Sol–gel preparation of efficient red phosphor Mg2TiO4:Mn4+ and XAFS investigation on the substitution of Mn4+ for Ti4+ Tiannan Ye, Shan Li, Xueyan Wu, Miao Xu, Xiao Wei, Kaixue Wang, Hongliang Bao, Jianqiang Wang and Jiesheng Chen J. Mater. Chem. C, 1 (2013) 4327

Introduction The valence, location and distribution of the Mn species in the host lattices determine their luminescent properties. ex) Zn2SiO4:Mn2+:Green-emitting => green emission 4T1 → 6A1 transition of Mn2+ Mn3+-doped garnet crystal: red to near-infrared region => red emission 5T2 → 5E transition of Mn2+ 3.5MgO▪0.5MgF2▪GeO2:Mn4+: deep red emission => red emission 2E → 4A2 transition of Mn2+ Phosphors prepared using a sol–gel method may possess narrow particle size distributions, homogeneous, compositions and uniform luminescent-center distributions, which are critical for phosphor performance. Mg2TiO4 has an inverse spinel structure, in which the titanium atoms occupy octahedral sites and the magnesium atoms occupy both tetrahedral and octahedral sites.

Experimental Powder samples of Mg2TiO4:Mn4+ nMn/(nMn + nTi) = 0, 0.01, 0.05, 0.10 and 0.25% were prepared through a sol–gel route. A solution of butyl titanate precursor in acetic acid was added into an ethanol solution of magnesium nitrate and manganese nitrate. stirring at 40 °C in a water-bath for 24 h, the yellow solution obtained was dried in an oven at a temperature of 120 °C. The yellowish solid obtained was then calcined in a tube furnace at 900~1300°C for 8 h, followed by annealing at 570°C for 16 h with flowing oxygen.

Results & discussion Fig. 1 XRD patterns of the samples (A) magnesium titanates at elevated calcination temperatures: (a) 900 °C, (b) 1100 °C, (c) 1300 °C; (B) Mg2TiO4:Mn4+ with nMn/(nMn + nTi): (a) 0%, (b) 0.01%, (c) 0.05%, (d) 0.10%, (e) 0.25%.

Results & discussion ionic size Ti4+ : 0.605Å Mn4+ : 0.53Å Mg2+ : 0.72Å Lattice volume decreased with increasing concentrations of Mn

Results & discussion Fig. 2 (a) Excitation spectrum, (b) emission spectra, (c) optical photos according to (b), and (d) concentration dependence of the relative emission intensity of the Mg2TiO4:Mn4+ samples. nMn/(nMn + nTi)= 0%, 0.01%, 0.05%, 0.10% and 0.25% respectively.

Results & discussion Fig. 3 The emission spectra of the as-prepared sample Mg2TiO4:Mn4+ nMn/(nMn + nTi) ¼ = 0.10% phosphor and the reference product Mg2TiO4:Mn4+ nMn/(nMn + nTi) = 0.10% prepared from solid-state reaction under 475 nm excitation.

Results & discussion Scheme 1. Schematic illustration of the coordination model used for (a) the as-prepared sample Mg2TiO4:Mn4+ nMn/(nMn + nTi) = 0.10% phosphor and (b) the reference product Mg2TiO4:Mn4+ nMn/(nMn + nTi) = 0.10% prepared from solid state reaction. The distribution of the Mn4+ luminescent centers is indicated by the white dotted boxes

Results & discussion Fig. 4 (a)The room-temperature UV-vis absorption spectra and (b) the plot of (αhv)2 versus hv based on the direct gap of the sample Mg2TiO4:Mn4+, nMn/(nMn + nTi) = 0%, 0.01%, 0.05%, 0.10% and 0.25% respectively.

Results & discussion Fig. 5 XANES spectrum of the sample Mg2TiO4:Mn4+, nMn/(nMn + nTi) = 0.10% and the XANES spectra of a series reference compounds MnO, Mn2O3 and β-MnO2 respectively.

Results & discussion Fig. 6 (a) XANES and (b) EXAFS spectra recorded at the Mn and Ti K-absorption edge of the Mn-doped and undoped Mg2TiO4.

Results & discussion

Results & discussion Fig. 7 EPR spectra of the sample Mg2TiO4:Mn4+, nMn/(nMn + nTi) = 0%, 0.01%, 0.05%, 0.10% and 0.25% respectively.

Conclusions An efficient red phosphor Mg2TiO4:Mn4+ with different Mn4+ concentrations has been prepared through a simple sol–gel route. The successful substitution of Mn4+ for Ti4+ in the lattice of Mg2TiO4 was confirmed on the basis of XAFS spectroscopy. As revealed by XANES, EPR and UV-vis analyses, the oxidation state of the doped Mn was +4. The emission wavelength of this Mg2TiO4:Mn4+ phosphor was at approximately 655 nm, assigned to the 2E → 4A2. The emission intensity of the phosphor was approximately 1.5 times higher than that prepared through a solid-state reaction.