Nanooptics and photonics

The method for control Mn charge state in the MgTiO3 red phosphors
L. Borkovska1, L. Khomenkova1, T. Stara1, I. Vorona1, V. Nosenko1, O.Gudymenko1, V. Kladko1, K. Kozoriz1, C. Labbe2, J. Cardin2, J.-L. Doualan2 and T. Kryshtab3
1 V. Lashkaryov Institute of Semiconductor Physics of the NAS of Ukraine, 45 Prospect Nauky, 03028 Kyiv, Ukraine. 
E-mail: l_borkovska@ukr.net
2 CIMAP, CEA-CNRS-ENSICAEN, Normandie Université, 6 Blvd Maréchal Juin, Caen, France.
 3 Instituto Politécnico Nacional – ESFM, Av. IPN, Ed.9 U.P.A.L.M., 07738 Mexico D.F., Mexico.

Recently, magnesium titanates have become important for applications in solid state lighting. Specifically, their activation with Mn4+ ions results in a red emission which can be excited by light of blue LED chip. The temperature dependent characteristics of red emission enable application of these materials as optical thermal sensors. However, the control of manganese charge state remains one of the major challenges of Mn doped compounds.
 In the present work, the simple method for manipulating Mn charge state in the Mn-doped MgTiO3 red phosphor is proposed. The samples were fabricated through a solid-state reaction using different molar ratio of MgO and TiO2. The concentration of Mn ions was 0.1 mol %. 
The phosphors produced according to a stoichiometric ratio of MgO and TiO2 demonstrated weak red photoluminescence (PL) at 701 nm caused by 2Eg→4A2g transition of the Mn4+ ions substituted Ti4+ in MgTiO3. The PL decay curve showed single exponential behavior with relatively short relaxation time of about 17 μs. Low intensity of room temperature Mn4+ red PL was explained by its thermal quenching, partial absorption of UV excitation light by MgTi2O5 side phase and low concentration of Mn4+ centers. The electron spin resonance (ESR) study proved that manganese incorporated mainly as Mn2+ ion on Mg2+ site. In turn, the samples fabricated with excess MgO showed the 30-50 times increased Mn4+ red PL, enhanced Mn4+ optical absorption and significantly decreased Mn2+ ESR signal. Besides, an excess MgO suppressed formation of MgTi2O5 side phase but resulted in appearance of Mg2TiO4 side phase. It is concluded that excess MgO can be used for control of Mn charge state in magnesium titanates.