Rare earth free white emitting Na₂TiSi_1−xGexO₅ phosphors were synthesized by a conventional solid-state reaction method, and the photoluminescence properties were characterized. The phosphors obtained in the present study showed broad bluish-white emission with a peak at 485 nm due to the charge transfer transition between O²⁻ and Ti⁴⁺ and the emission peak intensity was effectively enhanced by the Ge⁴⁺ doping in the Na₂TiSiO₅ crystal lattice. The emission peak intensity of Na₂TiSi_0.5Ge_0.5O₅ was 1.35 times higher than that of Na₂TiSiO₅ and the CIE chromaticity coordinate values for the phosphor were x = 0.29 and y = 0.38.

Keywords: White emission phosphor, Rare earth free, Na2TiSiO5

Recent year, transition metal ions have been widely investigated as a luminescence ion an alternative to the rare earth ions [1-5], because rare earth elements are expensive compared with another metal and the price of rare earth ions go up every year. In particular, polyhedral oxides consisting of transition metal ions without electron in the outermost d orbital such as Ti⁴⁺, V⁵⁺, and W⁶⁺ have attracted considerable attention as activator in rare earth free phosphors and these transition metal ions doped phosphors can show good luminescence efficiency [6-12]. Among them, Ti⁴⁺ ions contained materials have extensively investigated and used in tunable solid-state laser materials [13], nonlinear optical materials [14], phosphors [15-18], and long-lasting phosphorescence materials [19], due to their optical properties and excellent chemical and thermal stability. Ti⁴⁺ ions contained materials are reported that these materials exhibit two different type luminescence under UV excitation such as blue emission corresponding to the charge transfer transition from 2p orbits of O²⁻ to 3d orbit of Ti⁴⁺, i.e., O²⁻-Ti⁴⁺ → O⁻-Ti³⁺, and yellow emission due to the oxygen defects in titanium octahedron [16, 17, 20]. In addition, the yellow emission in the Ti⁴⁺ contained materials is observed only at low temperature. According to the studies on luminescence properties of Ti⁴⁺ contained materials, however, some materials show the yellow- or white-emission at room temperature under UV excitation and these materials consist TiO₅ square pyramids in the crystal structure. It is well known that the luminescence corresponding to the charge transfer transition from 2p orbits of O²⁻ to 3d orbit of Ti⁴⁺ in the TiO₅ square pyramids has larger Stokes shift than that for the TiO₆ octahetra contain materials [9, 11, 21], and these materials show white-emission with high emission color purity.
Considering this, we have focused on Na₂TiSiO₅ as Ti⁴⁺ contained white emission phosphor, because the Na₂TiSiO₅ consist TiO₅ square pyramids in the crystal structure and TiO₅ pyramidal shared corners with the SiO₄ tetrahedral, which is separated by Na⁺ ions in the direction to the c axis (Fig. 1) [11, 22]. In addition, Na₂TiSiO₅ has high chemical and thermal stability, which is advantageous for obtaining phosphor particles of high crystallinity. It is previously reported that Na₂TiSiO₅ exhibits the bluish-white emission at low temperature (4.2 k). In this study, in order to further enhance the emission efficiency of the Na₂TiSiO₅ phosphor, Si⁴⁺ ions in the Na₂TiSiO₅ crystal lattice were partially substituted with Ge⁴⁺, because Na₂TiGeO₅ has similar crystal structure to that of Na₂TiSiO₅, which indicates that Ge⁴⁺ can be substituted in the Na₂TiSiO₅ lattice as solid solution form.

X-ray powder diffraction (XRD) patterns of the Na₂TiSi_1-xGe_xO₅ (0 ≤ x ≤ 1.0) phosphors are shown in Fig. 2. The standard XRD patterns of Na₂TiSiO₅ (#166623) and Na₂TiGeO₅ (#20129) from the inorganic crystal structure database (ICSD) are also shown in Fig. 2 as a reference. The XRD patterns of the samples with x ≤ 0.5 were identical to a single phase of highly crystalline orthorhombic Na₂TiSiO₅ structure. On the other hand, the crystal structure of the samples transferred from Na₂TiSiO₅ to Na₂TiGeO₅ with increase in the Ge⁴⁺ content beyond x = 0.5 and these samples obtained in a single phase of Na₂TiGeO₅. However, there are no diffraction peak corresponding to any impurities in all XRD patterns, which is attributed to the same crystal structure of both Na₂TiSiO₅ and Na₂TiGeO₅ [11,22]. In addition, a peak shift to a lower diffraction angle was observed with increase in the Ge⁴⁺ content, because Si⁴⁺ (ionic radius: 0.026 nm for 4 coordination [23]) sites in the Na₂TiSiO₅ lattice is substituted with the larger Ge⁴⁺ (ionic radius: 0.026 nm for 4 coordination [23]) to form solid solutions.
Fig. 3 shows the emission spectra of the Na₂TiSi_1-xGe_xO₅ (0 ≤ x ≤ 1.0) phosphors with excitation at 254 nm. The emission spectra of the samples with x ≤ 0.5 exhibit a broad bluish-white emission band with a peak at 490 nm due to the charge transfer (CT) transition from 2p orbits of O²⁻ to 3d orbit of Ti⁴⁺, i.e., O²⁻-Ti⁴⁺ → O⁻-Ti³⁺ in the isolated TiO₅ polyhedron in the Na₂TiSiO₅ lattices [16, 17, 20]. The emission intensity of Na₂TiSiO₅ was effectively enhanced by the Ge⁴⁺ doping into the crystal lattice, and the emission intensity of Na₂TiSi_0.5Ge_0.5O₅ was 1.35 times higher than that of Na₂TiSiO₅. In the samples with x > 0.5, however, the emission band shifted to shorter wavelength (higher energy) side with increase in the Ge⁴⁺ content, which is considered to be due to the phase transition from Na₂TiSiO₅ to Na₂TiGeO₅. The emission peak wavelength corresponding to charge-transfer (CT) transition strongly depends on the excitation energy for the electron transfer between O²⁻ and Ti⁴⁺ in the crystal structure. Since the electronegativity of Ge⁴⁺ (2.01 [24]) is higher than that of Si⁴⁺ (1.90 [24]), the electronic attractive force between O²⁻ and Ti⁴⁺ in the Na₂TiGeO₅ is smaller than that in the Na₂TiSiO₅, which leads to shift the emission band to shorter wavelength (high energy) side due to increase the excitation energy for the electron transfer from O²⁻ to Ti⁴⁺ in the crystal structure. In addition, the Ti⁴⁺-O²⁻ bond length in the Na₂TiGeO₅ lattice is larger than that in Na₂TiSiO₅ lattice (Table 1), which also leads to shift the emission band to shorter wavelength side.
Fig. 4 presents the Commission International de I'Eclairage (CIE) chromaticity diagram for the Na₂TiSi_0.5Ge_0.5O₅ phosphor. Results for a conventional Ti⁴⁺-contained white emitting Ba₂TiP₂O₉ phosphor is also included for comparison. The CIE chromaticity coordinate values of the Na₂TiSi_0.5Ge_0.5O₅ phosphor were x = 0.29 and y = 0.35 under excitation at 254 nm UV light, which is close to the standard white chromaticity (x = 0.33, y = 0.33) for the NTSC system. In addition, the x and y values of the Na₂TiSi_0.5Ge_0.5O₅ phosphor are more close to white light compared with a conventional Ti⁴⁺ contained Ba₂TiP₂O₉ phosphor (x = 0.29, y = 0.38).

Rare earth free white emitting Na₂TiSi_1−xGe_xO₅ phosphors were in a single phase form synthesized by the conventional solid-state reaction method. These phosphors exhibited a broad bluish-white emission centered on 490 nm under excitation at 254 nm and the photoluminescence emission intensity was successfully enhanced by the Ge⁴⁺ doped into the Si⁴⁺ site in the Na₂TiSiO₅ lattice. The emission intensity of Na₂TiSi_0.5Ge_0.5O₅ was 1.35 times higher than that of Na₂TiSiO₅ and the CIE chromaticity coordinate values for the phosphor were x = 0.29 and y = 0.35 under excitation at 254 nm UV light.

This work was supported by a project from NEDO, New Energy and Industrial Technology Development Organization (Rare Metal Substitute Materials Development Project Development of Technology for Reducing Tb and Eu Usage in Phosphors for Fluorescent Lamp by High-speed Material Synthesis and Evaluation).