence and Crystal Field Calculations of Mn4+- Doped Li2TiO3 Deep Red Phosphor

Abstract

Red emission of the Mn4+ ions incorporated in different materials is used in various applications, such as lighting, holographic recording, thermoluminescence dosimetry, thermometry, etc. Mn4+ emission can improve color-rendering-index of phosphor-converted white-light-emitting diodes, and it is a promising candidate to replace the well-established rare-earth Eu activator in red phosphors. Here, synthesis, structure, detailed spectroscopic and crystal field analysis of Mn4+ doped Li2TiO3 are presented. We have successfully synthesized Li2Ti0.995Mn0.005O3 powder by a solid state synthesis. All precursors were mixed with stoichiometric ratio Li : Ti = 2 : 1 and calcined at 800⁰C for 5 h to produce pure-phase. Structural analysis was done by X-ray diffraction. It confirmed monoclinic Li2TiO3 structure with space group C2/2, in which the octahedral sites are occupied by Ti4+ and Mn4+. Optical characterization included measurements of photoluminescence excitation and emission spectra, emission decay analyses and diffuse reflection measurements. Optical spectra showed emission and absorption bands originating from d–d electronic transitions (d3 configuration). Kubelka-Munk function of the measured diffuse reflectance spectrum showed characteristic absorption of Mn4+. Deep red emission at 680 nm under λexc= 490 nm is a result of the spin-forbidden 2Eg→4A2 and 4T2→ 4A2 electron transition of the tetravalent manganese ions. The energy levels of the Mn4+ ions in Li2TiO3 were calculated using the crystal field exchange-charge model. Three parameters are needed to describe completely each energy level: the Racah parameters B and C, which determine the positions of the free ion energy levels, and the crystal field strength denoted by Dq, which describes the splitting of the free ion terms. These parameters were calculated and showed good agreement with the experimental photoluminescence excitation and emission spectra. Figure 1