Control of Luminescence in Eu2+-Doped Orthosilicate

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Control of Luminescence in Eu2+-Doped OrthosilicateOrthophosphate Phosphors by Chainlike Polyhedra and Electronic Structures Lizhu He, Zhen Song,* Xionghui Jia, Zhiguo Xia, and Quanlin Liu* The Beijing Municipal Key Laboratory of New Energy Materials and Technologies, School of Materials Sciences and Engineering, University of Science and Technology Beijing, Beijing 100083, China S Supporting Information *

ABSTRACT: A series of Eu2+-doped orthosilicate-orthophosphate solid-solution phosphors, KxBa1.97−x(Si1−xPx)O4:0.03Eu2+, have been synthesized via the conventional solid-state reaction. Using varying compositions, the lowest-energy excitation can be tuned from 470 to 405 nm, with an emission from 515 to 423 nm. We determined how chainlike cation polyhedra controlled excitation- and emission-band features by introducing in-chain characteristic length d22 and outside-chain characteristic length d12 and that there was a nearly linear relationship between the lowest-energy-excitation position and the ratio of d22 to d12. This influence of chainlike polyhedra on luminescence can be understood through the inductive effect. Luminescent thermal properties are improved remarkably by the cosubstitution of K+ and P5+ ions for Ba2+ and Si4+ ions with a T1/2 over 200 °C. We have established the host-referred-binding-energy (HRBE) and vacuum-referred-binding-energy (VRBE) schemes for the electronic structure of the series of lanthanide-doped phosphors according to the Dorenbos model and given a thermalquenching mechanism for this series of phosphors.

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composition (46% Sr) of (Ba1−xSrx)2SiO4:Eu2+ possesses the best resistance to the thermal quenching of luminescence because of its enhanced structural rigidity.10 We have introduced cation ordering to elucidate an enhancement to the resistance of the thermal quenching of luminescence in the (Ba1−xSrx)2SiO4:Eu2+ system and revealed a positive correlation between the thermal-quenching-of-luminescence temperature T1/2 and cation ordering.11 Besides orthosilicate phosphors, a variety of orthophosphate phosphors could also be applied for wLEDs. Orthophosphate phosphors have the general formula ABPO4 (A and B represent monovalent and divalent cations, respectively), whose structures are dependent on the choices of A and B. For ABPO4 phosphates, the reported structures are monoclinic for A = Na and B = Sr,12,13 orthorhombic for A = Li and B = Ba14,15 and for A = K and B = Sr or Ba,16,17 and hexagonal for A = Li and B = Ca or Sr18,19 and for A = Na and B = Ca or Ba.20,21 It is worth noting that KBaPO4 is of the orthorhombic structure with the space group Pnma (No. 62), which is isotopic with Ba2SiO4. Considering that the highest thermal-quenching temperature was observed in the intermediate composition of the SrxBa2−xSiO4:Eu2+ solid solutions, in this paper we designed and synthesized a series of Eu2+-doped KxBa2−x(Si1−xPx)O4 solid solutions. Then, the influence of the cosubstitution of K+ and P5+ for Ba2+ and Si4+ on crystal structure, luminescence,

INTRODUCTION In the recent years, continuing progress has been achieved in phosphor-converted white-light-emitting diodes (pc-wLEDs), which are used in the solid-state lighting area on account of their advantages, such as their low energy consumption, high efficiency, long service lifetimes, and harmlessness to the environment, which puts forward a pressing demand for highperformance phosphors.1−3 For wLED applications, the requirements for ideal phosphors are as follows: high quantum efficiency, an appropriate excitation spectrum that falls in the range where blue-LED-chip emission is most efficient, high chemical stability, and a resistance to the thermal quenching of luminescence.4−6 Because the phosphor layers are operating above 400 K in commercially available products, the effect of the thermal quenching of luminescence plays a pivotal role in the application of phosphor. A widely applied class of phosphors is that of the alkalineearth-metal orthosilicates with the general composition of M2SiO4 (M = Ba or Sr), which crystallize in the orthorhombic system with the space group Pnma (No. 62).7 The composition of the Ba/Sr ratio for (Ba1−xSrx)2SiO4 can be varied continuously from 0 to 1. When Eu2+ ions are doped into the M2SiO4 host lattice, these compounds can be efficiently excited by UV to blue light and emit wavelength-tunable light from green to yellow. The two end-member compositions (Ba2SiO4 and Sr2SiO4) possess brilliant luminescent properties, such as high emitting brightness and pronounced chemical stability.8,9 Additionally, Denault et al. have reported that the intermediate © XXXX American Chemical Society

Received: September 22, 2017

A

DOI: 10.1021/acs.inorgchem.7b02431 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry and thermal-quenching behavior was investigated. We found that a structure of chainlike polyhedra plays a key role in controlling excitation- and emission-band features. Furthermore, we established the host-referred-binding-energy (HRBE) and vacuum-referred-binding-energy (VRBE) schemes for the electronic structure of the series of lanthanide-doped phosphors according to Dorenbos model,22−27 which gave a satisfactory explanation of the thermal-quenching-of-luminescence behavior.

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EXPERIMENTAL SECTION

Synthesis. A series of Eu2+-doped orthosilicate-orthophosphate solid-solution phosphors, KxBa1.97−x(Si1−xPx)O4:0.03Eu2+, have been synthesized via the conventional solid-state reaction. The raw materials are BaCO3 (99.9%, Aladdin), K2CO3 (99.9%, Aladdin), SiO2 (99.9%, Aladdin), NH4H2PO4 (99.9%, Aladdin), and Eu2O3 (99.9%, Aladdin) powders, with 6 wt % KCl (99.9%, Aladdin) added as flux. The starting materials were ground with alcohol for half an hour and transferred to corundum crucibles. The mixtures were first sintered in air at 850 °C for 4 h and then sintered in a high-temperature tubular furnace at 1250 °C for 4 h in a weakly reducing atmosphere (95% N2 and 5% H2). The obtained samples were ground into powders again for the following measurements. Characterization. The X-ray diffraction data were collected on a Rigaku TTR III diffraction instrument (Cu Kα radiation) under the test condition of 40 kV and 35 mA. We adopted a continuous-scan mode with a scanning speed of 8°/min, and the test range was 2θ = 15−70°. The room-temperature photoluminescence spectra were measured by an Edinburgh Instruments FLS920. The temperaturedependent photoluminescence spectra were measured by an Edinburgh Instruments FLS920 with a heating accessory. The excitation spectra in the VUV−UV range were measured on the beamline 4B8 of the Beijing Synchrotron Radiation Facility (BSRF) under normal operating conditions by using the spectrum of sodium salicylate (o-C6H4OHCOONa) as a reference.

Figure 1. (a) XRD patterns of KxBa1.97−x(Si1−xPx)O4:0.03Eu2+ with the x values varying from 0 to 1, the standard data of the Ba2SiO4 phase (PDF#70-2113) and KBaPO4 phase (PDF#84-1462). (b) Magnified XRD curves in the range of 25−35°.

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RESULTS Synthesis and Phase Identification. Ba2SiO4 and KBaPO4 possess the same type of crystal structure with the general formula M1M2AO4 (M1 = K or Ba, M2 = Ba, and A = P or Si). The compound crystallizes in the orthorhombic β-K2SO4-type structure, space group Pnma (No. 62).7 Two sites for large cations (K and Ba) exist in the structure: the 10-coordinate M1 site and the 9-coordinate M2 site. The volume of the M1 site is larger than that of the M2 site. Considering the size effect, K+ ions (RK+ = 1.59 Å, CN = 10; RK+ = 1.55 Å, CN = 9) occupy the larger M1 sites, and P5+ ions (RP5+ = 0.17 Å, CN = 4) occupy the A sites, replacing Si4+, when they are doped into a Ba2SiO4 crystal lattice, where the Ba2+ ions (RBa2+ = 1.52 Å, CN = 10; RBa2+ = 1.47 Å, CN = 9) are in both the M1 and M2 sites, and the Si4+ ions (RSi4+ = 0.26 Å, CN = 4) are in the A sites.28,29 The X-ray diffraction (XRD) profiles of the KxBa1.97−x(Si1−xPx)O4:0.03Eu2+ (x = 0, 0.1, ..., 1) samples along with the magnified range of 25−35° are shown in Figure 1. The phase purities of all the samples indicate the formation of completely miscible solid solutions with K+ and P5+ cosubstituting for Ba2+ and Si4+. The Rietveld refinements have been performed using the FullProf program30 to obtain the detailed crystal structure information. The atomic coordinates, temperature factors, and occupancies for the KxBa1.97−x(Si1−xPx)O4:0.03Eu2+ solid solutions are listed in Table S3, and the corresponding bonelengths are listed in Table S4. The obtained cell parameters and cell volumes are depicted in Figure 2 as functions of x for the KxBa1.97−x(Si1−xPx)O4:0.03Eu2+ solid solutions. All the cell parameters change in a linear fashion as the x value grows from 0 to 1, which further confirms the formation of a solid solution.

Figure 2. Cell parameters and volumes of KxBa1.97−x(Si1−xPx)O4:0.03Eu2+ as functions of x.

In the enlarged region of Figure 1b, it is interesting to note that the (202) diffraction peak shifts to a low angle, whereas the (020) diffraction peak moves to a high angle, which indicates an anisotropic change of the chainlike polyhedra with the changing composition; the details as well as the effects on luminescence will be discussed in the following section. Controllable Tuning of Photoluminescence. The normalized excitation spectra shown in Figure 3a cover a very broad range, from 250 to 480 nm, which is ascribed to the typical transitions of Eu2+ between its ground state, 4f7, and crystal-field-split, 4f65d, configuration and is analogous to the excitation spectra reported by Xia et al. in (Na1−xCax)(Sc1−xMgx)Si2O6:Eu2+ solid solutions.31 It can be seen that as the x value increases, the excitation bands shift to the high-energy region and become narrower. For the normalized emission spectra of KxBa1.97−x(Si1−xPx)O4:0.03Eu2+, the evolution of the emissionpeak wavelengths (denoted as λem in Figure 3c) can be divided into three stages, as shown in Figure 3b. In Stage 1, the peak wavelengths are red-shifted slightly from 505 to 515 nm as x increases from 0 to 0.3. However, a blue shift occurs in Stage 2 from composition x = 0.4 to 0.9, with the peak wavelength reaching 473 nm. For x = 1, the emission band has a very large blue jump of 50 nm with the peak located at 423 nm. The key spectra data are extracted, listed in Table S1, and represented as B

DOI: 10.1021/acs.inorgchem.7b02431 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

269, 320, 360, and 383 nm for KBaPO4:Eu2+. The excitation bands labeled A at 176 nm for Ba2SiO4:Eu2+ and 164 nm for KBaPO4:Eu2 are attributed to the host exciton-absorption edge, corresponding to the excitation of an electron from the top of the valence band to the energetically lowest bound exciton state. The five distinguished bands (B−F) can be used to calculated crystal-field splitting and the centroid shift. Band A can be used to estimate the host band gap in the construction of the HRBE and VRBE diagrams for electronic structure, which will be discussed in detail in a later section. Thermal-Quenching Properties. Figure 5 shows the normalized integrated photoluminescence intensities of the

Figure 3. (a) Normalized excitation spectra of KxBa1.97−x(Si1−xPx)O4:0.03Eu2+ monitored under optimal emission. (b) Normalized emission spectra under a 395 nm excitation. (c) λcs (the wavelength at which the integrated intensity of the excitation spectrum reaches half of the total integrated intensity), λcp, and λem vs x. (d) Stokes shift (SS) and fwhm of the emission bands as functions of x.

a function of x in Figure 3c,d. The excitation-spectrum-centroidwavelength position (λcs) shows a monotonous, decreasing trend as the x value increases, whereas the cross point of the excitation and emission bands (λcp) first slightly increases and then decreases. This variation is due to the Stokes-shift change in the composition x, as shown in Figure 3d. The Stokes shift almost monotonously increases when the compositions vary from x = 0 to 0.9 and has an abrupt drop at x = 1. The trend of the fwhm of the emission bands is very similar to that of the Stokes shift. The explanation for the above variations will be given in following discussion. To evaluate the 5d energy level location of Eu2+ in KxBa1.97−x(Si1−xPx)O4:0.03Eu2+, we needed to analyze the excitation spectra in detail. As shown in Figure 3a, the broad-band excitation spectra make it difficult to distinguish the five crystal-splitting components of the 5d energy level, so we measure the excitation spectra in the VUV−UV range using the beamline 4B8 of the Beijing Synchrotron Radiation Facility (BSRF) under normal operating conditions, as shown in Figure 4. The bands

Figure 5. Normalized integrated photoluminescence intensities of KxBa1.97−x(Si1−xPx)O4:0.03Eu2+ in the range of 25 to 200 °C at different compositions of x with the corresponding T1/2 in the inset graph.

KxBa1.97−x(Si1−xPx)O4:0.03Eu2+ series as functions of temperature, and the inset presents T1/2 versus x. T1/2 represents the thermal-quenching temperature, which is the temperature at which the emission intensity drops to half of that at room temperature. As shown in the inset, T1/2 linearly increases as x increases, suggesting that the thermal luminescence stability of this phosphor can be improved remarkably by the cosubstitution of K+ and P5+ ions for Ba2+ and Si4+ ions. For end member KBaPO4 (x = 1), the T1/2 reaches a temperature over 200 °C, which is much higher than those reported for other phosphates.32,33

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DISCUSSION Evolution of the Chainlike Polyhedral Structure and Excitation-Spectra Control. Generally, the excitation spectrum of the Eu2+ f−d transition is mainly determined by the 5d centroid (εc) and crystal-field splitting (εcfs). Dorenbos has conducted a systematic study of the energies of the lowest 4fn−15d1 states of Eu2+/Ce3+ ions.22,34 The centroid shift εc depends on the covalency between the lanthanide ions and the anionic ligands of the host lattice, that is, it depends on the orbital overlap between Eu2+ and the surrounding O2− ligands. The crystal-field splitting, εcfs, is associated with the coordination environments of the RE ions, that is, it is mainly determined by the shape and size of the polyhedron of Eu2+/Ce3+ ions. Herein, for the KxBa1.97−x(Si1−xPx)O4:0.03Eu2+ series, we can see that the lowest-energy excitation band gradually shifts toward a shorter wavelength and becomes narrower as the x value increases, as shown in Figure 3a. However, as depicted in Figure 2, the cell volume shrinks monotonously as x increases,

Figure 4. (a,c) Synchrotron radiation VUV−UV excitation and (b,d) PLE spectra of (a,b) Ba2SiO4 (λem = 505 nm) and (c,d) KBaPO4 (λem = 423 nm) at room temperature.

B, C, D, E, and F are assigned to the transitions from 4f ground state to the five excited 5d levels of Eu2+ ions. They are located at 256, 288, 343, 396, and 430 nm for Ba2SiO4:Eu2+ and 250, C

DOI: 10.1021/acs.inorgchem.7b02431 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry which is usually followed by enhanced crystal-field splitting. In this case, a red shift that is opposite to the blue shift of the lowest-energy excitation band is expected. In the following discussion, we will show that this anomalous behavior arises from the evolution of chainlike polyhedra in the structure. The replacement of Ba2+ and Si4+ with K+ and P5+ induces not only monotonous cell-volume shrinkage but also an anisotropic change of the cell parameters, that is, the a axis elongates, whereas the b and c axes shrink, as shown in Figure 2. These variations are also reflected by the fact that the (020) peak shifts gradually to a higher angle, whereas the (202) peak moves to a lower angle, as shown in Figure 1b. This anisotropy could be understood by the buildup of the crystal structure in view of the coordination polyhedra along different axes, as shown in Figure 6. KxBa1.97−x(Si1−xPx)O4:0.03Eu2+ can be

Figure 7. (a) Bond lengths of M1−O1, A−O2, and M1−A; the sum of them; and the cell parameter of the a axis vs x. (b) Bond length of M1−O3 and the cell parameter of the b axis as functions of x.

earth ions in rows gave emissions at long wavelengths, and they proposed a model to explain the long-wavelength emission.35−37 In a chainlike polyhedral structure, an Eu2+ ion in the chain experiences positive charges because of its cationic neighbors in the chain direction in addition to the negative charges from its nearest anionic neighbors. The positive charges can orient one 5d orbital preferentially, lowering its energy and resulting in the Eu2+ longer-wavelength emission. Herein, we present the in-chain characteristic length as d22, which refers to the distance between the two nearest M2 sites along the chain, and denote the outside-chain characteristic length as d12, which means the distance between the M2 site and the M1 site outside the chain. We have calculated the d22 and d12 values for the KxBa1.97−x(Si1−xPx)O4:0.03Eu2+ series and listed them in Table 1. d22 increases gradually with increasing x, whereas d12

Figure 6. Crystal structures of M1M2AO4 (M1 = K or Ba, M2 = Ba, and A = P or Si). (a) Along the a direction, the [M1−O] and [A−O] polyhedra are arranged alternately. (b) The length of the b axis is mainly determined by the bond length of M1−O3. (c) The [M2−O] polyhedra, which the Eu2+ ions occupy, form a zigzag chain along the a axis with two neighboring M1 polyhedra on both sides of the chain.

represented as M1M2AO4 (M1 = K or Ba, M2 = Ba, and A = P or Si) according to the Wyckoff positions. As shown in Figure 6a, along the a direction, the [M1−O] and [A−O] polyhedra are arranged alternately, and the a length is almost equal to the sum of the distances of M1−O2, A−O2, and M1−A along the a direction. On the basis of the Rietveld-refinement results, the distances of M1−O2, A−O2, and M1−A and the total sum are calculated and displayed in Figure 7a. It can be seen that the sum matches well with the values of the cell parameter a. Considering that the variation in the A−O2 bond length is very small (within 0.1 Å), we believe that the elongation of the a axis is mainly determined by an increase in the M1−O2 bond length. Similarly, as shown in Figure 6b along the b axis, the [M1−O] polyhedra connect with each other by sharing corners (the O3 atoms) to form a chain. Figure 7b shows the bond lengths of M1−O3 and the b lengths as functions of x. These two curves almost coincide with each other, indicating that the cell parameter b is mainly determined by the M1−O3 bond length. In addition, changes in the M1−O3 distance have important effects on the luminescent properties, which will be discussed later. We next address the arrangement of the M2−O polyhedra, which determines the characteristics of the excitation and emission bands. As depicted in Figure 6c, the [M2−O] polyhedra, in which the Eu2+ ions are incorporated, form a zigzag chain along the a axis with two neighboring [M1−O] polyhedra at different distances on both sides of the chain. Poort et al. reported that Eu2+ ions in silicate-host lattices with alkaline

Table 1. Distances between the Two Nearest M2 Sites (d22) and between the M2 and M1 Sites (d12) As the x Values Increase x

d12 (Å)

d22 (Å)

0 0.2 0.4 0.5 0.6 0.8 1

4.201 4.198 4.180 4.170 4.164 4.148 4.010

3.918 3.938 3.956 3.962 3.972 3.988 4.011

decreases monotonously, as shown in Table 1. On the basis of these data, we plotted the lowest 5d energy level and the ratio of d22 to d12 as functions of x, as shown in Figure 8. One can see that the lowest 5d energy level and the ratio of d22 to d12 have similar variation tendencies with x. When we plotted the lowest 5d energy level as a function of the ratio of d22 to d12, as shown in the inset of Figure 8, a nearly linear relation between them was obtained. This indicates that the D

DOI: 10.1021/acs.inorgchem.7b02431 Inorg. Chem. XXXX, XXX, XXX−XXX

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The crystal field splitting of Ba2SiO4:Eu2+ is 15807 cm−1, larger than the 13890 cm−1 of KBaPO4:Eu2+. Emission Spectra and Stokes Shift. The emission band location is determined by the lowest-energy excitation band and Stokes shift. As it follows from the above discussions, the lowest-energy excitation band shifts to the blue region gradually as x increases in KxBa1.97−x(Si1−xPx)O4:0.03Eu2+. However, the emission peak has a slight red shift for samples with x = 0−0.3, whereas it blue-shifts gradually for samples with x = 0.4−1.0, as shown in Figure 3b. Correspondingly, the Stokes shift slightly increases as x goes from 0 to 0.9 but abruptly decreases for endmember KBaPO4:Eu2+. This variation of the Stokes shift with composition is often observed in solid-solution phosphors, such as (Ba, Sr)2SiO4:Eu2+ and xSr2Ca(PO4)2−(1−x)Ca10Li(PO4)7.11,42 The reason lies in the degradation of structural rigidity for intermediate compositions due to the presence of more types of atoms compared with that in end-member host lattice. Electronic Structure and Control of the Thermal Quenching of Luminescence. Temperature-dependent luminescence behaviors are of great importance for phosphors and guide their possible applications. Denault et al. studied the thermal quenching properties of (Ba,Sr)2SiO4:Eu2+ solid solutions and found that the intermediate composition possessed the highest thermal-quenching temperature because of its enhanced structural rigidity.10 Therewith, we calculated the cation ordering in the (Ba,Sr)2SiO4:Eu2 system and found a linear relationship between the cation ordering and thermal-quenching temperature.11 Wang. et al. studied the temperature-dependent luminescence behaviors of Ba2Si5N8:Eu2+phosphors in detail.43 The thermal quenching property of the most widely used phosphor, YAG:Ce3+, has garnered significant attention in recent years. Bachmann et al. ascribed the thermal quenching of it to thermally activated concentration quenching (for highly doped systems) and temperature dependence of the oscillator strength (for low-doping concentrations).5 Later, Song et al. conducted a careful investigation into the thermal-quenching-of-luminescence resistance and persistent luminescence of Y3(AlxGa1−x)5O12:Ce3+ and discovered a negative correlation between them with varying Ga content, which is determined by the electronic structures.44 In the present system, KxBa1.97−x(Si1−xPx)O4:0.03Eu2+, the thermal-quenching temperature, T1/2, increases monotonously with x, as shown in Figure 5. Generally, there are two mechanisms that account for the thermal quenching of luminescence of d−f emissions for Ce3+/Eu2+. One mechanism is related to the thermal stimulated-ionization process from the 5d energy level to the conduction band.45 This thermal-quenching behavior is mainly determined by the energy difference between the 5d excitation level and the conduction-band bottom. The other process of the thermal quenching of luminescence arises from a displacement between the ground- and excited-state curves in the configurational coordinate diagram.46 This thermalquenching behavior is generally related to structural rigidity and the Stokes shift.11,47 Usually, the phosphor with the larger Stokes shift has weak structural rigidity and therefore a lower thermal-quenching-of-luminescence temperature. However, for KxBa1.97−x(Si1−xPx)O4:0.03Eu2+, the T1/2 grows as the x value increases, and the Stokes shift also increases. Therefore, thermalquenching behavior is mainly affected by the thermal ionization process of the 5d electrons and is highly correlated to the electronic band structure, that is, the host-referred-binding-energy (HRBE) and vacuum-referred-binding-energy (VRBE) schemes. Figure 9 illustrates the VRBE and HRBE schemes of Ln2+/Ln3+ with n 4f electrons doped in a Ba2SiO4 host. The bottom of the

Figure 8. Lowest 5d energy level and the ratio of d22 to d12 as functions of the x values.

lowest 5d energy level of Eu2+ is closely related to the d22 to d12 ratio. The phenomenon that the Eu2+ ions in the chains show emissions at long wavelengths can be explained by the inductive effect in solid-state chemistry,38 that is, the nearest-neighboring cations can modify the bond-covalency degree. When Eu2+ ions are substituted to the M2 (Ba2+) sites in Ba2SiO4, the nearestneighboring cations along the chain direction (d22 = 3.928 Å) are much closer than those outside the chain direction (d12 = 4.201 Å). Thus, the nearest-neighboring Ba ion can more easily donate an electron and exert an electron pressure on the Eu−O bond along the chain direction, resulting in an increase in the covalent contribution of this bond. As a result, the oriented 5d orbital along the chain direction becomes the lowest-energy 5d orbital, determining the position of lowest-energy excitation band. In KxBa1.97−x(Si1−xPx)O4:0.03Eu2+, as x increases, d22 increases gradually in parallel with the monotonous d12 decrease. Consequently, the d orbital oriented along the chain direction (the d22 direction), that is, the lowest-energy d orbital, will be elevated more in energy, causing the blue shift of the excitation band, as show in Figure 8. For the end member, KBaPO4:Eu2+, d22 is nearly equal to d12, so the effect of the preferential orientation of the d orbital completely vanishes, leading to the excitation band with the largest blue-shift and the narrowest fwhm, as shown in Figure 3a,d. The other reason for the blue shift of the excitation spectrum arises from the centroid-shift decrease due to the replacement of the P5+ ions with Si4+ ions, which leads to a larger average cation electronegativity.39 Pauling type40 electronegativity values were chosen to elucidate this point, with χBa = 0.89, χK = 0.82, χSi = 1.9, and χP = 2.19.41 As K+ and P5+ substitute for Ba2+ and Si4+, the difference between χBa and χK (0.89−0.82 = 0.07) becomes much smaller than that of χSi and χP (2.19−1.9 = 0.29). Thus, the average cation electronegativity increases gradually for the KxBa1.97−x(Si1−xPx)O4:0.03Eu2+ series with the substitution level increasing, resulting in a decrease in the 5d centroid shift and the blue shift of the excitation band, as listed in Table S1. On the basis of the measured synchrotron radiation VUV−UV excitation spectra, the crystal-field splitting and the centroid shifts of the end members of Ba2SiO4:Eu2+ and KBaPO4:Eu2+ can be calculated to provide further justifications. The centroid shifts of Ba2SiO4:Eu2+ and KBaPO4:Eu2+ are 0.36 and 0.3 eV, respectively, which are consistent with the predicted trend. E

DOI: 10.1021/acs.inorgchem.7b02431 Inorg. Chem. XXXX, XXX, XXX−XXX

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given by Dorenbos.22 This U (6, A) value is an important supplement to Dorenbos’s data collection.27 Second, the absolute 4f-electron binding energy of Eu2+ in the Ba2SiO4 host lattice, E4fabs(7, 2+, A), can be calculated by26 E4fabs(7, 2 + , A) = −24.92 +

18.05 − U (6, A) 0.777 − 0.0353U (6, A) (3)

where A = Ba2SiO4, and the result is −4.08 eV. Therefore, the HRBE energy scale can be obtained by pinning the 4f groundstate energy level of Eu2+ to −4.08 eV, as depicted in Figure 9. The detailed energy-level data of the Ln2+/Ln3+ ions in the HRBE and VRBE diagrams of Ba2SiO4 are compiled in Table S2. The HRBE and VRBE diagrams of KBaPO4 can be constructed in a similar way. The band gap of KBaPO4 is estimated to be 8.27 eV, which is larger than that of Ba2SiO4, 7.36 eV. For the intermediate compositions, it is reasonable to assume that the band gap varies linearly, as depicted in Figure 10. Generally,

Figure 9. VRBE and HRBE diagrams of Ln2+/Ln3+ in a Ba2SiO4 host.

conduction band is determined by 1.08 × Eex, in which Eex denotes the host excitonic absorption energy.22 The excitation band at 182 nm (6.81 eV)48 is ascribed to Eex, and therefore the band gap is 7.36 eV. In addition, the charge-transfer (CT) energies of the trivalent lanthanides (Ln3+) determine the 4f-level energy locations of the corresponding divalent lanthanides (Ln2+) relative to the top of the valence band.23 Similarly, the pinning of the 4f-state positions of the Ln3+ ions needs the CT energies of the Ln4+ ions. In Ba2SiO4, the CT energy of Eu3+ is determined to be 4.23 eV (represented by arrow 1 in Figure 9), which is also confirmed by other reports,49,50 and that of Ce4+ (arrow 2 in Figure 9) is 2.44 eV.45 With this value known, all the CT energies of the lanthanide elements in Ba2SiO4 host could be obtained by E CT(n , 3 + , Ba 2SiO4 ) = E CT(6, 3 + , Ba 2SiO4 ) + ΔE(n + 1, 7, 2 + )

(1)

Figure 10. HRBE and VRBE diagrams of KxBa1.97−x(Si1−xPx)O4:0.03Eu2+ with increasing x values.

The energy differences, ΔE(n + 1, 7, 2+) and ΔE(n, 6, 3+), are generally independent of the host, and the detailed values are listed in ref 24. Following this procedure, the characteristic double zigzag curves through the lanthanides series could be obtained as shown in Figure 9 (curves a and b). The upper zigzag curve a represents the 4f ground state of the Ln2+ ions, and curve b represents the 4f ground state of the Ln3+ ions relative to the valence-band top. After the 4f ground states of Ln2+ and Ln3+ have been pinned, the 5d excited states can be located in a similar manner. The transition energies from the 4f ground states to the lowest 5d states of Eu2+ (arrow 3) and Ce3+ (arrow 4) in the Ba2SiO4 host lattice are found to be 2.89 and 3.46 eV, respectively, the former of which is very close to the value reported by Dorenbos (2.86 eV for Eu2+).25 By adding ΔEfd(n + 1, 7, 2+) and ΔEfd (n, 6, 3+) to the lowest 5d energies of Eu2+ and Ce3+,23 the energy locations of the lowest 5d levels of both divalent (curve c) and trivalent lanthanides (curve d) could be obtained. The 5d curves are relatively flat compared with the 4f curves among the lanthanides and are determined by the nature of 5d orbital.22 In the end, the HRBE diagram is obtained. The VRBE can be constructed on the basis of the HRBE. First, the 4f−4f Coulomb repulsion energy, U (6, A), (arrow 5) is calculated by U (6, A) = E4f (7, 2 + , A) − E4f (6, 3 + , A)

the value of U (6, A) does not change much in an oxide host. For instance, the U (6, A) value for the silicate LiYSiO4 and the phosphate YPO4 are 7.05 and 7.00 eV, respectively.27 Here, we adopt U (6, A) = 7.03 eV for all the compositions by which the HRBE diagrams, shown on the left of Figure 10, can be transferred to VRBE diagrams, shown on the right. The energy differences between the lowest 5d levels and the bottom of the conduction band (Edc) is 0.24 eV for Ba2SiO4 and 0.83 eV for KBaPO4. This means that in Ba2SiO4, it is much easier for a 5d electron to be ionized to the conduction band, which results in poor thermal-quenching-of-luminescence behavior. Therefore, we expect a higher T1/2 in KBaPO4 than that in Ba2SiO4. For KxBa1.97−x(Si1−xPx)O4:0.03Eu2+, as x increases, the bandgaps of the compounds increase gradually, and the gaps between the 5d levels and conduction-band bottoms increase monotonously. Consequently, the ionization possibility for an electron at the 5d level decreases with an increase in x, leading to an improvement in the thermal luminescence stability of this phosphor series.

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CONCLUSIONS In this study, KxBa1.97−x(Si1−xPx)O4:0.03Eu2+ solid solutions have been synthesized and investigated. The substitution of Ba and Si with K and P results in an anisotropic change in the lattice parameters, with a monotonous shrinkage of cell volumes. We have found that the chainlike-cation-polyhedra structure plays a key role in controlling excitation- and emission-band

(2)

In this case, A = Ba2SiO4 and U (6, A) = U (6, Ba2SiO4) = 7.03 eV, which falls in the range of 6.4 to 7.2 eV for oxides F

DOI: 10.1021/acs.inorgchem.7b02431 Inorg. Chem. XXXX, XXX, XXX−XXX

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(4) Kim, Y. H.; Arunkumar, P.; Kim, B. Y.; Unithrattil, S.; Kim, E.; Moon, S. H.; Hyun, J. Y.; Kim, K. H.; Lee, D.; Lee, J. S.; Im, W. B. A zero-thermal-quenching phosphor. Nat. Mater. 2017, 16, 543−550. (5) Bachmann, V.; Ronda, C.; Meijerink, A. Temperature Quenching of Yellow Ce3+ Luminescence in YAG:Ce. Chem. Mater. 2009, 21, 2077. (6) McKittrick, J.; Hannah, M. E.; Piquette, A.; Han, J. K.; Choi, J. I.; Anc, M.; Galvez, M.; Lugauer, H.; Talbot, J. B.; Mishra, K. C. Phosphor Selection Considerations for Near-UV LED Solid State Lighting. ECS J. Solid State Sci. Technol. 2013, 2, R3119−R3131. (7) Barry, T. L. Fluorescence of Eu2+-Activated Phases in Binary Alkaline Earth Orthosilicate Systems. J. Electrochem. Soc. 1968, 115, 1181. (8) Kim, J. S.; Park, Y. H.; Kim, S. M.; Choi, J. C.; Park, H. L. Temperature-dependent emission spectra of M2SiO4:Eu2+(M = Ca, Sr, Ba) phosphors for green and greenish white LEDs. Solid State Commun. 2005, 133, 445−448. (9) Shao, Q.; Lin, H. Y.; Dong, Y.; Jiang, J. Q. Temperaturedependent photoluminescence properties of (Ba,Sr) 2SiO4 :Eu2+ phosphors for white LEDs applications. J. Lumin. 2014, 151, 165−169. (10) Denault, K. A.; Brgoch, J.; Gaultois, M. W.; Mikhailovsky, A.; Petry, R.; Winkler, H.; DenBaars, S. P.; Seshadri, R. Consequences of Optimal Bond Valence on Structural Rigidity and Improved Luminescence Properties in SrxBa2−xSiO4:Eu2+ Orthosilicate Phosphors. Chem. Mater. 2014, 26, 2275−2282. (11) He, L. Z.; Song, Z.; Xiang, Q. C.; Xia, Z. G.; Liu, Q. L. Relationship between thermal quenching of Eu2+ luminescence and cation ordering in (Ba1‑xSrx)2SiO4:Eu phosphors. J. Lumin. 2016, 180, 163−168. (12) Yim, D. K.; Song, H. J.; Cho, I.-S.; Kim, J. S.; Hong, K. S. A novel blue-emitting NaSrPO4:Eu2+ phosphor for near UV based white light-emitting-diodes. Mater. Lett. 2011, 65, 1666−1668. (13) Zhang, S. Y.; Nakai, Y.; Tsuboi, T.; Huang, Y. L.; Seo, H. J. The Thermal Stabilities of Luminescence and Microstructures of Eu2+Doped KBaPO4 and NaSrPO4 with β-K2SO4 Type Structure. Inorg. Chem. 2011, 50, 2897−2904. (14) Sun, J. Y.; Zhang, X. Y.; Xia, Z. G.; Du, H. Y. Luminescent properties of LiBaPO4:RE (RE = Eu2+, Tb3+, Sm3+) phosphors for white light-emitting diodes. J. Appl. Phys. 2012, 111, 013101. (15) Zhang, S. Y.; Nakai, Y.; Tsuboi, T.; Huang, Y. L.; Seo, H. J. Luminescence and Microstructural Features of Eu-Activated LiBaPO4 Phosphor. Chem. Mater. 2011, 23, 1216−1224. (16) Tang, Y. S.; Hu, S. F.; Lin, C. C.; Bagkar, N. C.; Liu, R. S. Thermally stable luminescence of KSrPO4:Eu2+ phosphor for white light UV light-emitting diodes. Appl. Phys. Lett. 2007, 90, 151108. (17) Im, W. B.; Yoo, H. S.; Vaidyanathan, S.; Kwon, K. H.; Park, H. J.; Kim, Y.; Jeon, D. Y. A novel blue-emitting silica-coated KBaPO4:Eu2+ phosphor under vacuum ultraviolet and ultraviolet excitation,. Mater. Chem. Phys. 2009, 115, 161−164. (18) Zhang, X. G.; Mo, F. W.; Zhou, L. Y.; Gong, M. L. Properties− structure relationship research on LiCaPO4:Eu2+ as blue phosphor for NUV LED application. J. Alloys Compd. 2013, 575, 314−318. (19) Wu, Z. C.; Shi, J. X.; Wang, J.; Gong, M. L.; Su, Q. A novel blueemitting phosphor LiSrPO4:Eu2+ for white LEDs. J. Solid State Chem. 2006, 179, 2356−2360. (20) Qin, C. X.; Huang, Y. L.; Shi, L.; Chen, G. Q.; Qiao, X. B.; Seo, H. J. Thermal stability of luminescence of NaCaPO4:Eu2+ phosphor for white-light-emitting diodes. J. Phys. D: Appl. Phys. 2009, 42, 185105. (21) Zhang, S. Y.; Huang, Y. L.; Nakai, Y.; Tsuboi, T.; Seo, H. J. The Luminescence Characterization and Thermal Stability of Eu2+ IonsDoped NaBaPO4Phosphor. J. Am. Ceram. Soc. 2011, 94, 2987−2992. (22) Dorenbos, P. A review on how lanthanide impurity levels change with chemistry and structure of inorganic compounds. ECS J. Solid State Sci. Technol. 2013, 2, R3001−R3011. (23) Dorenbos, P. Systematic behaviour in trivalent lanthanide charge transfer energies. J. Phys.: Condens. Matter 2003, 15, 8417− 8434.

features. We have adopted the in-chain characteristic length d22 and outside-chain characteristic length d12 to represent the evolution of this chainlike polyhedral configuration of the compositions and further accounted for the changes of the excitation- and emission-bands, that is, the lowest-energy excitation peaks ranging from 470 to 405 nm and emission peaks ranging from 515 to 423 nm. The influence of the chainlike polyhedral geometry on luminescence is from the inductive effect along different directions. The cosubstitutions of K+ and P5+ ions for Ba2+ and Si4+ ions improve the thermal luminescence resistance by increasing the T1/2 from 118 to 222 °C. In order to understand this temperature-dependent luminescence, the HRBE and VRBE schemes have been constructed. These reveal that the energy gap between the 5d energy level of Eu2+ and the bottom of the conduction band is closely related to the thermal quenching of luminescence. This work shows a clear relationship between crystal structure, electronic structure, and luminescence. In addition, the HRBE and VRBE schemes could be used to predict the luminescence of other lanthanides.

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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.7b02431. Main parameters of the experimental values extracted from the luminescence spectroscopy of KxBa1.97−x(Si1−xPx) O4:0.03Eu2+; detailed energy-level data of Ln2+/Ln3+ ions in the HRBE and VRBE diagrams of Ba2SiO4; atomic coordinates, temperature factors, and occupancies for K xBa 1.97−x(Si 1−xP x)O 4 :0.03Eu 2+; bond lengths (Å) between cations and their coordination oxygen atoms in KxBa1.97−x(Si1−xPx)O4:0.03Eu2+; observed and calculated XRD profiles of KxBa1.97−x(Si1−xPx)O4:0.03Eu2+ (x = 0, 0.2, 0.4, 0.6, 0.8, and 1, respectively), the difference between them, and the expected positions (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Zhiguo Xia: 0000-0002-9670-3223 Quanlin Liu: 0000-0003-3533-7140 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Nos. 51472028 and 51602019). REFERENCES

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