Synthesis, characterization and photoluminescence properties of Tm 3+ -doped Ca 2 YTaO 6 blue-emitting phosphors

. A series of Ca 2 YTaO 6 : Tm 3+ blue-emitting phosphors were firstly prepared by the solid-state method. The phase formations and purity of Ca 2 YTaO 6 : x Tm 3+ ( x = 0.3%-5% mol) were verified by X-ray powder diffraction. The morphological characteristics of Ca 2 YTaO 6 : 0.005Tm 3+ were detected by scanning electron microscopy (SEM). Photoluminescence properties were discussed by emission ( λ em = 460 nm) and excitation ( λ ex = 359 nm) spectra. The critical doping concentration of the products was 0.005 mol. The proposed concentration quenching mechanism in Ca 2 YTaO 6 materials was the electric multipole interaction. Besides, the color coordinates (0.1408, 0.0891) of Ca 2 YTaO 6 : 0.005Tm 3+ were located in blue region. The results suggested the Ca 2 YTaO 6 : Tm 3+ phosphors can be promising blue-emitting components for the WLED applications.


Introduction
Attaching great importance of energy-saving and environment protection, the traditional fluorescent lamps and incandescent lamps in daily lighting were fade away. White light-emitting diodes (WLEDs) are getting growing attention for the merits of energy-saving, improving efficiency, etc. [1][2][3][4][5][6][7]. Nowadays, two common methods of WLED fabrication were widely used. (1) Combining the blue LED chips with Ce 3+ doped Y 3 Al 5 O 12 phosphors. However, the great limitations of the method are low color rendering index (CRI) and high correlated color temperature. And the low CRI was due to the lack of red composition [8][9][10]. Method (2) was mixing the tricolor (RGB) emission phosphors with a near-UV chip [11][12][13]. The quest for blue-emitting phosphors for lighting is an important and urgent challenge for investigators.
Ttantalates enjoy the merits of excellent chemical and physical stability, which have drawn widespread attention in the synthesis of phosphors. Recently, Eu 3+ , Sm 3+ , Dy 3+ , and Mn 4+ doped Ca 2 YTaO 6 phosphors were proved to be well applied in the WLED applications [14,15] [16][17][18][19]. In this work, Ca 2 YTaO 6 :Tm 3+ phosphors were firstly synthesized through solid-state reaction (at 1550 °C, in air condition). The XRD data of the products were collected and discussed. At the same time, the morphology, the concentration quenching mechanism, photoluminescence (PL) properties, and CIE color coordinates were investigated in detail.

Synthesis method
The Ca 2 YTaO 6 : xTm 3+ (x = 0.3%-5% mol) phosphors were obtained by solid-state reaction. Starting materials CaCO 3 (99.99%), Y 2 O 3 (99.99%), Ta 2 O 5 (99.5%), and Tm 2 O 3 (99.99%) were weighed according to the stoichiometric ratio. Following the general process, the mixed powder was ground for 15 minutes and was transferred into crucibles. Then, the reaction system was calcined at 1550℃ for 3 hours in the muffle furnace. After the calcination was finished, hard white phosphors were obtained and reground for the subsequent luminescence characterizations.

Characterization method
The phase purity of phosphor was detected by the XRD through a Bruker D2 PHASER X-ray diffractometer with Cu Kα radiation source (λ = 0.15405 nm), and the result was presented in 2θ range between 10° and 70°. The morphology was measured by a scanning electron microscope (JEOL JSM-6490). The PL spectra were achieved by Edinburgh FLS 980 spectrometer. The XRD patterns of Ca 2 YTaO 6 : xTm 3+ (x = 0.3%-5% mol) phosphors were presented in Figure 1. A comparison of the patterns revealed that the diffraction peaks of the products match well with the simulated card of Ca 2 YTaO 6 , indicating that the as-prepared products have a high phase purity, and the Ca 2 YTaO 6 : Tm 3+ phosphors were successfully synthesized. For the composition of Ca 2 YTaO 6 : 0.005Tm 3+ , the cell parameters are a = 5.814 Å, b = 8.050 Å, c = 5.578 Å, V= 261.07 Å 3 , respectively. As the ionic radii of the Y 3+ (0.90 Å ) are approximate to that of Tm 3+ ions (1.052 Å), while the coordination number equals 6 [20], suggesting Tm 3+ ions can take place of Bi 3+ ions in relatively low concentration in the Ca 2 YTaO 6 compound. The surface morphology and particle sizes of phosphors are important for WLED applications. The SEM micrograph of the typical Ca 2 YTaO 6 : 0.005 Tm 3+ product is shown in Figure 2. At 10000× and 5000× magnification, the particle sizes are distributed in the range of a few microns to dozens of microns. The products have an irregular surface morphology with uneven size distribution among the pieces, which was the result of high-temperature annealing.   [21,22]. And another strong peak at 359 nm is attributed to the characteristic 4f-4f transition of 3 H 6 → 1 D 2 of Tm 3+ . In the blue curve, the single emission band at 460 nm is attributed to the electronic dipole transition of 1 G 4 → 3 H 6 of Tm 3+ [23]. The energy level diagram of Tm 3+ illustrates the corresponding radiative process in the host. Under 359 nm excitation, the electrons in Tm 3+ transitioned from the 3 H 6 level of the ground state to the 1 D 0 level of the excited state. Subsequently, the electrons of Tm 3+ relaxed to 1 G 4 level and returned to the 3 H 6 level via radiative transitions and emitted blue light. The typical blue emission peaks of Ca 2 YTaO 6 : Tm 3+ suggest its potential applications as blue-emitting phosphors. The PL spectra of Ca 2 YTaO 6 : xTm 3+ (x = 0.3%-5% mol) are shown in Figure 4(a). All the emission curves have the same peak shapes while the doping concentration of Tm 3+ is different. In Figure 4(b), when the doping concentration of Tm 3+ reached 0.5% mol in the Ca 2 YTaO 6 host, the emission intensity of the Ca 2 YTaO 6 : 0.005Tm 3+ reached its maximum. Whereafter, the intensity gradually decreased with further increasing Tm 3+ doping concentration, which is caused by concentration quenching. The energy transfer mechanism in the Ca 2 YTaO 6 :

Results and discussion
Tm 3+ system is discussed in terms of critical energy transfer distance (Rc), which can be estimated by the equation (1) [24-28]: Here X c (0.005) represents the critical doping concentration. In the unit cell, V (261.07 Å 3 ) refers to the volume, and N (4) is the number of substitutable cations. According to equation (1), R c was calculated to be 29 Å, more than that of exchange interaction distance (5.0 Å). Thus, the electric multipole interactions between Tm 3+ ions are proposed for the concentration quenching mechanism.
Besides, the specific type of the energy transfer mechanism among Tm 3+ in Ca 2 YTaO 6 host is discussed according to Dexter's theory [29]: (2) Where Q is a certain constant, representing types of energy transfer interaction, which are the nearest-neighbor ions (Q=3), electric dipole-dipole (Q=6), dipolequadrupole (Q=8), and quadrupole-quadrupole (Q=10) interactions, respectively. x is the Tm 3+ doping concentration, K and β are constants. The experimental data of the relationship between lg(I/x) and lgx ( Figure 5) can be fitted by a linear relationship.
The slope (-Q/3) of the line is well fitted to be −1.11. Thus Q value is expected to be 3, suggesting the nearest neighbor ions interaction is the main mechanism for the concentration quenching in Ca 2 YTaO 6 : Tm 3+ compounds. CIE chromaticity coordinates are important parameters that should be characterized to determine the luminescent properties of materials. The color coordinates of Ca 2 YTaO 6 : xTm 3+ (x = 0.3%-5% mol) are displayed in Figure 6 with the uniform locations in blue region. The color purity of the products can be estimated by equation (3) [30,31]:

100
(3) Here (x, y) are the chromaticity coordinates of the phosphors; (x i , y i ) are the chromaticity coordinates of the white light source, equal to (0.333, 0.333); and (x d , y d ) are the chromaticity coordinates of the dominant wavelength points [32]. As shown in Figure 6, the color coordinates of the products were in the blue region. In the Ca 2 YTaO 6 : 0.05Tm 3+ compound, the color purity was calculated around 94.6%. When the Ca 2 YTaO 6 : Tm 3+ is excited by UV light, it can emit blue light effectively with a high color purity.

Conclusions
The Ca 2 YTaO 6 : xTm 3+ (x = 0.3%-5% mol) phosphors were firstly prepared through solid-state method. At 359 nm excitation wavelength, the Ca 2 YTaO 6 : Tm 3+ phosphors present strong emission peaks at 460 nm. The critical doping concentration of Tm 3+ is 0.5% mol in Ca 2 YTaO 6 : xTm 3+ . The concentration quenching mechanism is proposed for the interaction of the nearest-neighbor ions, while the relevant critical distance is around 29 Å. The CIE color coordinates near the standard blue position. The results indicate that the Ca 2 YTaO 6 : Tm 3+ products have blue-emission ability with good color purity. As a result, the Ca 2 YTaO 6 : Tm 3+ phosphors are promising blueemitting candidates for WLED applications.