Article Cite This: Inorg. Chem. 2018, 57, 4146−4154
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Consequence of Optimal Bonding on Disordered Structure and Improved Luminescence Properties in T‑Phase (Ba,Ca)2SiO4:Eu2+ Phosphor 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
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ABSTRACT: T-phase (Ba,Ca)2SiO4:Eu2+, showing excellent luminescent thermal stability, has a positionally disordered structure with the splitting of five atom sites, but until now the reason has remained unclear. Herein, we investigate the coordination environments of each cation site in detail to understand the origins of the atom site splitting. We find that the three cation sites in the split-atom-site model are optimally bonded with ligand O atoms compared to the unsplit-atom-site model. This atom site splitting results in larger room and smaller room for each splitting cation site, which just accommodates larger Ba2+ ions and smaller Ca2+ ions, respectively, leading to more rigid structure. Based on the X-ray diffraction data refinement, the boundary of the T-phase for (Ba1−xCax)2SiO4 is redetermined. The Eu2+-doped T-phase (Ba,Ca)2SiO4 phosphors show excellent luminescent thermal stability, which can be attributed to optimal bonding and more rigid structure with atom site splitting. These results indicate that T-phase (Ba,Ca)2SiO4:Eu2+ phosphors have promise for practical applications.
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configurational coordinate diagram17 and the thermal-stimulated ionization of the 5d electron to the conduction band.18 The former is associated with the structural rigidity and the Stokes shift,19,20 and the latter is mainly determined by the energy difference between the lowest 5d excitation level and the conduction-band bottom.11,18 The alkaline earth silicate-based phosphor with general formula M2SiO4 (M = Ca, Sr, Ba) has been studied extensively. The (Sr,Ba)2SiO4:Eu2+ phosphor is commercially applied in pcwLED. To overcome the deficiencies in Ba2SiO4:Eu2+ such as poor thermal stability, many efforts has been dedicated, and the most effective strategy is the solid solution use as a host. Denault et al. reported that the intermediate composition (46% Sr) in the (Ba,Sr)2SiO4:Eu2+ system possesses the best thermal stability due to the optimized bond-valence and the resulted rigid crystal structure.21 Later, we introduced the cation ordering concept for the (Ba,Sr)2SiO4:Eu2+ solid solutions analysis to further explain the enhanced thermal stability of the intermediate composition.19 The cosubstitution of K+ and P5+ ions for Ba2+ and Si4+ ions in KxBa1.97−x(Si1−xPx)O4:0.03Eu2+ solid solutions can also improve the thermal luminescence stability by increasing the energy difference between the lowest 5d excitation level and the conduction-band bottom.11 These
INTRODUCTION In recent years, phosphor converted white light LED (pcwLED) has received an astonishing amount of attention owing to its environmental friendliness, energy savings, long serving time, and chemical stability.1−5 Because phosphors play a key role in white LED devices, it is urgent to develop new phosphor materials with higher performance. To be a prospect for practical applications, a phosphor should have the following characteristics: suitable excitation wavelength that can well match to the blue or near-UV LED chips; suitable emission wavelength; high quantum efficiency; high thermal and chemical stability.4,6,7 Among these, the high thermal quenching stability is very important for the phosphor application, since, in the working state, the temperature of the phosphor layer can reach above 150 °C. The excitation/ emission properties of Eu2+/Ce3+-doped phosphors have been studied quite deeply and extensively,8,9 which can be tuned via the cation/anion substitution10−12 and energy transfer among multiple rare earth ions.13−15 For Eu2+-doped silicate glass, emission can be tuned by changing the excitation wavelength and adjusting the concentration of Eu2+ ions.16 The research on the mechanism of these luminescence modulations is quite abundant, but unfortunately, the research on the thermal quenching behavior is relatively rare. There are two mechanisms commonly considered to explain the thermal quenching of d-f emissions for Ce3+/Eu2+ ions: the large displacement between the ground and excited states in the © 2018 American Chemical Society
Received: February 8, 2018 Published: March 15, 2018 4146
DOI: 10.1021/acs.inorgchem.8b00362 Inorg. Chem. 2018, 57, 4146−4154
Inorganic Chemistry
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RESULTS AND DISCUSSION Local Structure and Bonding Analysis. Fukuda et al. first conducted a detailed study of the crystal structure of the Tphase.24 Figure 1 illustrates the representative crystal structure
results inspired us to explore the thermal quenching properties on the intermediate compositions in the (Ba,Ca)2SiO4 system. In the (Ba,Ca)2SiO4 system, there exist six phases from the Ba-end to the Ca-end, which were denoted as Ba2SiO4, Tphase, X-phase, and α-, β-, and γ-Ca2SiO4, respectively.22 Among them, the Eu2+-doped T-phase has attracted much attention because of the unique crystal structure and luminescent thermal quenching features.23 Fukuda et al.24 found that its crystal structure is disordered. There are five distinct cation sites in the structure, denoted as M1, M2, M3, M4, and M5 which are occupied by Ba2+ and/or Ca2+ ions with different occupancies. Si atoms occupy two crystallographic sites, and O atoms take five sites. It is interesting to note that M1, M2, O1, O2, and O3 sites possess splitting phenomenon.24 According to the research of Park et al. for (Ba1.20Ca0.7Eu0.1)SiO4, the thermal quenching temperature T1/2 (the temperature when the integrated emission intensity drops to half the initial intensity) can reach 175 °C, which is higher than that of the commercial (Ba,Sr)2SiO4 phosphor.23 The luminescence properties of T-phase (Ba,Ca)2SiO4:Eu2+, Mn2+ phosphors were also investigated.25−27 Since many atom sites in the structure of the T-phase are splitting, up to now the reason for this splitting is unclear. Therefore, the source of the excellent thermal quenching property of the Eu2+-doped T-phase phosphor is not clear. In this study, based on the crystallographic data of the representative T-phase (Ba0.65Ca0.35)2SiO4 reported by Fukuda et al., we carefully evaluate the coordination environments of all cation sites to understand the origin of the atom site splitting. By analyzing the cation site coordination environments by splitand unsplit-atom-site models, we find that the three cation sites in the split-atom-site structure are optimally bonded with ligand O atoms, leading to a more rigid structure. A series of Eu2+doped T-phase (Ba,Ca)2SiO4 phosphors were synthesized, and the compounds showed better luminescent thermal stability compared to end-member Ba2SiO4 and Ca2SiO4 crystals. This can be attributed to a consequence of atom site splitting that results in optimal bonding and a more rigid structure.
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Article
Figure 1. Crystal structures of T-phase (Ba,Ca)2SiO4 for the spiltatom-site model (a) and unsplit-atom-site models (b).
of the T-phase in (Ba,Ca)2SiO4 solid solutions. The crystal structure is trigonal with space group P3̅m1. There are five crystallographic sites for cations Ba and Ca, denoted as M1, M2, M3, M4, and M5. For composition (Ba0.65Ca0.35)2SiO4, one can refer to the detailed site occupancies for the five cation sites in ref 24. The M1−M4 sites are occupied by Ba2+/Ca2+ ions. On the contrary, the M5 site is solely occupied by Ba2+ ions. Si atoms take two sites, and the SiO4 tetrahedra are isolated without sharing any O atoms. O atoms occupy four sites, denoted as O1, O2, O3, and O4. It is worth noting that there exists the splitting of crystallographic sites in the crystal structure, not only for the cation sites, but also for the anion sites. As depicted in Figure 1a, the M1 site splits from Wyckoff position 2d into the 6i position. One M2 splits from 1a into six 6g sites. M3, M4, M5, and two Si sites do not split. One O1 site (6i) splits into two 12j sites. One O2 site (2d) splits into three sites of the 6i Wyckoff position. One O3 site splits into two sites with Wyckoff positions unchanged, which is 6i, but with different atomic fractional coordinates, denoted as O3a and O3b, respectively. O4 site does not split. For the split-atom-site model, the atom occupancies of M1, M2, O1, O2, O3a, and O3b are correspondingly adjusted to be 1/3, 1/6, 1/2, 1/3, 1/2, and 1/2, respectively, due to atom site splitting. On the basis of the atomic coordinates given by Fukuda et al., we can deduce the atomic fractional coordinates for the unsplit-atom-site model. For comparison, we list both of them in Table 1. Figure 1 depicts both the crystal structure of the real T-phase (Ba,Ca)2SiO4 and the deduced unsplit-atom-site structure. Because many sites in the crystal structure of the T-phase have splitting and occupancy for split-sites decreases correspondingly, it is very difficult to determine the coordination polyhedron and coordination number. To analyze local structure, we depict two-dimensional maps parallel to (001) at different z values, as shown in Figure 2. Figure 2a depicts the two-dimensional map at z = 0 containing M2 sites, which clearly shows that M2 sites are split into six positions, displacing from the 3-fold rotoinversion axis, to form hexatomic-rings. Each of the split positions is on the 2-fold rotation axes, which are parallel to [100] and [110]. Figure 2b shows the two-dimensional map at z = 0.16 including M1 and O1 sites. The M1 sites are split into three positions, displacing from the 3-fold rotoinversion axis. The O1 site departs from the
EXPERIMENTAL SECTION
Synthesis. Eu2+-doped T-phase (Ba,Ca)2SiO4 silicates with different Ba/Ca ratios were synthesized via the conventional high temperature solid state reaction method. The raw materials are highpurity BaCO3 (Sinopharm, 99.9%), CaCO3 (Sinopharm, 99.9%), SiO2 (Sinopharm, 99.9%), and Eu2O3 (Aladdin, 99.99%). The powders were weighed in a stoichiometric ratio and then transferred into an agate mortar. After grinding with absolute ethyl alcohol for half an hour, the mixture was transferred to the alumina crucible and then sintered in the high temperature tube furnace at 1420 °C for 4 h. A weak reductive atmosphere (95% N2 + 5% H2) was applied during the sintering process. The samples were cooled naturally to room temperature and ground to a powder again for the following measurements. Characterization. The powder X-ray diffraction (XRD) patterns were recorded on a D8 Advance diffractometer (Bruker Corporation, Germany) with working conditions 40 kV and 40 mA. The testing range is 2θ = 10−60° with testing rate 8°/min. The XRD data for Rietveld refinements were collected in the range of 2θ = 10−120°. The Rietveld refinement was performed by the Fullprof program.28 The room-temperature photoluminescence (PL) and photoluminescence excitation (PLE) spectra and temperature-dependent luminescence properties were measured with a FLS920 fluorescence spectrophotometer (Edinburgh Instruments Ltd., U.K.) equipped with a 450 W xenon lamp as the excitation source and a heating accessory. 4147
DOI: 10.1021/acs.inorgchem.8b00362 Inorg. Chem. 2018, 57, 4146−4154
Article
Inorganic Chemistry
Table 1. Atomic Coordinates of T-Phase (Ba,Ca)2SiO4 for the Spilt-Atom-Site Model and the Deduced Unsplit-Atom-Site Model, Respectivelya Split-atom-site model
Unsplit-atom-site model
Sites
Wyckoff position
x
y
z
Sites
Wyckoff position
x
y
z
M1 M2 M3 M4 M5 Si1 Si2 O1 O2 O31 O32 O4
6i 6g 2d 2c 1b 2d 2d 12j 6i 6i 6i 2d
0.654 0.085 1/3 0 0 1/3 1/3 0.145 0.715 0.812 0.828 1/3
−0.645 0 2/3 0 0 2/3 2/3 0.36 0.285 0.188 0.172 2/3
0.160 0 0.344 0.244 1/2 0.119 0.610 0.160 0.496 0.336 0.370 0.006
M1 M2 M3 M4 M5 Si1 Si2 O1 O2 O3 O4
2d 1a 2d 2c 1b 2d 2d 6i 2d 6i 2d
2/3 0 1/3 0 0 1/3 1/3 0.180 2/3 0.82 1/3
−2/3 0 2/3 0 0 2/3 2/3 0.360 1/3 0.18 2/3
0.160 0 0.344 0.244 1/2 0.119 0.610 0.160 0.496 0.353 0.060
a
For the split-atom-site model, the atom occupancies of M1, M2, O1, O2, O3a, and O3b are 1/3, 1/6, 1/2, 1/3, 1/2, and 1/2, respectively, due to atom site splitting.
peaks further indicates that the positionally disordered splitatom model is preferred. The refined atomic coordinates and occupancies for the split-atom-site model are listed in Table S1. Next, we address the reasons for atomic position splitting. We have conducted a careful in-depth investigation of the crystal structure of the T-phase (Ba0.65Ca0.35)2SiO4, especially the coordination environments of the five big cation sites. The refined data split-atom-site model by Fukuda et al. was adopted because, in their work, the three-dimensional electron density distribution (EDD) was determined by the maximum-entropy method (MEM) and a MEM-based pattern fitting method. The bond lengths between the coordinated oxygen atoms and the central cation ions are calculated and listed in Table 2. To investigate the origin of the splitting in selected crystallographic sites, the corresponding bond lengths for the unsplit-atom-site model are calculated, as listed in Table 3. To distinguish which
Figure 2. Two-dimensional map parallel to (001) at (a) z = 0, (b) z = 0.1, (c) z = 0.344, and (d) z = 0.5 with the corresponding bond lengths from the structure of T-phase (Ba,Ca)2SiO4.
mirror plane, which produces doublet atomic splitting. As the splitting phenomenon takes place only on the sites which accommodate Ba2+/Ca2+ ions, it is reasonable to speculate that the splitting is due to the significant difference in ionic radii between Ba2+ and Ca2+ ions, which will be discussed in detail later. Figure 2c gives the two-dimensional map at z = 0.344 including M1, O3a, and O3b sites. Figure 2d plots the twodimensional map at z = 0.5 including M5 and O2 sites. It can be observed intuitively that, compared to the unsplit-atom-site model, the O2 site displaces from the 3-fold rotoinversion axis, splitting into three sites. The T-phase (Ba0.65Ca0.35)2SiO4 sample was synthesized and the Rietveld refinement was performed to obtain crystallographic information. Figure S1a illustrates the XRD pattern for the split-atom-site model. The calculated profile gives good fits to the observed one, which indicates that the structure can be successfully expressed by the split-atom-site model. The atomic coordinates for the unsplit-atom-site model listed in Table 1 were used as starting parameters as well. However, the refined results for the unsplit-atom-site model are very unsatisfactory (Figure S1b). The intensities of the calculated diffraction peaks at 2θ = 21.54°, 30.22°, and 31.07° show relatively enormous differences with those of the corresponding observed diffraction peaks. Generally, the intensity of the diffraction peak is associated with the type, occupancy, and position of the atoms in the unit cell. Thus, the mismatch in the diffraction
Table 2. M1, 2, 3, 4, 5-O Bond Lengths (Å) in the T-Phase (Ba0.65Ca0.35)2SiO4 Crystal Structure for the Split-Atom-Site Model Bond M1 site
Bond length
M1−O11a/O16a M1−O11b/O16b M1−O12a/O15a M1−O12b/O15b M1−O13a/O14a M1−O13b/O14b M2 site M2−O11a/O14a M2−O11b/O14b M2−O12a/O15a M2−O12b/O15b M3 site M3−O1 (×6) M3−O2 (×3) M4 site M4−O1 (×6)
2.566 2.968 2.687 3.087 2.784 3.187
M1−O3a M1−O3a (×2) M1−O3b M1−O3b (×2) M1−O4
2.932 3.024 3.448 3.535 2.439
2.711 2.747 2.962 3.028
M2−O13a/O16a M2−O13b/O16b M2−O41 (×2) M2−O42 (×2)
3.225 3.255 2.906 3.357
3.112 2.399
M3−O3a (×6) M3−O3b (×6)
2.885 2.901
2.188
M4−O3a (×3) M4−O3b (×3)
2.521 2.309
2.839
M5−O3a (×6) M5−O3b (×6)
3.049 2.564
M5 site M5−O2 (×6)
4148
Bond
Bond length
DOI: 10.1021/acs.inorgchem.8b00362 Inorg. Chem. 2018, 57, 4146−4154
Article
Inorganic Chemistry Table 3. Bond Lengths (Å) between the Coordinated Oxygen Atom and the M1, M2, M3, M4, and M5 Cations in the T-Phase (Ba0.65Ca0.35)2SiO4 Crystal Structure for the Unsplit-Atom-Site Model along with the Average Bond Length (Å) and the Coordination Number in the Parentheses Bond M1 site (2.953, M1−O1 M1−O3 M2 site (3.135, M2−O1 M3 site (2.896, M3−O1 M3−O2 M4 site (2.291, M4−O1 M5 site (3.063, M5−O2
Bond length
Bond
Bond length
2.878 3.217
M1−O4
2.435
2.948
M2−O4
3.321
3.105 2.35
M3−O3
2.882
2.179
M4−O3
2.403
3.321
M5−O3
2.805
10)
12) 10)
6) 12)
atoms are bonded together, we adopt the following notations. For example, the multiplicity of the O1 site coordinating to the M1 site is six for the unsplit-atom-site model, and thus, the positions are denoted as O11, O12, O13, O14, O15, and O16. For the split-atom-site model, the six O1 sites displace from the mirror plane with double splitting to produce 12 O1 positions. After splitting, a and b are used for further classification, such as O11a and O11b (circled by dotted lines in Figure 3b and Figure 3d), which are from the split of the same O1 site. As listed in Table 2, the bond length data for M1−O1 can be classified into two groups. One has shorter bond lengths and the other one longer. This can be explained by the cooccupation of the Ba2+ and Ca2+ ions in M1 sites. Due to the significant difference in the ionic radii for Ba2+ and Ca2+ ions, it is more energetically favorable to provide larger space for Ba2+ ions and smaller space for Ca2+ ions. Since the atom occupancies of O1, O2, O3a, and O3b are 1/2, 1/3, 1/2, and 1/2, respectively, due to atom site splitting, they need to be considered in determining the coordination number of the cations. To understand the coordination environments of M1 sites, the M1−O1 bond lengths for the split-atom-site model are compared to the Ba−O and Ca−O bond lengths in some common barium and calcium-based silicates, as listed in Table S2. As mentioned above, the bond length data for M1−O1 can be classified into two groups; the average bond length for one group is 2.679 Å, which approximates that for Ca−O in Ca2SiO4 (2.53 Å), while the average bond length for another group is 3.081 Å, in agreement with that for Ba−O in Ba2SiO4 (2.98 Å). In the unsplit-atom-site model, the bond length for M1−O1 is 2.878 Å, which is too long for Ca atoms. Therefore, the origin of atom site splitting for M1 and O1 is to optimize the bonding between Ca/Ba and O atoms. Next, we want to find the coordination number of the M1 site. There are six O1 atoms coordinated to M1, as depicted at Figure 2b and Figure 3b. From a statistical point of view, if the M1 site is occupied by Ba2+ ion, O1 atoms will preferentially locate at the site resulting in a longer O1−M1 bond length. Meanwhile, if the M1 site is taken up by Ca2+ ion, the O1 atom will stay at the position leading to a shorter O1−M1 bond length. Only one O4 atom (unsplit atom site) is coordinated to M1 with the bond length of 2.439 Å, as shown in Figure 3b. One O3 site split into two sites, O3a and O3b, with 50% occupancy for each site. The
Figure 3. (a)−(g) Coordination environments of M1, M2, M3, M4, and M5 sites and (h) the form of cationic coordination polyhedrons in the crystal structure of the T-phase.
M1−O3a bond lengths are shorter and M1−O3b bond lengths are longer. From a statistical point of view, on one hand, if M1 are occupied by Ca atoms, the bond lengths of M1−O3b are too long (3.448 and 3.535 Å) for the Ca2+−O2− pair to form a valid chemical bond. Therefore, we can only consider the bond of M1−O3a for the Ca2+ ion. Although there are three O3a site atoms bonding to the M1(Ca1) site, the coordination number of M1−O3a only is set to be one or two because of the 50% atom occupancy of each O3a site. Therefore, the total coordination number for the Ca atom in the M1 site is determined to be 8 or 9. On the other hand, for Ba2+ ion in the M1 site, we can consider both O3a and O3b bonding with the M1 (Ba1) site, and so, the coordination number is 10. The detailed coordination environment is summarized in Table 4. The M2 site is coordinated by O1 and O4 atoms. As depicted in Figure 3c, M2 and O1 sites are split into six and two equivalent positions, respectively, while there is no splitting in the O4 site. The split of the M2 site can also be explained by the co-occupation of Ba2+ and Ca2+ ions. As mentioned above, the six splitting M2 sites are on the 2-fold rotation axes parallel to [100] or [110], which are denoted as the dashed straight line in Figure 3c. As a representation, the M21 atom is selected to investigate the coordination environment of the M2 site. The analysis process is consistent with that for the M1 site. After the excess bonds and atoms have been stripped away, the coordination configuration of the M2 site shown in Figure 3c can be transformed to the configuration shown in Figure 3d. Next, we address the cause of M2 position splitting. For the unsplit-atom-site model, the coordination number for M2 sites 4149
DOI: 10.1021/acs.inorgchem.8b00362 Inorg. Chem. 2018, 57, 4146−4154
Article
Inorganic Chemistry
in Table 2 and Table 3, respectively. As can be seen from the tables, for those sites that can accommodate both Ba2+ and Ca2+ ions (M1 and M4), the O3 site splits into O3a and O3b sites to make O3a closer to M1 and M4 sites (2.932 and 2.309 Å) compared to the unsplit O3 site (3.217 and 2.403 Å). For the split-atom-site model, the M1−O3a bond length (2.932 and 3.024 Å) is much shorter than the M1−O3b bond length (3.448 and 3.535 Å). For the unsplit-atom-site model, the M1− O3 bond length is calculated to be 3.217 Å, that is too large to stably accommodate Ca2+ ions. Therefore, it can be concluded that the split of the O3 site could strengthen the bonds with Ba2+/Ca2+ ions in different cation sites. As mentioned above, the M3, M4, and M5 sites do not split and the point symmetry of the M3, M4, and M5 sites is 2d, 2c, and 1b, respectively, which indicates that they are located on the rotation triad axis and mirror plane. So, the bond lengths between M3, M4, M5, and O atoms in the equivalent Wyckoff sites are the same. However, due to the split of the O1, O2, and O3 sites, the bonding between them is also optimized for the split-atom-site model. The M3 site is almost occupied by Ba2+ ions, and it is coordinated by three O1 atoms, one O2 atom, three O3a atoms, and three O3b atoms (Figure 3e and Table 2). The coordination number for the M3 site is 10 with an average bond length of 2.909 Å. The M4 site is almost occupied by Ca2+ ions, and it is coordinated by three O1 atoms and three O3a/O3b atoms. The coordination number for the M4 site is 6, and the average bond length is 2.302 Å. The M5 site is solely taken up by Ba2+ ions, and it is coordinated by two O2 atoms, three O3a atoms, and three O3b atoms. The coordination number is 8 and the average bond length is 2.815 Å. Table 4 and Figure 4 summarize the coordination environments of the five cation sites along with the corresponding
Table 4. Summary of the M1, 2, 3, 4, and 5-O Bond Lengths (Å) in the Coordination Polyhedrons along with the Corresponding Average Bond Lengths (Å) and Coordination Numbers for the Split-Atom-Site Model in the Parentheses for the T-Phase (Ba0.65Ca0.35)2SiO4 Bond
Count
Bond length
M1 (Ba) site (3.135, 10) M1−O11b/O14b 2
2.968
M1−O12b/O15b
2
3.087
M1−O13b/O16b
2
3.187
M1−O3b 2 M1−O3b 1 M1−O4 1 M2 (Ba) site (3.059, 10) M2−O11b/ 2 O4b M2−O12b/ 2 O5b M2−O13b/ 2 O6b M2−O41 2 M2−O42 2 M3 site (2.909, 10) M3−O1 3 M3−O2 1 M3−O3a/O3b 6 M5 site (2.815, 8) M5−O2 2 M5−O3a/O3b 6
3.448 3.535 2.439 2.747 3.028 3.255 2.906 3.357 3.112 2.399 2.893
Bond
Count
Bond length
M1 (Ca) site (2.701, 8 or 9) M1−O11a/ 2 2.566 O14a M1−O12a/ 2 2.687 O15a M1−O13a/ 2 2.784 O16a M1−O3a 1 2.932 M1−O3a 1/2 3.024 M1−O4 1 2.439 M2 (Ca) site (2.951, 8) M2−O11a/ 2 2.711 O14a M2−O12a/ 2 2.962 O15a M2−O13a/ 2 3.225 O16a M2−O41 2 2.906 M4 site (2.302, 6) M4−O1 3 M4−O3a/O3b 3
2.188 2.415
2.839 2.807
is 12 with the average bond length of 3.135 Å, as shown in Table 3. This space is too large to stably accommodate Ca2+ ion. After the splitting of the M2 site, the coordination number of the M2 site is optimized to be 10 with the average bond length of 3.059 Å for Ba2+ ion and 8 with the average bond length of 2.951 Å for Ca2+ ion. Compared with the Ca−O bond length in Ca2SiO4 (2.53 Å), the average M2−O bond length is much longer. This can be explained by the fact that the Ca2+ occupancy in M2 is only about 15%. Therefore, the reason for M2 site splitting is also to optimize the bonding between Ca/ Ba and O atoms. As mentioned above, O1 site splitting is due to optimal bonding between O1 and M1 sites. Next, we will explore why O2 sites split. The M5 site does not split and it is solely occupied by Ba2+ ions. The M5−O2 bond lengths for the splitatom-site model and the unsplit-atom-site model are calculated and listed in Table 2 and Table 3, respectively. For the unsplitatom-site model, the M5−O2 bond length is 3.321 Å. But in the split-atom-site model, the M5−O2 bond length shrinks to 2.839 Å due to the O2 site splitting (see in Figure 2d); that is, for the M5 site, the O2 atom is about 0.5 Å closer to the M5 site. Therefore, the O2 atom site splitting also arises from optimizing the bonding between Ba2+ ions in the M5 site and O2 atoms. Correspondingly, in the unsplit-atom-site model, the coordination number of the M5 atom decreases to 8 from 12. Next, we focus on the reason for O3 site splitting. The O3 site split into the O3a site and the O3b site, as listed in Table 2. The O3a and O3b atoms are bonded to M1, M3, M4, and M5 sites. The corresponding bond lengths for the split-atom-site model and the unsplit-atom-site model are calculated and listed
Figure 4. Average bond lengths and the coordination numbers of the different Ca2+/Ba2+ sites for the split-atom-site model (⊗) and unsplitatom-site model (⧫) for T-phase (Ba,Ca)SiO4, respectively.
average bond lengths. As shown in Figure 4, for M3 (almost occupied by Ba2+ ions) and M4 (almost occupied by Ca2+ ions) sites which do not split, the average bond length and coordination number for the spilt-atom-site model are almost the same as those for the unsplit-atom-site model. For the unsplit M5 site, which is solely taken up by Ba2+ ions, the average bond length and coordination number are modified from 3.063 to 2.815 Å and 12 to 8, respectively, due to the split of the O2 site. As mentioned above, due to the co-occupation of Ba2+/ Ca2+ ions, the M1 and M2 sites are split due to the 4150
DOI: 10.1021/acs.inorgchem.8b00362 Inorg. Chem. 2018, 57, 4146−4154
Article
Inorganic Chemistry
lattice parameters with the compositions based on the Vegard’s law.32 As evident from Figure 5, the composition range of Tphase for (Ba1‑xCax)2SiO4 is x = 0.28 - 0.4. When x = 0.20− 0.28, the samples are composed of T-phase and Ba2SiO4 impurity phase. When x = 0.4−0.5, the cell parameter of Tphase is almost constant within the possible error range, indicating that there exists another impurity phase. However, due to the low content, we cannot identify it in the XRD patters, as shown in Figure S2c. Photoluminescence and Thermal Quenching Properties of Eu2+-doped T-phase (Ba,Ca)2SiO4 samples. Figure 6a and 6b display, respectively, the normalized photo-
need for optimal bonding, thus resulting in two sets of coordination environment with larger room for Ba2+ ions and smaller room for Ca2+ ions, respectively. In Figure 4, the length values for the Ba2+ polyhedrons are connected by a solid line and the points related to the Ca2+ polyhedrons are connected by a dashed line. These results differ from the results reported earlier. As mentioned above, most authors take the six-splitting M2 site to be 6-coordinated while M1, M3, M4, and M5 as 10or 12-coordinated.23,25,26 As shown in this work, it is more reasonable to investigate the coordination environments of M1 and M2 sites accounting for the co-occupation of Ba2+ and Ca2+ ions. Because of the difference in the ionic radii, there must be some difference in the requirements of the distance between Ba2+/Ca2+ and oxygen ions to reach optimal bonding. For Eu2+/Ce3+ ions, luminescent behaviors are closely related to their surrounding crystal field environment, i.e., the coordination environment.29−31 Although the lanthanidedoped T-phase phosphor has been reported by many researchers, the detailed dopant preferential cation site has not been elucidated accurately due to the split of M1, M2, O1 and O2 sites. Generally, in the Eu2+ doped system, it is widely taken that the M1, M3, M4 and M5 sites are 10- or 12coordiated to accommodate Eu2+ ions. Table S2 also summarizes the average bond lengths in some Europiumsilicates. The data on Europium-silicate shows that Eu−O bond lengths are 2.87 Å for the 10-coordinated site and 2.70 Å for the 9-coordinated site in Eu2SiO4. These values are much larger than the M4-O bond length (2.291 Å). Thus, Eu2+ ions would favor the larger M1, M2, M3 and M5 sites rather than M4 site when they are doped into the T-phase (Ba1‑xCax)2SiO4 host lattice. The average bond lengths in the silicates listed in Table S2 are all extracted from the Crystallographic Information Files. The Phase Boundary of T-phase (Ba1‑xCax)2SiO4. To define the phase boundaries of T-phase in (Ba,Ca)2SiO4, the samples with different Ba/Ca ratio, (Ba1‑xCax)2SiO4 (x = 0.18− 0.5), were synthesized. The X-ray diffraction (XRD) patterns of the as-prepared samples are shown in Figure S2a ∼ S2c. All the major diffraction peaks of the (Ba1‑xCax)2SiO4 (x = 0.18−0.5) samples can be indexed to the corresponding standard data for the trigonal T-phase Ba1.31Ca0.69SiO4 (JCPDS#36−1449). However, in the range of x = 0.18 to 0.26, Ba2SiO4 can be detected as impurity phase as well. The Rietveld refinements have been performed using the FullProf program to obtain the detailed crystallographic parameters. The cell parameter variation with composition x is depicted in Figure 5. The phase boundary can be accurately determined by variance of
Figure 6. (a) Normalized PLE spectra monitored at the optimal emission wavelength and (b) normalized PL spectra at 340 nm excitation of T-phase (Ba0.99‑yCayEu0.01)2SiO4 samples as varying y values.
luminescence excitation (PLE) and Photoluminescence (PL) spectra of Eu2+ doped T-phase (Ba0.99‑yCayEu0.01)2SiO4 samples, as a function of Ca2+ doping content (y). The PLE spectra monitored at the optimal emission wavelength (460−500 nm) exhibit typical broad band of Eu2+ ions from 250 to 450 nm, which matches well with the n-UV chip for w-LED applications, indicating the potential application prospects. The profiles of the excitation spectrum for y = 0.32 to y = 0.40 are very similar while that for y = 0.28 is slightly red-shifted. Supposedly, the effect appeared because this composition is on the boundary of T-phase. On the other hand, the red-shift of the excitation band for y = 0.28 may be explained by a larger centroid shift for the electronegativity of Ba2+ ions (0.89) is smaller than that of Ca2+ ions (1.00).33−35 The PL spectra shown in Figure 6b are consist with asymmetric broad bands and located in the blue to bluish-green
Figure 5. Refined cell parameters of (Ba1−xCax)2SiO4 (x = 0.18−0.5) samples with varying x values. 4151
DOI: 10.1021/acs.inorgchem.8b00362 Inorg. Chem. 2018, 57, 4146−4154
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rigid structure is formed due to a rational distribution of the Ba/Sr/Eu cations over the crystallographic sites and the optimized bonding appearing between them and the ligands, leading to the enhanced thermal stability, as shown for the (Ba,Sr)2SiO4:Eu2+ system. Within the T-phase region (y = 0.28−0.40), the thermal stability of the samples decreased with Ca2+ contents increase. This can be explained by the increased Stokes-shift, as mentioned above, due to the Ca2+ ions substitution for Ba2+ ions (see in Table S3). Generally, if Eu2+ ion occupies a looser site, the emission will have a larger Stokes-shift and worse luminescent thermal stability.
region under the excitation of 340 nm, which are ascribed to the electric dipole-allowed transition of Eu2+ ions from the lowest 5d excited state to the 4f ground state.9 The asymmetric bands may indicate the existence of the potential multiple emission centers of Eu2+ ions, as mentioned above.27 With increasing y values, the emission peaks gradually shift to longer wavelength that is accompanied by considerable broadening in full width at half-maximum (fwhm). The red-shift and the broadening of the emission band can be mainly attributed to the increasing Stokes-shift (see Table S3). With increasing y values, more Ca2+ ions incorporated into the host lattice give preference to the smaller sites. In this case, Eu2+ ions have to occupy larger sites which are normally occupied by Ba2+ ions. This looser coordination environment will lead to the increase of the Stokes-shift of Eu2+ emission. In recent years, the widespread application of high-powered LEDs has presented new challenges to the thermal stability of phosphor powder. Although Eu2+-doped barium silicates possess excellent luminescence properties, they may suffer poor thermal stability as well. Many efforts have been dedicated to improve this property.11,19,21 Figure 7 shows the temperature
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CONCLUSIONS In summary, a series of T-phase (Ba,Ca)2SiO4 phosphors were synthesized via the conventional high temperature solid state reaction. By analyzing the coordination environments of each cation site in detail for split-atom-site and unsplit-atom-site models, we find that the origin of the splitting on M1, M2, O1, O2, and O3 sites is the need to optimize the bonding between Ba/Ca and O atoms. The two cation sites in the split-atom-site model are optimally bonded with ligand O atoms compared to the unsplit-atom-site model. Due to atom site splitting, there exist larger room and smaller room at M1 and M2 splitting cation sites, just appropriate for large Ba2+ ions and smaller Ca2+ ions, respectively, resulting in a more rigid structure. The T-phase range for (Ba1‑xCax)2SiO4 is reverified as x = 0.28−0.4. The PLE spectra of T-phase (Ba,Ca)2SiO4:Eu2+ present typical broad-band excitation characteristics of Eu2+, covering the range 250−450 nm, and a broad emission band with tunable peak wavelength from 460 to 500 nm. The T-phase (Ba,Ca)2SiO4:Eu2+ phosphors show better luminescent thermal stability as compared to end-member Ba2SiO4:Eu2+ and Ca2SiO4:Eu2+ and the best BaSrSiO4:Eu2+ phosphor in the (Ba,Sr)2SiO4 system. This can be ascribed to the fact that the bonds are optimized and the structure becomes more rigid with atom site splitting. For the compositions in the T-phase region, the luminescent thermal stability decreases with the Ca2+ contents increase. These results indicate that T-phase (Ba,Ca)2SiO4:Eu2+ phosphors have potential practical applications.
Figure 7. Dependence of the integrated intensity of (Ba0.99‑yCayEu0.01)SiO4 phosphors with increasing y values (the inset is the T1/2 vs y) on temperature.
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ASSOCIATED CONTENT
S Supporting Information *
dependence of the integrated intensities of (Ba0.99−yCayEu0.01)SiO4 phosphors under 365 nm excitation. The inset is T1/2 (the temperature where the integrated intensity of the sample drops to half the initial intensity) against Ca2+ contents (y). It is notable that the thermal stability of all T-phase (Ba0.99‑−CayEu0.01)SiO4 phosphors is superior to that of Ba2SiO4:Eu2+ (y = 0) and Ca2SiO4:Eu2+ (y = 0.99). Their integrated luminescence intensities maintain about 90% of the initial intensities while that of Ba2SiO4 starts to drop dramatically when the temperature is elevated to 375 K. It is inspiring to find in Figure 7 that the thermal quenching stabilities of samples with y = 0.28 and y = 0.32 are even superior to that of the BaSrSiO4:Eu2+ sample, which possesses the best thermal quenching stability in the (Ba,Sr)2SiO4:Eu2+ system.19,21 For the intermediate composition BaSrSiO4:Eu2+, smaller Sr2+ ions preferentially occupy the smaller M2 site (9coordinated) while larger Ba2+ ions prefer to occupy the larger M1 site (10-coordinated). Similarly, for T-phase (Ba0.99‑yCayEu0.01)SiO4 phosphors, Ba2+ ions preferentially take over larger space while Ca2+ ions smaller space. Thus, a more
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00362. Figure S1: Rietveld refinement results of the T-phase (Ba0.65Ca0.35)2SiO4 sample for the split-atom-site model and unsplit-atom-site model. Table S1: Refined structural parameters of the T-phase (Ba0.65Ca0.35)2SiO4 sample for the split-atom-site model. Table S2: Average Ba/Ca/Eu/O bond lengths (Å) with different coordination numbers in some common barium/calcium/europium-silicate compounds. Figure S2: XRD patterns of (Ba1−xCax)2SiO4 phosphors with varying compositions. Table S3: Stokes-shift (cm−1) and fwhm of the emission bands (cm−1) of T-phase (Ba0.99‑yCayEu0.01)2SiO4 samples with varying y values. (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. 4152
DOI: 10.1021/acs.inorgchem.8b00362 Inorg. Chem. 2018, 57, 4146−4154
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Inorganic Chemistry *E-mail:
[email protected].
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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 (No. 51472028 and 51602019) and Fundamental Research Funds for the Central Universities (FRF-TP-17-005A2).
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