A novel single-phase white light emitting phosphor Ca9La(PO4)5(SiO4)F2:Dy3+: synthesis, crystal structure and luminescence properties

A novel single-phase white light emitting phosphor Ca9La(PO4)5(SiO4)F2:Dy 3+ was prepared through traditional high-temperature solid state technology. The crystal structures of Ca9La(PO4)5(SiO4)F2 with or without Dy ions were refined by the Rietveld method. The diffuse reflection spectra, excitation spectra, emission spectra, and decay times were characterized to investigate the photoluminescence properties for application in white light-emitting diodes. The results showed that the Ca9La(PO4)5(SiO4)F2:Dy 3+


Introduction
2][3] Presently, the most prevalent w-LEDs in the market generally can be fabricated by a combination of a blue InGaN chip and the yellow-emitting YAG:Ce phosphor on basis of the phosphor-converted emission mechanism.However, this approach faces a low color rendering index and a high correlated color temperature due to the lack of sufficient red component. 4,5Accordingly, another alternative method on fabricating w-LEDs is generated by means of near-ultraviolet (n-UV) chip LED assigned with the mixture of tricolor (blue, green and red) phosphors.This method also faces some disadvantages such as low luminescent efficiency due to the reabsorption of the blue light by the red and green phosphors. 6Therefore, it is instant for the development of a single-phase white-light emitting phosphor to widespread use in w-LEDs application.
As an important member in solid state phosphor family, the apatite structure compounds have been extensively studied in solid-state illumination and display manufacturing owing to their stable and adjustable structure for optical materials. 7,8It is well known that the typical apatite compound has an isostructure to the nature mineral uorapatite Ca 10 (PO 4 ) 6 F 2 , which belongs to the hexagonal symmetry system (space group of P6 3 /m) with a general chemical formula as A 10 (PO 4 ) 6 Z 2 , where A oen represents divalent cations such as Ca 2+ , Mg 2+ , Ba 2+ , Fe 2+ , Sr 2+ , Mn 2+ , Pb 2+ and so on.Meanwhile, [PO 4 ] 3À can also be isomorphically substituted by [SiO 4 ] 4À , [BO 4 ] 5À , and [VO 4 ] 3À anion groups under different conditions.And Z is always denoted by F, Cl, Br, or O. [9][10][11][12] Since apatite-type compound possesses the capability of being substituted by versatile ions and forming host structure changeable solid solution, which may endow tunable luminescence and outstanding luminescent properties.That is to say, it is very interesting to build up new inorganic framework to obtain new compounds with apatite structure.Hitherto, several single-component white light emitting phosphors with apatite-type structure suitable for n-UV-pumped w-LED have been reported, such as, Ca 2 La 8 (GeO 4 ) 6 O 2 :Eu 3+ /Dy 3+ , 13 Mg 2 Y 8 (SiO 4 ) 6 O 2 :Ce 3+ /Tb 3+ /Sm 3+ (ref.14)  and Sr 3.5 Y 6.5 O 2 (PO 4 ) 1.5 (SiO 4 ) 4.5 :Ce 3+ /Tb 3+ /Mn 2+ . 157][18] Customarily, Dy 3+ ions generate two dominant emissions in blue (475-500 nm) and yellow (570-600 nm) regions, which correspond to 4 F 9/2 / 6 H 15/2 (magnetic dipole transition) and 4 F 9/2 / 6 H 13/2 (electric dipole transition) transitions, respectively. 19To our knowledge, the electric dipole transitions of Dy 3+ ions can be easily affected by the crystal eld environment of the host lattice rather than the magnetic dipole transitions of Dy 3+ ions.Thus, it is an effective strategy to obtain white light emission in Dy 3+ activated luminescent materials by adjusting the chemical environment of the matrix.Consequently, we demonstrated a novel single-component white-light phosphor Ca 9 La(PO 4 ) 5 (SiO 4 )F 2 :Dy 3+ with apatite structure for UV-light emitting diodes via a high-temperature solid-state technology.In addition, the concentration quenching as well as lifetime studies of Dy 3+ in Ca 9 La(PO 4 ) 5 (SiO 4 )F 2 phosphor was carried out in detail.

Experimental
Traditional high-temperature solid-state reaction method was employed to prepare Ca 9 La 1Àx (PO 4 ) 5 (SiO 4 )F 2 :xDy 3+ phosphors.CaCO 3 , (NH 4 ) 2 HPO 4 , SiO 2 , NH 4 HF 2 , and Dy 2 O 3 were mixed and ground according to the given stoichiometric ratio.An additional amount of NH 4 HF 2 was mixed as ux and balanced the loss of F source at high temperature.Stoichiometric amounts of reactants were thoroughly mixed by grinded together in an agate mortar with a small amount of ethanol, then the nal mixture was placed into an alumina crucible and annealed at 1450 C in a reducing atmosphere (5% H 2 + 95% N 2 ) for 5 h.Aer this, the samples were furnacecooled to room temperature, and ground again into powder for measurement.
X-ray diffraction (XRD) patterns were recorded using an X-ray powder diffractometer (XD-3, PGENERAL, China) with Cu-Ka radiation (l ¼ 0.15406 nm) operated at 40 kV and 30 mA.The continuous scanning rate (2q ranging from 10 to 70 ) used as phase determination was 8 (2q)/min.The powder diffraction data of Ca 9 La(PO 4 ) 5 (SiO 4 )F 2 and Ca 9 La 0.9 Dy 0.1 (PO 4 ) 5 (SiO 4 )F 2 for Rietveld analysis were collected at room temperature with a Bruker D8 ADVANCE powder diffractometer (Cu-Ka radiation) and linear VANTEC detector.The step size of 2q was 0.02 , and the counting time was 3 s per step.Rietveld renement was performed by using TOPAS 4.2. 20The measures of PL and photoluminescence excitation (PLE) spectra were carried out by a uorescence spectrophotometer (F-4600, HITACHI, Japan) equipped with a photomultiplier tube operating at 400 V, and a 150 W Xe lamp was used as the excitation lamp.A 400 nm cutoff lter was used in the measurement to eliminate the second-order emission of source radiation.The diffuse-reectance spectra (DRS) were recorded by UV-Visible spectrometer (TU-1901, China) with integration sphere.The roomtemperature luminescence decay curves were obtained from a spectrouorometer (Horiba, Jobin Yvon TBXPS) using a tunable pulse laser radiation as the excitation.All measurements were carried out at room temperature.

Results and discussion
3.1 Crystal structure of Ca 9 La(PO 4 ) 5 (SiO 4 )F 2 Fig. 1a shows the XRD proles of as-prepared Ca 9 La(PO 4 ) 5 (SiO 4 ) F 2 , Ca 9 La 0.95 (PO 4 ) 5 (SiO 4 )F 2 :0.05Dy 3+ and Ca 9 La 0.85 (PO 4 ) 5 (SiO 4 ) F 2 :0.15Dy 3+ phosphors.It is obvious that all the observed diffraction peaks are well indexed to the phases of Ca 5 (PO 4 ) 3 F (JCPDS No. 15-0876) and no second phase is observed, which conrmed that the doping Dy 3+ ions are completely incorporated into the host lattice and do not cause signicant changes in the host.
In order to further check the crystal structure of the synthesized phosphor, the Rietveld structural renements for Ca 9 La(PO 4 ) 5 (SiO 4 )F 2 and Ca 9 La 0.90 (PO 4 ) 5 (SiO 4 )F 2 :0.10Dy 3+ compounds were performed based on the TOPAS 4.2 program.Almost all peaks were indexed by hexagonal cell (P6 3 /m) with parameters very close to Ca 9 La(PO 4 ) 5 (SiO 4 )F 2 published earlier. 21,22Therefore this crystal structure was taken as starting model for Rietveld renement.In this study, two independent sites of Ca were occupied by Ca, La and Dy ions with rened occupation and we assume the sum of occupancies in each site equals 1. Occupancies of P and Si in site were xed according to suggested formula.No detectable impurity phase was observed in the obtained samples and we found all the observed peaks satisfy the reection condition.The observed, calculated peaks and their difference for the Rietveld renement are shown in Fig. 1b and c.Moreover, the nal rened residual factors and rened structural parameters are summarized in Tables 1-3.It can be seen that all of the observed peaks satisfy the reection condition and converge to R exp ¼ 3.33%, R wp ¼ 5.63%, R p ¼ 4.10%, c 2 ¼ 1.69 in the renement of the Ca 9 La(PO 4 ) 5 (SiO 4 )F 2 compound and R exp ¼ 3.63%, R wp ¼ 5.55%, R p ¼ 4.03%, c 2 ¼ 1.53 in the renement of the Ca 9 La 0.9 Dy 0.1 (PO 4 ) 5 (SiO 4 )F 2 compound, respectively.The unit cell parameters obtained for Ca 9 La(PO 4 ) 5 (SiO 4 )F 2 are a ¼ 9.432 Å, c ¼ 6.926 Å, and V ¼ 533.61 Å3 , which are larger than those for Ca 9 La 0.9 Dy 0.1 (PO 4 ) 5 (SiO 4 )F 2 shown in Table 1.The larger cell parameters are ascribed to the larger-sized La atoms as compared with Dy atoms.
As mentioned previously, Ca 9 La(PO 4 ) 5 (SiO 4 )F 2 belongs to apatite-type compound, which crystallizes in a hexagonal symmetrical system with space group P6 3 /m and the detailed crystal structure diagram of Ca 9 La(PO 4 ) 5 (SiO 4 )F 2 compound is given in Fig. 1d.In this structure, there are two kinds of cationic sites labeled Ca/La(I) and Ca/La(II), the Ca/La(I) site is dened as being nine-fold coordinated with C 3 point symmetry, and Ca/ La(II) is dened as being seven-fold coordinated with C s point symmetry, respectively.As depicted in Fig. 1d 3), and a uorine atom forming an irregular CaO 6 F-polyhedron. 22Considering the effective ionic radii and charge balance of cations with different coordination number (CN), the activators Dy 3+ ions are predictable to occupy the La 3+ sites randomly in the Ca 9 La(PO 4 ) 5 (SiO 4 )F 2 host, because the effective ionic radii of Dy 3+ (r ¼ 0.97 Å for CN ¼ 7 and r ¼ 1.083 Å for CN ¼ 9) is closest to that of La 3+ (r ¼ 1.10 Å for CN ¼ 7 and r ¼ 1.216 Å for CN ¼ 9). 23The ne local structures of Ca 9 La(PO 4 ) 5 (SiO 4 )F 2 are further examined by HRTEM, and the fast Fourier transform (FFT) images shown in Fig. 1e-g.It can be seen in Fig. 1e, the lattice fringes with a d spacing of 0.255 nm through the solved structure of the compound by renement, which could be assigned to (301) plane.The d (301) enlarged (d (301) of 0.251 nm for Ca 5 (PO 4 ) 3 F compound) slightly owing to the increase of the cell parameters with the substitution of La 3+ /Si 4+ for Ca 2+ /P 5+ ions as compared with the Ca 5 (PO 4 ) 3 F compound.
The UV-Vis reection spectra of Ca 9 La(PO 4 ) 5 (SiO 4 )F 2 host and Ca 9 La 0.85 (PO 4 ) 5 (SiO 4 )F 2 :0.15Dy 3+ are described in Fig. 2c.The Ca 9 La(PO 4 ) 5 (SiO 4 )F 2 matrix exhibits an energy absorption in the region with wavelength less than 300 nm and a high reection ranging from 300 to 800 nm.The band gap of the virgin Ca 9 La(PO 4 ) 5 (SiO 4 )F 2 can be estimated according to the following formula: 25,26 [ where hn is the photon energy, A is a proportional constant, E g is the value of the band gap, n ¼ 2 for a direct transition or 1/2 for an indirect transition, and F(R N ) is Kubelka-Munk function which is dened as: 25,26 where R, K and S are the reection, the absorption and the scattering coefficient, respectively.As illustrated in the insert of  inset of Fig. 3a, the optimal doping concentration of Dy 3+ in Ca 9 La 1Àx (PO 4 ) 5 (SiO 4 )F 2 :xDy 3+ is xed at x ¼ 0.15, then the PL intensity begins to decrease, which could be ascribed to the internal concentration quenching effect via energy transfer from one activator to another.The interaction type between sensitizers or between sensitizer and activator can be estimated via the following equation: 27-29 in which x is the concentration of activation, which is more than the critical concentration K and b are constants under the same excitation condition of host lattice, I/x is the emission intensity (I) per activator concentration (x) and q is a function of electric multipolar character.q ¼ 6, 8, 10 for dipole-dipole (d-d), dipole-quadrupole (d-q), quadrupole-quadrupole (q-q) interactions, respectively.Fig. 3b shows the tting lines of log(I/x) vs. log(x) in Ca 9 La 1Àx (PO 4 ) 5 (SiO 4 )F 2 :xDy 3+ phosphors with the different wavelength (485 nm and 580 nm) beyond the quenching concentration.It is clear that the tting curves of lg(I/x) vs. lg(x) can be well matched as a relatively linear dependence and the slopes were determined to be À1.97, and À2.08, which corresponds to the 4 F 9/2 / 6 H 15/2 and 4 F 9/2 / 6 H 13/2 transition of doped Dy 3+ ions.Herein, the values of q can be calculated as 5.91 and 6.24, respectively.Both of the obtained q values are close to 6, demonstrating that the interaction type in Ca 9 La(PO 4 ) 5 (SiO 4 )F 2 :Dy 3+ is dipole-dipole interactions.
The PL decay curves and lifetimes of Dy 3+ for Ca 9 La 1Àx (PO 4 ) 5 (SiO 4 )F 2 :xDy 3+ (x ¼ 0.01-0.50)phosphors under the excitation of 352 nm by monitoring different emission bands corresponding to the wavelength at 485 and 580 nm are shown in Fig. 4. It is found that all of the decay curves could be tted successfully with a typical second-order exponential decay equation as follows: where I(t) is the luminescence intensity at time t, A 1 and A 2 are constants, t is the time, and s 1 and s 2 are rapid and slow times for the exponential components.Herein, the decay processes of these samples are characterized by average lifetime (s*), which was dened as following formula: An obvious phenomenon is found that the decay time begins to decrease gradually with the increasing of Dy 3+ concentration.The decay lifetimes at 485 nm are estimated to be 674.66us, 643.96 us, 630.21 us, 599.67 us, 557.12 us, 493.96 us, and 473.75 us.Meanwhile, the decay lifetimes at 580 nm are estimated to be 704.66us, 647.23 us, 641.48 us, 602.87 us, 577.62 us, 512.21 us, and 485.93 us.The restrained lifetime is related to the total relaxation rate, which can be expressed by: 30 here s 0 is the radiative lifetime, A nr is the nonradiative rate due to multiphonon relaxation, and P t is the energy transfer rate between activator Dy 3+ ions.Considering the distance between Dy 3+ -Dy 3+ ions decreases in pace with increasing Dy 3+ doped concentrations, the probability of energy transfer to luminescent killer sites increased.][32] The variation of chromaticity coordination (x, y) of Ca 9 -La 1Àx (PO 4 ) 5 (SiO 4 )F 2 :xDy 3+ (x ¼ 0.01-0.50)excited under 352 nm are calculated based on the corresponding PL spectrum in the CIE 1931 chromaticity, and the chromaticity coordinates and  CCT are summarized in Fig. 5 and Table 4, respectively.It can be seen that the emission colors of all of the as-prepared samples locate in the white light region.As we known, the standard white chromaticity coordination was selected with x ¼ 0.333 and y ¼ 0.333 in the CIE 1931 chromaticity.Generally, it exhibits a better white emitting quality when the chromaticity coordination closes to the ideal white chromaticity coordination (x ¼ 0.333, y ¼ 0.333). 33Therefore, the optimal chromaticity coordinates and CCT are designed as (x ¼ 0.338, y ¼ 0.336) and 5262 K, which indicates that Ca 9 La(PO 4 ) 5 (SiO 4 )F 2 :Dy 3+ could serve as a potential single phase white-emitting phosphor in solid-state lighting for white-light n-UV LED devices.
The thermal stability of phosphor is one of the important issues for potential applications in high-power LEDs.Fig. 6 shows the temperature-dependent emission spectra for the selected Ca 9 La 0.85 (PO 4 ) 5 (SiO 4 )F 2 :0.15Dy 3+ phosphor under 390 nm excitation, and the relative emission intensities as a function of temperature were given in the inset of Fig. 6.It can be seen that the PL intensity slowly decreases with increasing temperature.Generally, the phosphors of white LED are require to sustain their emission efficiency up to 150 C over a long term because the temperature of a LED package rises by the heat generation of LED itself during LED operation.When the temperature rose from room temperature to 150 C, the PL intensities of Ca 9 La 0.85 (PO 4 ) 5 (SiO 4 )F 2 :0.15Dy 3+ phosphor decreased to 71% of the initial PL intensity, which indicates that the as-prepared phosphors has great potential application to meet the application requirements for n-UV LEDs.

Conclusions
In summary, a series of novel white-light-emitting phosphors Ca 9 La 1Àx (PO 4 ) 5 (SiO 4 )F 2 :xDy 3+ were synthesized via a conventional solid-state reaction.Upon 352 nm excitation, these phosphors exhibits two intense emission bands centered at 485 and 580 nm, which ascribes to 4 F 9/2 / 6 H 15/2 and 4 F 9/2 / 6 H 13/2 transitions of Dy 3+ , respectively.The spectroscopic data and uorescence decay dynamics indicate that the energy transfer process takes place between Dy 3+ -Dy 3+ ions via a nonradiative dipole-dipole mechanism.All of the chromaticity coordinates of Ca 9 La 1Àx (PO 4 ) 5 (SiO 4 )F 2 :xDy 3+ are located in the proximity of the standard white chromaticity coordination, demonstrating Ca 9 La(PO 4 ) 5 (SiO 4 )F 2 :Dy 3+ can be regarded as a potential candidate in solid-state lighting combing with blue chip or near-UV chip.
, the Ca/La(I) site is surrounded by nine O atoms: three O(1), three O(2) and three O(3), forming a CaO 9 -polyhedron with six short and three long Ca-O bonds.While the Ca/La(II) site is coordinated by six oxygen atoms O(1), O(2), four O(

Table 3
Fractional atomic coordinates and isotropic displacement parameters of the Ca 9 La(PO 4 ) 5 (SiO 4 )F 2 compound