Article pubs.acs.org/IC
Four-Dimensional Incommensurate Modulation and Luminescent Properties of Host Material Na3La(PO4)2 Dan Zhao,* Fa-Xue Ma, Shu-Qi Ma, Ai-Yun Zhang, Cong-Kui Nie, Min Huang, Lei Zhang, and Yun-Chang Fan* College of Chemistry and Chemical Engineering, Henan Polytechnic University, Jiaozuo, Henan Province 454000, China S Supporting Information *
ABSTRACT: A series of orthophosphates Na3Ln(PO4)2 (Ln = lanthanoids) have for a long time been known as good luminescent materials, yet their crystal structures have not been studied in full detail. In this work, compound Na3La(PO4)2 was prepared using molten salt (flux) method and for the first time was structurally determined on X-ray single-crystal diffraction data. Interestingly, it crystallizes in the four-dimensional incommensurately modulated structure with orthorhombic superspace group Pca21(0β0)000 and modulation wave vector q = 0.387b*. Furthermore, to evaluate the potentiality of Na3La(PO4)2 to be used as a luminescent host material, 5 mol % Eu3+, Tb3+, and Dy3+ doped phosphors were prepared, respectively. The excitation spectra, emission spectra, decay time, quantum efficiency, and the color purity of prepared phosphors, Na3La0.95Eu0.05(PO4)2, Na3La0.95Tb0.05(PO4)2, and Na3La0.95Dy0.05(PO4)2, were studied.
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INTRODUCTION In recent years, an increasing number of scientists have shown strong interest to explore new phosphors in various host materials for their applications in solid-state lighting technologies including white light-emitting diodes (LED) and Hgfree lamps.1−5 Inorganic phosphates are one of the good host series, because they can produce plenty of crystal field environments and have many advantages such as high chemical stability, simple preparation method, structural diversity, and suitable band gap.6,7 Among them, a series of vitusite-(Ce)8 type of orthophosphates with the general formula Na3Ln(PO4)2 (Ln = trivalent rare-earth metal) have been extensively studied for their good photoluminescence (PL) properties. This family of compounds features Na3La(VO4)3-type9 basic structure with the orthorhombic space group Pbc21 (No. 29) and unit cell of a ≈ 5.522 Å, b ≈ 14.05 Å, c ≈ 18.44 Å; they are built on isolated LnOx polyhedra, NaOx polyhedra, and PO4 tetrahedra. Since recent years, a large amount of literature concerning the luminescent properties of compounds Na3Ln(PO4)2 has been reported.10−12 However, the structural details of Na3Ln(PO4)2 family were not fully studied especially for some early rare earths despite some reports have already discussed this matter in early years.13−16 To general surprise, the detailed crystal structure of Na3La(PO4)2 is still unknown. This was the incentive for us to start a study on the crystal structure of compounds Na3Ln(PO4)2 to establish the relation between their crystal structure and physical properties. In our work, we found that the structure solution of Na3Ln(PO4)2 through single-crystal X-ray diffraction (SC© XXXX American Chemical Society
XRD) method was complicated by incommensurate modulation. High-dimensional crystallography provides a powerful tool for solving modulated crystals whose three-dimensional (3D) periodic repetition of the unit cell is not in existence for the displacement of atoms from their average position.17 The lacking periodicity can be restored by transforming the data to a superspace higher than physical 3D space. The diffraction pattern of modulated crystals consists of two types of reflections: strong main reflections that correspond to the basic structure, and weak satellite reflections that correspond to the modulation wave. Modulation wave functions can be characterized by a wave vector q that can be given with respect to the direction and wavelength of the wave, that is, q = αa* + βb* + γc*, where {a*, b*, c*} are the basis vector (reciprocal lattice) of basic structure. The past 20 years witnessed a large number of compounds with incommensurate modulation in literature, such as Na ≈ 2/3 FePO 4 , 18 Cs2TB4O9(T = Ge, Si),19 SrPt2Al2,20 KEu(MoO4)2,21,22 KNd(MoO 4 ) 2 , 2 3 PbBiNb 5 O 1 5 , 2 4 LiCuVO 4 , 2 5 [CaNd]2[Ga]2[Ga2O7]2,26 Pr2NiO4,27 etc. In our earlier work, the commensurately modulated structures of KSbOB2O528 and ZnNb2O629 were successfully solved in the superspace groups Pmn21(0β0)s00 and Pbcn(α00)00s, respectively. In this paper, we present the synthesis, structural determination, UV−vis spectrum, and PL properties activated by Eu3+, Tb3+, and Dy3+. Received: September 17, 2016
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DOI: 10.1021/acs.inorgchem.6b02241 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
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EXPERIMENTAL SECTION
Materials and Instrumentations. Raw materials Na 2CO3 (≥99.8%), La2O3 (≥99.95%), Eu2O3 (≥99.99%), Tb4O7 (≥99.9%), Dy2O3 (≥99.9%), and NH4H2PO4 (≥99.5%) were purchased from the Sinopharm Chemical Reagent Shanghai Co., Ltd. and were used without further purification. The powder X-ray diffraction (XRD) analyses were fulfilled on a Rigaku DMax2500 PC powder dffractormeter with using graphite-monochromated Cu Kα characteristic radiation in the range of 2θ = 5−60° (0.02°/step). Ultraviolet− visible (UV−vis) spectrum was measured using a JASCOV-550 double beam spectro-photometer in the range from 200 to 1200 nm. Photoluminescence (PL) spectra and the lifetime test were performed using an FLS920 Edinburgh Analytical Instrument apparatus. The steady-state measurements were performed using a standard 450 W continuous-wave xenon lamp as the excitation source. The step width 1 nm and integration time 0.2 s were used for the PL excitation and emission spectra measurements. The lifetime measurement was fulfilled by using a standard microsecond flash lamp μF920H with the time-correlated single-photon counting technique. The flash lamp operated at 200 Hz pulse frequency with a pulse width of 2 μs. The external quantum efficiency was determined on the same instrument equipped with a barium sulfate coated integration sphere as a reflectance standard. Synthetic Procedures. The high-temperature molten salt method, that is, flux method, was used to prepare small single crystals of compound Na3La(PO4)2. Additional reactants Na2O and P2O5 in a proper molar ratio were used as the flux to avoid impurities. The raw materials, Na2CO3 (2.212 g, 20.87 mmol), La2O3 (0.5666 g, 1.739 mmol), and NH4H2PO4 (3.000 g, 26.09 mmol), were mixed and put into an arc platinum crucible after carefully grinding in an agate mortar. It was pretreated in muffle furnace at 400 °C for 5 h to release volatile gas (CO2, NH3, and H2O). A necessary regrinding was performed to ensure the homogeneity of mixture. After that, the temperature was slowly increased to 1100 °C to melt the mixture completely. After holding the temperature at 1100 °C for 40 h, the solution was allowed to cool to 700 °C at a rate of 4 °C·h−1 to grow small single crystals. Finally, the production was washed by hot water to get rid of the addition flux. A few small single crystals of compound Na3La(PO4)2 can be carefully selected using a light microscope. Polycrystalline sample of compound Na3La(PO4)2 was prepared using traditional high-temperature solid-state reaction method. Raw materials, Na2CO3 (2.765 g, 26.09 mmol), La2O3 (2.833 g, 8.696 mmol), and NH 4 H 2 PO 4 (4.000 g, 34.78 mmol) with the stoichiometric molar ratio of 3:1:4, were thoroughly ground in an agate mortar to ensure the best homogeneity. After a pretreatment at 400 °C for 5 h, the mixture was calcined at 1100 °C for 50 h. It is very important to perform several intermediate grindings and mixings for the mixture to complete the reaction. The samples of 5 mol % Ln3+doped(Ln = Eu, Tb, Tb) Na3La(PO4)2 were prepared by similar method except for 5 mol % dopants of Eu3+, Tb3+, and Dy3+. Structure Determination. SC-XRD analysis was performed using the Bruker Smart Apex2 CCD device under the homeothermic condition of 20 °C. The data were collected in the range from 2.06° to 28.9°, the exposure time was set as 10 s/deg, and the scan width was set as 0.5°. Using this strategy, 1466 frames were collected in all for ∼6 h. Diffraction patterns were composed of strong main reflections and weak satellite reflections, clearly indicating a modulated structure. Structural modulation can be clearly observed using the reciprocal viewer tool provided by software Apex2.30 As shown in Figure 1, the main reflections and the first-order satellite reflections were drawn as red and turquoise spots, respectively, and the main lattice reflections can be selected and indexed as an orthorhombic cell a = 14.0835(4) Å, b = 5.3518(11) Å, c = 18.7296(14) Å. The additional satellite spots that are regularly distributed among the main reflections can be indexed as incommensurate modulation wave vector q⃗ = 0.387b⃗. The cell parameters and the modulation wave vector were then refined during data integration by SAINT software (involved in Apex2 package). 3026 observed main reflections, 4577 observed first-order satellite reflections, and 322 observed second-order satellite reflections
Figure 1. Reciprocal lattice view of compound Na3La(PO4)2, which is constructed from the experimental single-crystal diffraction data. Red and turquiose spots represent main and first-order satellite reflections, respectively. were embodied in the integration to generate the hklf six-type output file. The structure solution was fulfilled by charge-flipping algorithm31 (CFA) using software Jana2006.32 Table 1 presents the experimental details for data collection and structural refinement details of compound Na3La(PO4)2. The detailed crystal date was embraced in a CIF file deposited in Inorganic Crystal Structure Database (No. CSD-430499).
Table 1. Experimental Details for the Data Collection and Structural Refinement Details of Na3La(PO4)2 chemical formula Mr crystal system, space group temperature (K) modulation wave vector a, b, c (Å) V (Å3) Z abs coeff μ (mm−1) crystal size (mm) diffractometer absorption correction radiation type wavelength (Å) range of h, k, l, (m)
No. of measured and unique reflns No. of obsd reflns No. of obsd main reflns No. of obsd first-order satellites No. of obsd second-order satellites criterion for obs reflns Rint (sin θ/λ)max (Å−1) R, Rω (obs reflns) R, Rω (main reflns) R, Rω (first-order satellite reflns) R, Rω (second-order satellite reflns) GOF (obs) No. of reflections/parameters Δρmax, Δρmin (e·Å−3) computer programs B
Na3La(PO4)2 397.8 orthorhombic; Pca21(0β0)000 293 q = 0.386580 b* 14.0830(4), 5.3517(11), 18.7291(14) 1411.6(3) 8 6.70 0.20 × 0.10 × 0.10 Bruker Apex2 CCD multiscan Mo Kα 0.710 73 −18→ h → +18 −7→ k → +7 −24→ l → +24 −2→ m → +2 65 382, 17 514 7925 3026 4577 322 I > 3σ(I) 0.080 0.667 0.0391, 0.0403 0.0304, 0.0358 0.0492, 0.0457 0.1200, 0.1561 1.36 17 514/532 1.35, −1.57 Jana2006 DOI: 10.1021/acs.inorgchem.6b02241 Inorg. Chem. XXXX, XXX, XXX−XXX
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RESULTS AND DISCUSSION Synthesis. It is well-known that SC-XRD structure determination is one of the most effective methods to study the detailed structure of an unknown compound, especially for some complicated cases such as incommensurate modulation. However, the preparation of single crystals suitable for SC-XRD is usually not very easy. In our work, we chose hightemperature flux technique33−35 to grow small single crystals of compound Na3La(PO4)2 with the size of approximately 0.20 × 0.10 × 0.10 mm. The reaction can be shown by the following equation: 3Na 2CO3 + La 2O3 + 4NH4H 2PO4 → 2Na3La(PO4 )2 + 3CO2 + 4NH3 + 6H 2O
Unfortunately, the yield of single crystals using this way was low, and some unknown impurities were difficult to remove. Therefore, pure powder pattern of Na3 La(PO 4 ) 2 was synthesized by solid-state reaction at 1150 °C. The purity and crystallinity were studied by powder XRD experiments. As shown in Figure 2, the experimental XRD of powder sample fits well with that calculated from single-crystal data, revealing that it was successfully prepared as pure phase.
Figure 3. Average structure of Na3La(PO4)2 to show the linear array of −Na−La− and −Na−P−.
carefully checked the difference-Fourier maps (Figure 4), assigned the modulation waves, and realized that it was more suitable to introduce the first-order crenel (Legendre polynomials in crenel interval type) modulation wave amplitudes to Na1, O6, O12, and O16 atoms to avoid a large number of harmonic position modulation functions. Although the second-order satellite reflections is very low that only 322 reflections were observed with I > 3σ (I), it would lead to a better refinement when we applied some second-order positional modulations for heavy atoms La1, La2, P1, P2, P3, and P4. Finally, some first- or second-order anisotropic displacement parameters (ADP) for some atoms were added to reduce the residual electron densities around these atoms. The structure of Na3La(PO4)2 can be approximated to a hexagonal structure and can be described as being a distorted αK2SO436 (or glaserite K3Na(SO4)237) type of structure in which K and S atoms are substituted by (La, Na) and P atoms, respectively. The structure of α-K2SO4 with a hexagonal symmetry can be viewed as an ideal model in which the −K−S− line lies in the sixfold axis and the −K− line lies in the threefold axis, as shown in Figure 5a. In α-K2(SO4)2, large K+ cations possess a 12-coordinated environment, whereas in Na3La(PO4)2, small La3+ and Na+ cations have to adopt a lower coordination numbers of NaO6−8, and LaO8. To accommodate the cations of different size and charge, the PO4 tetrahedra are then twisted. As a result, the −Na−La− and −Na−P− lines are no longer straight along the a-axis compared with the ideal K3Na(SO4)2 structure for the pushing of positional modulation. Then the hexagonal symmetry was broken, and the atomic successions distortion led to a complicated orthorhombic incommensurately modulated structure. We think that the structure modulation may be caused by the irregular arrangement of Na and La atoms to accommodate the α-K2(SO4)2 type of framework, whereas the twisting (or rotating) of the PO4 groups is merely an adaptation to the changing of ions. The atom positions affected by the structural modulation could be viewed by using the commensurate approximant and drawing an approximant supercell. We could see that the vector q = 0.387b* ≈ 3/8b* ≈ 5/13b*, and thus a 1a × 8b × 1c or a 1a × 13b × 1c superlattice could be drawn for demonstrating
Figure 2. Experimental and simulated powder XRD diffraction patterns of Na3La(PO4)2 and 5 mol % (Eu, Tb, Dy)-doped Na3La(PO4)2 in the 2θ range of 5−60°.
Average Structure. The average structure of Na3La(PO4)2 features a 3D framework containing isolated PO4 tetrahedra, NaO6 octahedra, NaO7 polyhedra, NaO8 polyhedra, and LaO8 polyhedra. In this structure, these groups adopt two types of arrays running along the a-axis, that is, a line of −Na−P− (more accurately −Na−PO4−) and a line of −Na−La−, as shown in Figure 3. The Na atoms can be divided into two groups: two Na atoms in the −Na−La− line that are surrounded by six O atoms to form NaO6 octahedra, and four Na atoms in the −Na−P− line that are attached by O atoms forming NaO7 or NaO8 polyhedra. La atoms are surrounded by eight O atoms from PO4 tetrahedra to form LnO8 polyhedra. Incommensurately Modulated Structure. The output of Superflip gives most of the atoms, and the rest, several ones, can be found from the difference Fourier peak utility. To reach the best refinement of incommensurate structural model, we C
DOI: 10.1021/acs.inorgchem.6b02241 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 4. Positional modulations of La1 (a), La2 (b), Na1 (c), Na2 (d), Na3 (e), Na4 (f), Na5 (g), Na6 (h), P1 (i), P2 (j), P3 (k), and P4 (l) atoms in Na3La(PO4)2 as functions of the internal x4 axis through the superspace.
coordination environment.38−40 Relatively speaking, the distribution of Na−O and La−O bond lengths is much larger than P−O bond length, because the longer ones are usually more affected by modulation, which is a common feature in modulated compounds. This phenomenon also underlies within the same bond type. For example, the modulation imposed to the longest La−O bonds (La1O15iii) is much larger than the shortest La−O bonds (La1O16ii). The
the modulated structure, as show in Figure 5b. If one focuses only on the −Na−La− lines distribution projected on the bc plane, one can simply observe the deviation from the α-K2SO4 basic structure (Figure 5a). The Na−O, P−O, and La−O interatomic distances are strongly affected by modulation, resulting in more reasonable values and no ambiguous features. As shown in Figure 6, the P−O bond distances range from dmin = 1.468(10) Å to dmax = 1.577(7) Å, which are typical values for P atoms in a tetrahedral D
DOI: 10.1021/acs.inorgchem.6b02241 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 5. (a) Crystal structure of α-K2SO4 to show the linear array of −K−and −K−S−; (b) approximant cell of incommensurately modulated Na3La(PO4)2 viewed in the 8 × b supercell to show the linear array of −Na−La− and −Na−P−.
radius of Ln3+ are similar, so we confirm that the Eu3+, Tb3+, or Dy3+ dopants are well-incorporated in Na3La(PO4)2 host lattice. Eu3+ ion is a famous activator in a variety of host lattices.45,46 The excitation spectrum of Na3La(PO4)2:Eu3+ was recorded with 590 nm emission (Figure 8a). The spectrum includes a weak broad band at ∼275 nm, which can be attributed to the charge-transfer (CT) band from O2− to Eu3+ ions. There exist some sharp peaks in the visible light range from 310 to 550 nm, which can be assigned to the f→f characteristic transition of Eu3+, that is, 7F0→5H5 at 317 nm, 7D0→5D4 at 360 nm, 5 D0→5L7 at 378 nm, 7F0→5L6 at 392 nm, 7F0→5D3 at 414 nm, 7 F0→5D2 at 464 nm, and 7F0→5D1 at 525 nm.47 The strongest one is located at near-UV region of 392 nm (7F0→5L6). Figure 8b shows the emission spectrum of Na3La(PO4)2:Eu3+ in the range from 570 to 730 m, which originates from the 5D0→7Fj (j = 0, 1, 2, 3) transitions. The first and second strongest emissions at 590 and 616 nm originate in 5D0→7F1 and 5 D0→7F2 transitions of Eu3+ activator, respectively. It is wellknown that the substituent position of activator Eu3+ in host lattice significantly affects the relative strengths of emissions. The 5D0→7F2 transition corresponding to the electrical dipole moment transition is more sensitive to the crystallographic site symmetry of activator ions than the 5D0→7F1 transition corresponding to the magnetic dipole moment. Therefore, if Eu3+ ions locate in a low symmetry site, the 5D0→7F2 emission dominates the emission spectrum, whereas the 5D0→7F1 emission will be the strongest emission if Eu3+ occupies a high symmetry site.48,49 Furthermore, the comparative intensity ratio of them reflects the crystallographic sites of Eu3+ activator, which can be calculated to be 1.20 using the equation50
detailed average bond distances and their distributions are listed in Table S2, given as Supporting Information. In addition, the bond valence sum (BVS) was calculated for assessing the correctness our refined incommensurately modulated structure model of compound Na3La(PO4)2.41 The average BVS values of atoms La1, La2, P1, P2, P3, P4, Na1, Na2, Na3, Na4, Na5, and Na6 are 3.1467(9), 3.0456(8), 4.958(4), 5.154(5), 4.853(4), 5.094(5), 1.1969(6), 0.9761(4), 0.9692(1), 1.2559(17), 0.9328(1), and 1.0119(4), respectively. These values are common values and fall in the acceptable range compared with other similar inorganic rare-earth phosphates.42,43 Optical Absorption Spectrum. Figure 7 presents the UV−vis diffuse reflectance spectrum of powdered Na3La(PO4)2 ranging from 200 to 1200 nm. The Kubelka−Munk (K−M) function, α/S = K/S = F(R) = (1 − R)2/2R44 (in which the symbols α, S, K, and R are the absorption coefficient, scattering coefficient, absorption, and reflectance, respectively), was used to convert the spectrum for evaluating the optical absorption cutoff edge and the band gap. We can confirm that compound Na3La(PO4)2 does not absorb light energy in the range of 420− 1200 nm, and the absorption peak mainly locates in the UV light region. This conclusion is very much consonant with the white appearance of powdered Na3La(PO4)2. To get more accurate value of the band gap of compound Na3La(PO4)2, we plot the (K/S)-versus-E scheme (inset of Figure 7), in which the intersection point of tangent and horizontal axis presents the band gap of 3.50 eV. We tentatively put forward that the optical transition of Na3La(PO4)2 may originate from the O 2p orbital to the Na 3s, P 3s, or La 5d empty orbitals. Photoluminescence Properties. To evaluate the potentiality of Na3La(PO4)2 to be used as a luminescent host material, Eu3+, Tb3+, and Dy3+ doped in Na3La(PO4)2 matrix were prepared with the Ln3+ concentration of 5 mol % relative to the La3+ content. XRD studies (Figure 2) revealed that the dopants of Ln3+ do not cause any significant changes of the host lattice. It is well-known that the chemical properties and ionic
R=
∫ I( 5D0 →7 F2) ∫ I( 5D0 →7 F1)
According to the above crystal-structure analysis, the structure of Na3La(PO4)2 is derived from the α-K2SO4 basic E
DOI: 10.1021/acs.inorgchem.6b02241 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 6. Evolution of La1−O (a), La2−O (b), Na1−O (c), Na2−O (d), Na3−O (e), Na4−O (f), Na5−O (g), Na6−O (h), P1−O (i), P2−O (j), P3− O (k), and P4−O (l) distances vs the internal parameter t for compound Na3La(PO4)2.
orthorhombic. More specially, the K atoms on the sixfold of αK2SO440 with an inversion center symmetry are substituted by La atoms, which deviate from the high symmetry sites and occupy a general site without an inversion center in Na3La(PO4)2. However, this deviation is slight and inessential, and it cannot break the basic framework. In other words, the La atoms occupy general site but close to site with a high symmetry. We tentatively put forward that this fact leads to the comparable intensity of 5D0→7F2 and 5D0→7F1 transitions. Furthermore, there are three weak emission peaks at 577, 650, and 697 nm, which can be attributed to the 5D0→7F0, 5D0→7F3, and 5D0→7F4 transitions of Eu3+, respectively. It is worth mentioning that the number of 5D0→7F0 lines is determined by the number of crystallographic sites of Eu3+ activator, because of the nondegenerate characteristic of both ground state 7F0 and excited state 5D0. However, La atoms occupy two crystallographic sites in Na3La(PO4)2 host lattice as well as Eu3+ activators. This contradiction can be attributed to the similar coordination environments of two La atom sites. Thus, the splitting of 5D0→7F0 peak is very blurry and insignificant. The excitation spectrum of Na3La(PO4)2:Tb3+ monitored at the maxima emission (546 nm) is presented in Figure 9a in the range of 320−420 nm. The appearance of PL emission
Figure 7. Experimental UV−vis absorption spectrum of Na3La(PO4)2 ranging from 200 to 1200 eV.
structure with a hexagonal symmetry. The incompatible ion radius of substation atoms makes them deviate from their “ideal” sites and finally reduce the hexagonal symmetry to F
DOI: 10.1021/acs.inorgchem.6b02241 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 8. Excitation (a) and emission (b) spectra of 5 mol % Eu3+-doped Na3La(PO4)2.
Figure 9. Excitation (a) and emission (b) spectra of 5 mol % Tb3+-doped Na3La(PO4)2.
Figure 10. Excitation (a) and emission (b) spectra of 5 mol % Dy3+-doped Na3La(PO4)2.
Dy3+-activated phosphors usually show strong fluorescent transitions in the blue (around 485 nm) and yellow (around 575 nm) emissions corresponding to 4F9/2→6H15/2 and 4 F9/2→6H13/2 transitions, respectively.57−59 Therefore, a white phosphor can be produced using Dy3+ ion as the sole activator by adjusting the yellow/blue intensity ratio to a suitable value. The excitation spectrum for Dy3+-activated Na3La(PO4)2 phosphor was measured by monitoring the 4F9/2→6P7/2 (573 nm) emission, as shown in Figure 10a. There is a series of sharp peaks located at 325, 350, 365, and 387 nm, corresponding to the intrinsic 4f→4f transitions of Dy3+ activator, that is, from the ground state of 6H15/2 to the excited states of 4L19/2 (325 nm), 6P7/2 (350 nm), 6P5/2 (365 nm), and 4I13/2 (387 nm), respectively.60,61 The PL emission spectrum of Na3La-
spectrum proves that there exists a series of peaks ranging from 330 to 390 nm, corresponding to the spin-forbidden 4f→4f transitions of Tb3+ activator.51,52 The strongest emission locates at ∼375 nm can be assigned to the 7F6→5G6 or (5D3) transitions of Tb3+. Excited at the wavelength of 375 nm, Na3La(PO4)2:Tb 3+ shows strong green emission spectrum, as shown in Figure 9b. Four typical transitions from 5D4 level down to 7FJ levels of 4f8 configuration of Tb3+ were observed, that is, 5D4→7F6 (around 488 nm), 5D4→ 7F5 (around 546 nm), and 5D4→ 7F3 (around 621 nm), and the strongest of them is the green emission at ∼546 nm.53−56 The excitation and emission spectra indicate that phosphor Na3La(PO4)2:Tb 3+ exhibits a strong greenish emission. G
DOI: 10.1021/acs.inorgchem.6b02241 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 11. Fluorescent decay (black) and fitting (red) curves of Na3La(PO4)2:Eu3+ (a) and Na3La(PO4)2:Dy3+ (b).
(PO4)2:Dy3+ phosphor excited by 350 nm (Figure 10b) UV light consists of two main emission regions ranging from 450 to 650 nm, blue (around 485 nm) and yellow (around 576 nm) emissions corresponding to the 4F9/2→6H15/2 and 4F9/2→6H13/2 transitions, respectively. It is generally known that crystallography sites of Dy3+ activator in host matrix affects the relative strengths of blue and yellow emissions. If Dy3+ activator locates in site without an inversion center, the 4F9/2→6H13/2 yellow electric dipole transition dominates the emission spectrum, whereas the 4F9/2→6H15/2 blue magnetic emission is the strongest one if Dy3+ occupies a crystallography site without an inversion center. The reason for this is that the magnetic transition is less sensitive to the atom site in host matrix than electric dipole transition.62,63 For Na3La(PO4) 2:Dy3+ phosphor, clearly the peak at ∼576 nm is much stronger than the peak at ∼485 nm, indicating that Dy3+ activator occupies a crystallographic site without an inversion center symmetry. Compared to Eu3+ ion doped Na3La(PO4)2 phosphor, it seems that Dy3+ ion dopant is more appropriate to occupy a lower symmetry. This point can also be supported by the PL kinetics studies. As shown in Figure 11a, the experimental decay curve of phosphor Na3La(PO4)2:Eu3+ is fitted with a single-exponential function, y = y0 + A1 × exp(−x/τ), where τ is calculated to be 3.245 ms for representing the lifetime. Somewhat differently, the experimental decay curve of phosphor Na3La(PO4)2:Dy3+ is fitted with a two-exponential equation (Figure 11b), y = y0 + A1 × exp(−t/τ1) + A2 × exp(−t/τ2), where τ1 and τ2 are the lifetimes, and the average lifetime τ can be calculated to be 1.067 ms by the equation τ =
A1τ12 + A 2 τ2 2 64−66 . A1τ1 + A 2 τ2
smaller Dy3+ ions doping in Na3La(PO4)2 host might increase the distortion from the α-K2SO4 basic structure, whereas larger Eu3+ ions dopant would decrease such distortion. Furthermore, the color purity for the prepared phosphors were evaluated on the basis of the Commission International de L’ Eclairage (CIE) functions.70,71 The calculation results for them are (x1 = 0.620, y1 = 0.378) excitation at 392 nm falling in the orange region for Eu3+ activated, (x2 = 0.345, y2 = 0.573) excitation at 378 nm falling in the greenish region for Tb3+ activated, and (x3 = 0.394, y3 = 0.345) excitation at 350 nm falling in the yellowish region for Dy3+ activated (Figure 12).
Usually, the
index of exponential functions agrees with the number of Ln3+ ion sites in the host lattice. Hence, the fitting of decay curves suggests that one Eu3+ and two Dy3+ crystallographic distinct ions are present in the Na3La(PO4)2 host, respectively.67−69 Considering the fact that the La3+ sites in Na3La(PO4)2 host possess low symmetry sites but close to a centering symmetry sites, the Eu3+ and Dy3+ dopants will occupy such La3+ sites in very different ways because of their different ion radius. We may interpret this that larger Eu3+ dopant in host lattice tends to occupy La3+ sites in a more “regular” manner, leading to the enhancement of magnetic dipole 5D0→7F1 transition and single-exponential fitting function. Conversely, the smaller Dy3+ dopants prefer to occupy lower symmetry sites, resulting in the enhancement of electric dipole 4F9/2→6H13/2 transition and double-exponential fitting function. In other words, the
Figure 12. CIE chromaticity diagram of Na3La(PO4)2:Eu3+ (a), Na3La(PO4)2:Tb3+ (b), and Na3La(PO4)2:Dy3+ (c).
The quantum efficiency (QE) of a phosphor is an important parameter to be considered for practical application. The QE can be measured and calculated according to the following equation: ηQE = H
∫ LS ∫ ER − ∫ ES DOI: 10.1021/acs.inorgchem.6b02241 Inorg. Chem. XXXX, XXX, XXX−XXX
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where LS represents the emission spectrum, ES represents the excitation spectrum, and ER represents the background. Upon excitation at 392, 378, and 350 nm, the corresponding QE of phosphors Na3La0.95Eu0.05(PO4)2, Na3La0.95Tb0.05(PO4)2, and Na3La0.95Dy0.05(PO4)2 were 34.57%, 33.67%, and 54.72%, respectively. Since the significant factors in QE value (such as doping concentration, sintering temperature, and synthetic method) were not optimized, we may expect that the three phosphors have certain application value in white-LED light source.
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CONCLUSIONS In summary, single crystal of sodium lanthanum orthophosphate Na3La(PO4)2 was prepared by flux method and was structurally determined by SC-XRD method for the first time. Its structure can be attributed to the α-K2(SO4)2 type of family, and it features an incommensurately modulated structure with (3 + 1) dimensional orthorhombic space group Pca21(0β0)000. To our knowledge, it represents the first example of refined modulated crystals in this family of compounds. The average structure contains two one-dimensional lines of −Na−La− and −Na−P−, which are no longer straight along the a-axis compared with the ideal α-K2(SO4)2 type of structure. Furthermore, 5 mol % Eu3+, Tb3+, and Dy3+ activated phosphors were prepared for PL properties studying. The phosphors show the typical characteristic emission of activators, with CIE chromaticity color coordinates of (x1 = 0.620, y1 = 0.378; orange) for Na3La(PO4)2:Eu3+, (x2 = 0.345, y2 = 0.573; greenish) for Na3La(PO4)2:Tb3+, and (x3 = 0.394, y3 = 0.345; yellowish) for Na3La(PO4)2:Dy3+. The QE of the three phosphors are 34.57%, 33.67%, and 54.72%, indicating that they are efficient, and thus compound Na3La(PO4)2 is a promising host material for multiple activators.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02241. Atomic coordinates and Ueq values of Na3La(PO4)2; bond lengths (Å) of Na3La(PO4)2 showing interatomic distances influenced by structural modulation (PDF) Check CIF (CIF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Dan Zhao: 0000-0002-1573-4299 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Project Nos. 21201056 and 21307028). REFERENCES
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