Nanooptics and photonics

The influence of thermal decomposition of matrix on the photoluminescence of Mn4+ activated Mg2TiO4 red phosphors
L. Borkovska1, T. Stara1, I. Vorona1, L. Khomenkova1, O. Gudymenko1,         V. Kladko1, C. Labbé2, X. Portier2, J.Cardin2, T. Kryshtab3
1V. Lashkaryov Institute of Semiconductor Physics of the NAS of Ukraine, 45 Prospect Nauky, 03028 Kyiv, Ukraine. 
E-mail: l_borkovska@ukr.net
2CIMAP, Normandie Univ, ENSICAEN, UNICAEN, CEA, CNRS, 6 Blvd. Maréchal Juin, 14050 Caen, France.
3Instituto Politécnico Nacional – ESFM, Av. IPN, Ed.9 U.P.A.L.M., 07738, Ciudad de Mexico, Mexico.

Over the last decade, the magnesium titanates have attracted a great attention as the excellent microwave dielectric materials. Recently, they become important for optical applications too. In particular, the Mn4+ activated Mg2TiO4 has been proposed as a cheaper and more environmentally friendly alternative for rare earth doped nitride red phosphors used in commercial white LEDs. The Mg2TiO4 can be easily obtained by solid state reaction method. However, at temperatures lower than 1400 ºC it is unstable and tends to decompose on MgTiO3. In this report, we present the influence of thermal decomposition of Mg2TiO4 on the Mn4+ red photoluminescence (PL) efficiency. The phosphors were sintered at 1150 °C in air and cooled down to room temperature slowly (during ~3 h) and quickly (during one min). The X-ray diffraction showed that quick cooling resulted in formation of almost pure Mg2TiO4 phase, while slow cooling produced up to 15 % of MgTiO3 phase in addition to Mg2TiO4. 
All phosphors demonstrated red PL due to 2Eg→4A2g transition of the Mn4+ ions substituted Ti4+ in crystal lattice of magnesium titanates. At the same time, the Electron paramagnetic resonance (EPR) spectra revealed the sextets caused by Mn2+ ions in both Mg2TiO4 and MgTiO3 phases. No EPR signals from Mn4+ ions were found. In turn, no PL originated from Mn2+ ions was detected indicating that Mn2+ ions acted as the centers of non-radiative recombination. The phosphors subjected to slow cooling demonstrated the intensity of Mn4+ red PL up to 3 times higher than those produced under quick cooling. They also showed smaller intensity of EPR signal of Mn2+ ions in Mg2TiO4. It is proposed that enhanced red PL is caused by partial thermal decomposition of Mg2TiO4. The latter produces Mg-rich Mg2TiO4 and MgTiO3 phases, in which Mn tends to incorporate in +4 charge state and the concentration of non-radiative centers is decreased.