Photoluminescence mechanism of self-activated titanate phosphors investigated by x-ray absorption spectroscopy under UV irradiation

Abstract

The electronic states of self-activated titanate phosphors (Ba 2 TiSi 2 O 8 , Sr 2 TiSi 2 O 8 , and Na 2 TiSiO 5 ) were investigated by Ti K-edge x-ray absorption spectroscopy under ultraviolet (UV) irradiation in order to unveil the photoluminescence mechanism in the absence of particular atomic color centers. The overall spectral features of all samples were identical, main edges and large pre-edges corresponding to the Ti 4p and Ti 3d states, respectively. The pre-edge peak of Sr 2 TiSi 2 O 8 shifted toward lower energy by 0.2 eV compared with the other two samples, corresponding to a red shift of the photoluminescence spectrum. In the region between these features, a small hump was observed for each sample, which gradually diminishes as a function of UV power. The hump can be attributed to the Ti 4p state based on the calculated electron density of states determined by multiple scattering theory, and this hump serves as a trap for UV-excited electrons.

Local Structural Investigation of Near-Infrared-Reflective Black Ca2(Mn,Ti)O4 Pigments Using Synchrotron Radiation X-Ray and Density Functional Theory Calculations

“…To elucidate the origin of the change in the pre-edge peak by doping with Ti 4+ , the LDOS for the final local structure models of Ca 2 (Mn 1– x Ti x )­O 4 ( x = 0 and 0.25) were also calculated, and the results are shown in Figure . In the pre-edge region ( E – E F = 0–5 eV; where E F is the Fermi energy), peaks were observed due to the hybridization of the Mn 3d and 4p orbitals. ,, After doping with Ti 4+ , the maximum LDOS for Mn 4p was increased, and those positions at the peaks 1 and 2 regions were shifted to the higher and lower energy sides, respectively. Accordingly, the intensities and positions of pre-edge peaks 1 and 2 changed because of the change in the LDOS for the Mn 4p orbital.…”

“…In the pre-edge region (E − E F = 0−5 eV; where E F is the Fermi energy), peaks were observed due to the hybridization of the Mn 3d and 4p orbitals. 30,43,45 After doping with Ti 4+ , the maximum LDOS for Mn 4p was increased, and those positions at the peaks 1 and 2 regions were shifted to the higher and lower energy sides, respectively. Accordingly, the intensities and positions of preedge peaks 1 and 2 changed because of the change in the LDOS for the Mn 4p orbital.…”

“…For example, in the analysis of Ca–Mn–O oxides, the valence states of Mn in the compounds were analyzed and the corresponding changes resulting from doping by impurity cations was investigated using XAFS. − Furthermore, the local structures and the Mn–O and Mn–Mn atomic distances were revealed by using the radial distribution functions (RDF) obtained from EXAFS spectra. Moreover, in recent years, computational XANES simulations have been utilized as a theoretical tool. − By comparison of the experimental and theoretical data, the electronic states of the components in compounds were analyzed, and the simulations were used to evaluate the validity of the obtained structural model. In this study, the valence states of Mn and Ti atoms in Ca 2 (Mn,Ti)­O 4 black pigments and their average and local crystal structures were analyzed by using synchrotron radiation X-ray.…”

Structures, photoluminescence, and principles of self-activated phosphors

“…In titanate phosphors, the Ti 4+ ions in the TiO 6 octahedra can be excited through charge transfer (CT) from the O 2À ions to Ti 4+ ions upon UV irradiation, and then emit blue to green light (400-500 nm), which is ascribed to the CT from Ti 4+ back to O 2À . 42 Fan 43 studied the properties of self-activated titanate phosphors (Ba 2 TiSi 2 O 8 , Sr 2 TiSi 2 O 8 , and Na 2 TiSiO 5 ). The overall spectral features of all the samples were identical, with the dominant edges and large pre-edges corresponding to the Ti 4p and Ti 3d states, respectively.…”