Crystal Structure and Luminescence Properties of a Thermally Stable Single-Phase White Emitting Phosphor CaSr 2 (PO 4 ) 2 : Dy 3+ , Li +

Single-phase CaSr 2 (PO 4 ) 2 :Dy 3+ ,Li + phosphors were prepared using the high-temperature solid-state method in the air. To characterize the luminescence properties of the synthesized phosphors, Powder X-ray diffraction patterns (XRD), scanning electron microscopy images (SEM), photoluminescence spectra, and concentration-dependent emission spectra were measured to characterize the luminescence properties of the synthesized phosphors. The results showed that the CaSr 2 (PO 4 ) 2 :Dy 3+ ,Li + phosphors exhibited white luminescence, and the emission spectra of the phosphors consisted of two sharp peaks at ≈ 486 and ≈ 578 nm (the most intense one). The optimum concentration of Dy 3+ doping was determined to 0.06 mol.%. On the basis of the Dexter's theory, the mechanism of energy transfer between the Dy 3+ ions was determined to dipole–dipole interactions. The results of the temperature-dependent luminescence conrmed that the as-prepared phosphors are proved to be promising UV-convertible material capable of white light emitting in UV-LEDs due to its excellent thermal stability and luminescence properties. Luminescence intensity and decay time of the CaSr 2 (PO 4 ) 2 :Dy 3+ ,Li + phosphors were improved remarkably with the addition of charge compensators (Li + ions), which would promote their applications in white light-emitting diodes based on the near-UV chip.


Introduction
In recent years, white light-emitting diodes (LEDs) have been widely applied to various purposes due to their superior performance, like device indicators, automobile headlights and general illumination [1][2][3].
Commercial manufacture of white LEDs is typically achieved through coating a yellow-emitting phosphor on a blue LED chip, because of its lower cost and higher technical development [4,5]. However, these white LEDs exhibit a low color rendering index (CRI < 80) and high correlated color temperature (CCT) due to the de ciency of the red component [6]. Nowadays, many researchers have attempted to obtain the white LEDs by combining a near-UV LED chip with red, green and blue-emitting (RGB) phosphors [7,8].
These RGB systems, however, are expensive to manufacture, and may have a low e ciency of blue emission due to its reabsorption by the red and green-emitting phosphors [9][10][11]. Therefore, e cient, durable, and single-phase white light-emitting phosphors attract increasing research interest because of these disadvantages.
Here, a series of CaSr 2-2x (PO 4 ) 2 :xDy 3+ , xLi + phosphors was synthesized by a high-temperature solid-state reaction method in an air atmosphere. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence excitation (PLE) and emission (PL) spectra, including time-resolved spectroscopy (luminescence decay curves). The XRD pattern of the CaSr 2 (PO 4 ) 2 phase is similar to the whitlockite mineral that crystallizes in the space group R3c. In addition, the asprepared phosphors have good thermal stability, con rmed by temperature-dependent emission spectra.
The obtained results indicate that the as-prepared CaSr 2 (PO 4 ) 2 :Dy 3+ ,Li + phosphor can act as a UV convertible, white phosphor for w-LEDs.

Materials and synthesis
Powder samples CaSr 2-2x (PO 4 ) 2 :xDy 3+ ,xLi + (x= 0-0.4) were synthesized by a high temperature solid-state method. The starting materials, CaCO 3 (99.9%), SrCO 3 (99.9%), Li 2 CO 3 (99.9%), (NH 4 ) 2 HPO 4 (99.9%), and Dy 2 O 3 (99.999%) were purchased from Aldrich. First, according to stoichiometric ratios, these starting materials were mixed and thoroughly ground in an agate mortar. The pre-sintered samples were then transferred evenly to a tube furnace and calcined at 800 ℃ for 1 h of decomposition of the calcium carbonate and strontium carbonate in air. Finally, the as-prepared powders were calcined again at 1250 ℃ for 5 h in an air atmosphere, and the furnace was cooled down to room temperature before their removal.
The PL and PLE spectra of the phosphors were measured on a F-4600 spectro uorometer (HITACHI, Japan) with a photomultiplier tube operating at 500 V, and a 150 W Xe lamp used as an excitation source. A 400 nm cut-off lter was used to eliminate the second-order emission. Temperature-dependent PL spectra were also measured using the same spectro uorometer with the home-made heating controller. The CaSr 2 (PO 4 ) 2 were observed by SEM using the HitachiS-520 instrument. The roomtemperature luminescence decay curves were recorded with a spectro uorometer (Horiba, JobinYvon TBXPS), using a tunable pulse laser radiation as the excitation source.

Crystal structure
The valence charge is unbalanced when Mn + ions are substituted by M (n+1) . This is undesirable for the phosphor materials and decreases the luminescence intensity. To avoid the charge unbalance and the formation of vacancy in the sample, Li + ions were employed as charge compensators and added along with Dy 3+ ions. The mechanism of charge compensation is based on the fact that two Sr 2+ ions are replaced by one Dy 3+ ion and one Li + ion. Therefore, the crystallinity of CaSr 2 (PO 4 ) 2 :Dy 3+ ,Li + improves because the doping with Li + lowers the crystallization temperature. The XRD patterns of the synthesized and CaSr 2 (PO 4 ) 2 are shown in Fig. 2. It is clearly seen that all the XRD patterns can be well tted with the standard patterns of the Ca 3 (PO 4 ) 2 (JCPDS no. 09-0169) belonging to the trigonal crystal system, with the space group of R3c (161). The introduction of Li + did not have any signi cant in uence on the structure of the as-prepared samples, which suggests that all samples were crystallized in a single phase. All diffraction peaks shifted to the smaller 2θ angle side (larger d-spacing, i.e. interplanar distances), which can be ascribed to the substitution of Ca 2+ by larger Sr 2+ . Dy 3+ and Li + ions have been successfully embedded into the crystal lattice, and also the Dy 3+ and Li + dopant ions had negligible in uence on the structure of the host CaSr 2 (PO 4 ) 2 in varying amounts of doping.
The crystal structure of CaSr 2 (PO 4 ) 2 and different coordination environments for Ca/Sr atoms have already been discussed in the literature. [32] The crystalline structure of CaSr 2 (PO 4 ) 2 is a result of the lattice deformation of Ca 3 (PO 4 ) 2 by partial substitution of Ca 2+ with Sr 2+ . In the crystal structure of CaSr 2 (PO 4 ) 2 , the Ca 2+ (Sr 2+ ) ions are distributed between ve crystallographic sites, four of which can be occupied by both Ca 2+ and Sr 2+ ions in various ways de ned by chemical composition, and one site only can be occupied by Ca 2+ ions, which are too small to be occupied by Sr 2+ ions. If the number of Sr 2+ ions continues to increase in the Ca 2−x Sr 1+x (PO 4 ) 2 structure, the replacement of Ca 2+ by Sr 2+ in all these compounds does not change the space group, and the crystal structure remains unchanged. If all crystallographic sites are occupied by Sr 2+ ions, the structure of CaSr 2 (PO 4 ) 2 would change to Sr 3 (PO 4 ) 2 , which is different from the former one. In the crystal structure of Sr 3 (PO 4 ) 2 there are two nonequivalent crystallographic sites for the Sr 2+ ions (Sr1 and Sr2). The Sr1 atoms are located on the threefold axis, and show 10 coordination sites with 6 oxygen atoms as nearest neighbors belonging to the PO 4 group and site symmetry C3v slightly closer than other coordination sites with 8 oxygen atoms. The other Sr2 atoms in the unit cell show an octahedral con guration, coordinated with eight oxygen atoms. The Sr2 site has 12 coordination numbers and is the largest site in this structure from the PO 4 tetrahedral on the six-fold axis.
It was found that the formation of CaSr 2 (PO 4 ) 2 structure is more favorable than the formation of Sr 3 (PO 4 ) 2 [33]. This phenomenon can be explained by the fact that the formation energy of CaSr 2 (PO 4 ) 2 from Ca 2 Sr(PO 4 ) 2 is lower by 17.34 eV than the formation energy of CaSr 2 (PO 4 ) 2 from Sr 3 (PO 4 ) 2 [34]. In the crystal structure of CaSr 2 (PO 4 ) 2 , the Ca 2+ /Sr 2+ ions are distributed between ve crystallographic sites -all of them are occupied by both Ca 2+ and Sr 2+ ions in various ways. The Sr1/ Ca1, Sr2/Ca2, Sr3/Ca3, Sr4/Ca4 and Sr5/Ca5 positions are coordinated with six, six, seven, three and six oxygen atoms, respectively. It is worth noting that the Ca4 is three0fold coordinated, suggesting weak bonding and the formation of crystal defects.
To investigate the composition and morphology of the material, the CaSr 1.88 (PO 4 ) 2 : 0.06Dy 3+ , 0.06Li + phosphor was selected as a representative example for measurements. Fig. 3(a) displays the results elemental analysis of the sample measured by the EDS method, and the inset shows SEM images of the CaSr 1.88 (PO 4 ) 2 : 0.06Dy 3+ , 0.06Li + sample with elemental mapping of CaSr 1.88 (PO 4 ) 2 : 0.06Dy 3+ , 0.06Li + phosphor provided in Fig. 3(b). The EDS results indicate that the synthesized phosphor is composed of Ca, Sr, P, O, and Dy, which is consistent with the composition of CaSr 1.88 (PO 4 ) 2 : 0.06Dy 3+ , 0.06Li + material. Besides, the results revealed the contents of each element in Table 1. The SEM image reveals that the as-prepared samples are well-crystallized. The substances synthesized by solid-state method are usually agglomerated, but the sample obtained consist of irregular crystal sizes, which meet well the requirements of phosphor used in w-LEDs. Moreover, the mapping results showed that the distribution of the elements in this material is very uniform. assigned to the f-f transitions of Dy 3+ from its ground state 6 H 15/2 to the excited states 4 M 17/2 , 6 P 7/2 , 6 P 3/2 , 4 F 7/2 and 4 G 11/2 , respectively. Excitation peaks between 320 and 400 nm indicate that CaSr 2 (PO 4 ) 2 :Dy 3+ phosphors can be effectively excited by near-UV LED-chips. Fig.5 shows the emission spectra of the CaSr 2-2x (PO 4 ) 2 :xDy 3+ ,xLi + (x = 0.01-0.3) phosphors, measured at room temperature, wavelength with the excitation of 403 nm ( 6 H 15/2 → 6 P 3/2 ); the inset shows the dependence of the emission intensity at 486 and 578 nm on the Dy 3+ doping concentration. Different from other Dy 3+ -doped phosphors [35], as the Dy 3+ amount increases, the emission intensity increases and reaches a maximum at 6 mol.% Dy 3+ doping content, which is considered as the optimum concentration. A higher Dy 3+ ion concentration results in a reduction of the luminescence intensity associated with concentration quenching phenomenon. This is because when the concentration of Dy 3+ increases, the distance between the ions reduces and enhances the energy transfer cross-relaxation processes between the dopant ions. Fig. 6(a) shows the emission spectra of the CaSr 2-2x (PO 4 ) 2 :xDy 3+ ,xLi + (x = 0.01 and 0.03) and CaSr 2-2x (PO 4 ) 2 :xDy 3+ ,xNa + (x = 0.01 and 0.03) phosphors, under 351 nm excitation. The emission spectra of CaSr 2 (PO 4 ) 2 :0.03 Dy 3+ ,Li + / Na + samples include some peaks centered at 486 and 578 nm, which are similar to those mentioned in Fig. 5. This indicates that co-doping with Li + enhances the luminescence intensity of phosphors and it is more effective than co-doping with Na + . The maximum luminescence intensity is achieved with the content of 3% Li + doping. Fig. 6(b) illustrates the emission spectra of CaSr 2-2x (PO 4 ) 2 :xDy 3+ , xLi + and CaSr 2-1.5x (PO 4 ) 2 :xDy 3+ phosphors, showing the bene cial effect of Li + co-doping on the luminescence intensity of the samples. The phosphors need to produce ion defects to maintain the charge balance because the replacement of Sr 2+ with Dy 3+ is not equal. However, too many defects produce crystal lattice distortions that reduce the luminescence intensity. Instead, the introduction of Li + ions means that the charge compensation takes place according to the following formula: 2Sr 2+ = Dy 3+ + Li + . This limits the number of crystal defects and leads to the enhanced luminescence intensity. Moreover, Li + co-doping changes symmetry of the local coordination environment and the related crystal eld strength. The reduced site symmetry may also lead to the enhanced luminescence intensity.

Energy transfer mechanism
The quenching of the emission intensity depends on the critical distance Rc, which is the shortest average distance between the nearest dopant Dy 3+ ions at a critical concentration x c . The critical distance Rc is described by the following equation [36]: in which V stands for the volume of the unit cell, x c is the critical concentration of activator ion (Dy 3+ ) beyond the concentration quenching, and N represents the number of host cations in one unit cell. In our case, N = 6 V was estimated to be 3744.14 Å, and x c is 0.06 according to the above discussions.
According to the eq. (1), Rc was changed to 27.08 Å (x c = 0.06). It is well-known that exchange interactions play a crucial role in the energy transfer mechanism when the critical distance between the sensitizer and the activator ions is less than 4 Å. With a much higher Rc value, the energy transfer mechanism is considered to be an electric multipolar interaction. Based on the Dexter's theory, if the energy transfer occurs by electric multipolar interactions, then the relationship between the luminescent intensity (I) and the activator concentration (x) can be expressed by the following equation [37]: where x is the activator concentration, K and β are constants for each interaction at the same excitation. θ is a multipolar interaction constant equal to 3, 6, 8 or 10, corresponding to the nearest-neighbor ions, i.e., dipole-dipole (d-d), dipole-quadrupole (d-q) and quadrupole-quadrupole (q-q) interactions, respectively. We chose the peaks at 486 and 578 nm was plotted and depicted in Fig. 7. All q values are close to 6, hence we can conclude that the quenching mechanism between Dy 3+ ions in the CaSr 2 (PO 4 ) 2 samples conforms to the dipole-dipole (d-d) interactions.

Temperature-dependent luminescent properties
It is well-known that the luminescence intensity for most phosphors decreases if the operating temperature exceeds a certain value (thermal quenching).
[38] Thus, the thermal stability of the phosphor is a key issue for high-power w-LEDs. One of the key requirements for a good phosphor is to maintain the performance at the operating temperature of the device. Usually, the luminescence intensity of the phosphors at 423 K with respect to that at room temperature is used to assess the thermal stability. The reason for this has been reported as an increase in the non-radiative transition probability in the con gurational coordinated diagram. [39] Fig. 8 shows the temperature dependent emission spectra of the CaSr 1.88 (PO 4 ) 2 :0.06Dy 3+ ,0.06Li + phosphor from 298 to 523 K, at 351 nm excitation; the inset shows the relative emission intensities at 486 and 578 nm as a function of temperature. The shape of the emission bands remains unchanged with increasing temperature (Fig. 8), suggesting that the phosphor has excellent color stability, which is crucial in LEDs or high temperature LEDs. Upon heating the phosphor samples in the temperature range from 298K to 523K, the emission intensity decreased slightly, since the probability of nonradiation is increased and luminescent c enter is released through the crossing point between the excitation state and the ground state, causing the luminescence quenching.
[40] Besides, Fig. 9 illustrated the PL intensity of CaSr 1.88 (PO 4 ) 2 :0.06Dy 3+ ,0.06Li + phosphor with respect to time, monitored under 351 nm excitation continuously for 60 min at (a) 100˚C and (b) 150˚C for 30 min at each temperature with a time interval of 5 min, and the emission pro le of phosphor maintained at 100˚C and 150˚C continuously for 30 min at each temperature showed that the emission intensity are very stable and remain unchanged, as well as without variation of the emission wavelength. In addition, the emission intensities of CaSr 1.88 (PO 4 ) 2 :0.06Dy 3+ ,0.06Li + maintains 77% of the initial emission intensity corresponding to a temperature of 423K, revealing that the CaSr 1.88 (PO 4 ) 2 :0.06Dy 3+ ,0.06Li + phosphors have good thermal stability, which con rms the stable chromaticity coordinates of CaSr 1.88 (PO 4 ) 2 :0.06Dy 3+ ,0.06Li + phosphors. respectively. This is due to the decreasing distance between Dy 3+ -Dy 3+ ions, resulting in the observed concentration quenching phenomenon (enhanced cross-relaxations processes), as well as enhanced probability of energy transfer to the luminescence killer sites. Thereby, the luminescence lifetimes of Dy 3+ ions are shortened due to the favorable nonradiative energy transfer processes when the Dy 3+ concentration increases. The observed bi-exponential character of the decay curves could be because of the non-equal occupation of the cation sites by the emitting activator Dy 3+ that has a concentrationdependent preferential occupation in one of the sites as well.

Luminescence decay curves and chromaticity coordinates
The Commission International de I′Eclairage (CIE) chromaticity coordinates for the representative sample (Ca 2 Sr 1.88 (PO 4 ) 2 :0.06Dy 3+ ,0.06Li + ) were calculated based on the corresponding emission spectra, and the results are shown in Fig. 11(a). It can be seen that the coordinates (x = 0.3450, y = 0.3787) are located in the near-white region. In addition, we also calculated the CIE chromaticity coordinates of Ca 2 Sr 1.88 (PO 4 ) 2 :0.06Dy 3+ ,0.06Li + at different temperature values (Table 1), and we found that with the temperature rising the phosphor exhibits almost no change in the color of emission, which is showed in Fig. 11(b). Thus, the CaSr 2-2x (PO 4 ) 2 :xDy 3+ ,xLi + material can be potentially used as a white phosphor for w-LEDs in solid-state lighting applications.

Conclusions
In summary, a series of CaSr 2-2x (PO 4 ) 2 :xDy 3+ ,xLi + , CaSr 2-2x (PO 4 ) 2 :xDy 3+ ,xNa + and CaSr 2-1.5x (PO 4 ) 2 :xDy 3+ phosphors were prepared by a conventional solid-state reaction. The phase structure, luminescence properties, thermal quenching and emission decay curves were investigated. Under the excitation of 351 nm, the CaSr 2-2x (PO 4 ) 2 :xDy 3+ ,xLi + phosphors showed two emission bands centered at 486 and 578 nm, characteristic of Dy 3+ . The excitation spectra showed narrow excitation bands from 250 to 550 nm with a maximum at 403 nm. This means that the phosphor can be effectively excited by UV chips for potential applications in w-LEDs. The optimum dopant concentration of Dy 3+ ions is 0.06 mol.%. Furthermore, the quenching mechanism between the Dy 3+ ions was recognized as dipole-dipole (d-d) interactions. The dependence of the emission spectra on temperature indicated that the phosphor has a good thermal stability in both emission color and intensity. The CIE chromaticity coordinates of the selected Ca 2 Sr 1.88 (PO 4 ) 2 :0.06Dy 3+ ,0.06Li + sample were calculated (x =0.3450, y = 0.3787), and they are located in the white-light region. These results indicate that CaSr 2-2x (PO 4 ) 2 :xDy 3+ ,xLi + phosphors have a good potential for their use as white-emitting luminophores for the phosphor-converted w-LEDs. Thus, the results of this work indicate the potential applications of these effective phosphors in white light-emitting diodes, excited with a near-UV chip.

Con icts of interest
There are no con icts to declare.        The PL intensity of CaSr1.88(PO4)2:0.06Dy3+,0.06Li+ phosphor with respect to time, monitored under 351 nm excitation continuously for 60 min at (a) 100˚C and (b) 150˚C for 30 min at each temperature with a time interval of 5 min.