Thesis Summary
Photoluminescence Properties of Distorted Titanates Investigatedby X-ray Absorption Spectroscopy(烧䶂⏠⍶↮⃱㱽̬͉͌㬒̡͔͹ͷΫ愠⊾䈑̯΍͡΀ΣΗ΅ͳΫͱ̯䞼 䨞)                    FAN DONGXIAO
The titanium oxygen polyhedral, TiO 4 with tetrahedron structure, TiO 5 with square pyramidalstructure, and TiO 6 with octahedron structure, commonly exist in titanate materials from themicroscopic scale point of view. The physical properties of titanates are highly dependent on
the local structure of these titanium oxygen polyhedra. For example, Ba 2 TiO 4 , which contains aseries of isolated TiO 4 tetrahedra, has excellent CO 2 sorption properties. Fresnoite structurematerials containing TiO 5 polyhedron exhibit strong second harmonic generation response andhave been widely used for second-order nonlinear optical materials. A TiO 6 octahedron is morefamous than TiO 4 and TiO 5 and contained in the titanates with perovskite structure, which is a
hot research topic nowadays. The typical example is the ferroelectricity in BaTiO 3 and PbTiO 3 ,which is produced by displacement of a titanium ion in an octahedron cage. In terms ofstructural similarity, TiO 5 can be considered to result from the TiO 6 octahedron with Ti ion
displacement along the z-axis. The local structural nature of these polyhedra in materials isessential for the physical property.
The self-activated titanate phosphors with TiO 5 polyhedra show excellent photon
emission property under UV irradiation, i.e., photoluminescence (PL). This phenomenon wasalso seen in the BaTiO 3 with distorted TiO 6 octahedron. It is interesting to see the PL andferroelectric properties result from the distortion of TiO 6 octahedron. The “self-activatedphosphors” term is from no ion-doped existed in the phosphors, while the conventional
phosphors are created by doping ions in a matrix as the color center. The function of the doped
ions is to form in-gap states. To figure out the role of the TiO 5 polyhedron in self-activatedtitanate phosphors, the investigation of local electronic states in the TiO 5 polyhedron isrequired. In addition, the incipient ferroelectric materials SrTiO 3 , which is known ascentrosymmetric for the whole temperature range, also show PL property with faint blue lightemission below 37 K. This motivates us the investigation of the local structure of TiO 6 with theinfluence of temperature and UV irradiation. Beyond that, to explain the reports about theappearance of the dipole moment and the the increase of dielectric constant in SrTiO 3 under
UV irradiation also requires the local structure information. Due to the sensitivity of the atomic
species and high resolution of spatial distribution, XAFS is an ideal approach for theinvestigation of electronic states of self-activated phosphors and the local structure of SrTiO 3

with UV influence.
The Ti K-edge XANES of self-activated titanate phosphors (Ba 2 TiSi 2 O 8 , Sr 2 TiSi 2 O 8 , andNa 2 TiSiO 5 ) were measured with UV irradiation, two features in the pre-edge region wereobserved: A giant pre-edge feature and hump. The electronic states of features are determinedby the theoretical calculation. The electronic state of pre-edge originated from thehybridization from Ti 3d, Ti 4p, and O 2p acts as in-gap states to produce visible light emission
in the PL process. The electronic state of hump originated from the sub -band of Ti 4p acts aselectron traps, which receive the excited electrons by UV. These results confirm the color
center nature of TiO 5 polyhedron. The energy diagram of self-activated titanate phosphors wasproposed to explain the PL property.
To explore the local structure that benefits to produce PL property, Ti K-edge EXAFS ofSrTiO 3 was conducted with temperature from 22 K to 300 K and UV irradiation. By using thetetragonal structure model of SrTiO 3 to fit the data, the dynamic Ti ion displacement along thez-axis is confirmed, which illustrates the instantaneous existence of TiO 5 polyhedron. With UVirradiation, the Ti ion vibration along the z-axis is significantly enhanced, which can explain
the appearance of dipole moment and the increase of dielectric constant in SrTi O 3 .
The in-gap states are necessary to produce PL property in non-ion-doped titanates, theTiO 5 polyhedron can create the in-gap states. The distortion of TiO 6 octahedron in SrTiO 3
results in the TiO 5 polyhedron. As a result, the PL property can be a effective characterization
method to investigate the information about in-gap electronic states and determine the local
structure distortion in no ion-doped titanates. The simultaneous existence of PL andferroelectric property caused by the local structure distortion point out a new direction for
exploring new multifunctional materials.