Effects of Si Codoping on Optical Properties of Ce-Doped

Subscriber access provided by UNIV OF PITTSBURGH

Article

Effects of Si Codoping on Optical Properties of Ce-doped CaBaPO : Insights from First-Principles Calculations 6

4

17

Lixin Ning, Huang Xiaoxiao, Jiancheng Sun, Shizhong Huang, Mingyue Chen, Zhiguo Xia, and Yucheng Huang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11659 • Publication Date (Web): 29 Jan 2016 Downloaded from http://pubs.acs.org on February 2, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Effects of Si Codoping on Optical Properties of Ce-doped Ca6BaP4O17: Insights from First-Principles Calculations Lixin Ning,*,† Xiaoxiao Huang,† Jiancheng Sun,† Shizhong Huang,† Mingyue Chen‡, Zhiguo Xia,‡,* Yucheng Huang†

†

Center for Nano Science and Technology, Department of Physics, Anhui Normal University,

Wuhu, Anhui 241000, China ‡

School of Materials Sciences and Engineering, University of Science and Technology of

Beijing, Beijing 100083, China

*E-mail: [email protected]; [email protected]

*

To whom correspondence should be addressed. 1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract

It was recently reported that Ce-doped Ca6BaP4O17 displayed blue-green emission under excitation in the near-ultraviolet (UV) region and luminescence intensities can be greatly improved by codoping with Si. Here, a combination of hybrid density functional theory (DFT) and wave function-based CASSCF/CASPT2 calculations at the spin-orbit level has been performed on geometric and electronic structures of the material to gain insights into effects of Si codoping on its optical properties. It is found that the observed luminescence arises from 4f−5d transitions of Ce3+ occupying the two crystallograhically distinct Ca1 and Ca2 sites of the host compound with comparable probabilities, with the energy of the lowest 4f → 5d transition of CeCa1 being slightly higher than that of Ceca2. The codopant Si prefers to substitute for the nearest-neighbor (NN) P1 atom over the NN P2 atom around Ce3+, and this preference induces a blueshift of the lowest-energy 4f → 5d transition, consistent with experimental observations. The blueshift originates from a reduction in 5d crystal field splitting of Ce3+ associated mainly with electronic effects of the NN SiP1 substitution, while the contribution from the change in 5d centroid energy is negligible. On the basis of calculated results, the energy-level diagram for the 4f ground states and the lowest 5d states of all trivalent and divalent lanthanide ions on the Ca2+ sites of Ca6BaP4O17 is constructed and discussed in connection with experimental findings.

Keywords: Ca6BaP4O17 crystals; Ce3+ ions; Si codoping effects; 4f → 5d transitions; First-principles calculations

2


Page 2 of 25

Page 3 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. INTRODUCTION Phosphate luminescent materials activated by lanthanide ions have been extensively studied for their potential applications in lighting, flat panel displays, and bioimaging systems, due to the inherent characteristics of the host such as the large band gap and high thermal and chemical stability.1−9 Among these materials, Ce-doped phosphate compounds have attracted considerable attention owing to the electric-dipole allowed 5d → 4f transitions of Ce3+ with a radiative lifetime of several tens of nanoseconds. Recently, Komuro and coworkers reported the discovery of a new phosphate compound, Ca6BaP4O17, which, when doped with Ce3+, exhibited luminescence properties appealing for applications in white light-emitting diodes (LEDs).10,11 This Ce-doped compound displayed a strong emission in the blue-green spectral region upon excitation at around 400 nm,11,12 which matches well with the output of near-ultraviolet (UV) LED chips. This is remarkable in that the lowest-energy excitation bands observed for Ce-doped phosphates are mostly located in the UV spectral region at wavelengths below 350 nm.13 Ca6BaP4O17 adopts a monoclinic structure with space group C2/m (no. 12), containing two crytallographically distinct Ca sites (see Figure 1).10 The Ca1 site (Cs symmetry) is coordinated by eight oxygen atoms with the Ca−O distances ranging from 2.361 to 2.760 Å. The Ca2 site (C1 symmetry) is coordinated by seven oxygen atoms with the Ca−O distances in the range of 2.303−2.548 Å, and an additional long-ranged O atom at distance of 3.075 Å. The size of the Ca1 site is slightly larger than that of the Ca2 site, with the average Ca−O distances being 2.510 and 2.503 Å, respectively. From consideration of relative sizes of the dopant and host cations, the blue-green emission observed in Ce-doped Ca6BaP4O17 was ascribed to 5d → 4f transitions of Ce3+ occupying the two Ca2+ sites. It was further observed that codoping with Si4+ that was expected to occupy the P5+ site in Ca6BaP4O17 greatly 3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. Schematic representations of the atomic structure of the Ca6BaP4O17 crystal and the local coordinations of the Ca1 and Ca2 sites with experimental values (in Å)10 of the Ca−O bond lengths indicated. The two nonequivalent crystallographic sites of the P atoms are labelled as P1 and P2, and the O atoms that do not belong to any PO4 tetrahedra are denoted as Oiso.

enhanced the emission intensity of Ce3+.11 This can be understood as due to the decrease in the nonradiative transition rate of the emitting 5d state, resulting from reduction in energy migration along Ce3+ ions to killer sites as induced by charge imbalance of Ce3+ substitution on the Ca2+ sites. Interestingly, a blue shift of the lowest-energy Ce3+ 4f → 5d excitation band was also observed upon Si4+ codoping. By decomposing this excitation band into two Gaussian components which were assumed to arise from Ce3+ at the Ca1 and Ca2 sites and comparing their relative intensities in the cases of single doping and codoping, it was tentatively proposed that Si4+ codoping stimulated a preference of Ce3+ occupation on the Ca2 4


Page 4 of 25

Page 5 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


site over the Ca1 site.11 We notice however that, due to the strong interaction of 5d electron with the local crystalline surroundings, the excitation spectra of Ce3+ consist of zero-phonon transitions followed by a rich vibronic structure, which is generally difficult to analyze experimentally, especially when the local environment is complex. For such situations, first-principles calculations can be quite useful, because the applied electronic structure methods are able to provide the local structure of Ce3+, the energy levels involved in 4f−5d transitions, and also the relationship between them, as demonstrated in recent studies on different Ce-doped complex systems. 14−16 To elucidate luminescence properties of Ce, Si-codoped Ca6BaP4O17, first-principles calculations are carried out on geometric and electronic structures of the material. Hybrid DFT calculations within the supercell model are first conducted to obtain the local structures of Ce3+, and the relative preference of Ce substitution on the Ca1 and Ca2 sites is evaluated from the calculated DFT total energies. Wave function-based CASSCF/CASPT2 calculations at the spin-obit level are then performed on Ce-centered embedded clusters to derive the 4f1 and 5d1 energy levels of Ce3+. On the basis of calculated results, spectroscopic data as obtained experimentally are interpreted correctly, and new insights are gained into effects of Si4+ codoping on luminescence properties of Ce-doped Ca6BaP4O17. Finally, the energy-level diagram for the 4f ground states and the lowest 5d states of all trivalent (Ln3+) and divalent (Ln2+) lanthanide ions relative to the valence and conduction bands of the host is constructed and discussed.

2. COMPUTATIONAL METHODOLOGY The Ce, Si-codoped Ca6BaP4O17 was modeled with a 1 × 2 × 1 supercell containing 112 atoms, in which one of 24 Ca atoms was substituted by a Ce atom and one of 16 P atoms by a Si atom for charge compensation. The corresponding chemical formula is Ca6−xCexBaP4−xSixO17 (x = 0.25). For Ce at the Ca1 site, five symmetrically inequivalent CeCa1−SiP double 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

substitutions were considered, including four nearest-neighbor (NN) and one distant Ca1−P combinations within the undoped supercell while, for Ce at the Ca2 site, five NN and one distant CeCa2−SiP double substitutions were investigated. The two distant double substitutions were used to model the cases where Ce3+ is located at the Ca1 or Ca2 site without a nearby charge-compensating defect. The nearest distance between Ce3+ ions in the supercell systems is around 9.34 Å, which is large enough to neglect their mutual influence in the computational study of the localized 4f and 5d states of Ce3+. The lattice parameters and atomic coordinates of the supercells were optimized by periodic DFT calculations using the standard PBE0 hybrid functional that admixes 25% Hartree−Fock (HF) exchange with PBE exchange,17 as implemented in the VASP package.18,19 The Ce (5s25p64f15d16s2), Ca (3s23p64s2), Ba (5s25p66s2), P (3s23p3), Si (3s23p2), and O (2s22p4) were treated as valence electrons, and their interactions with the respective cores were described by the projected augmented wave (PAW) method.20 The geometry optimizations were performed until the total energies and the Hellmann−Feynman forces on the atoms were converged to 10−6 eV and 0.01 eV Å−1, respectively. Due to the large size of the supercells and the high computational cost of hybrid DFT with plane wave basis, only one k-point Г was used to sample the Brillouin zone, with the cutoff energy of the plane wave basis set to 530 eV. On the basis of the optimized geometries for the defective Ca6BaP4O17 supercells, the 4f → 5d transition energies of Ce3+ were computed with a wave function-based embedded cluster approach. The Ce-centered clusters were first constructed, each comprising the central Ce3+ ion and the oxygen ions in the first coordination shell. Their immediate surroundings within a sphere of radius 10.0 Å were represented by 709−719 ab initio model potentials (AIMPs)21 and the remainders of the surroundings were simulated by 79624−80927 point charges at lattice sites, which are generated with Lepetit’s method.22 Wave function-based CASSCF/CASPT2 calculations with the spin-orbit effect were then carried out to obtain the 4f1 and 5d1 energy levels of Ce3+ by using the program MOLCAS.23 In the CASSCF 6


Page 6 of 25

Page 7 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


calculations, a [4f, 5d, 6s] complete active space was adopted, and in the CASPT2 calculations, the dynamic correlation effects of the Ce3+ 5s, 5p, 4f, 5d electrons and the O2− 2s, 2p electrons were considered. More details of the calculations can be found in refs [14,15]. A relativistic effective core potential ([Kr] core) with a (14s10p10d8f3g)/[6s5p6d4f1g] Gaussian valence basis set from ref 24 was used for cerium, and a [He] core effective core potential with a (5s6p1d)/[2s4p1d] valence basis set from ref 25 was used for oxygen. Extra basis sets (11s8p)/[1s1p] and (10s7p)/[1s1p] were respectively added to the P and Si atoms in the second coordination shell of Ce3+, in order to improve the orthogonality of the cluster orbitals with the embedding environments.

3. RESULTS AND DISCUSSION 3.1. Structural and Electronic Properties of Undoped Ca6BaP4O17. The monoclinic structure of undoped Ca6BaP4O17 was first optimized with the DFT-PBE0 method. The calculated (experimental10) values of the lattice parameters are: a = 12.289 (12.303) Å, b = 7.095 (7.105) Å, c = 11.716 (11.716), and β = 134.340 (134.442) deg, with the deviations less than 0.14%. The calculated values of the atomic coordinates are listed in Table S1 of the Supporting Information (SI), showing a good agreement with the experimental values. With the optimized geometry, the calculated total and orbital projected densities of states (DOSs) for the Ca6BaP4O17 unit cell are plotted in Figure 2. The band gap energy (Eg) is predicted to be 6.59 eV, close to the experimental value of 6.63 eV, as estimated from the energy (5.79 eV) of the fundamental absorption onset measured at room temperature10 plus the electron−hole binding energy of the exciton.26 The top of the valence band is dominated by O 2p states with a large dispersion of 9.17 eV due to the existence of seven symmetrically distinct oxygen atoms in Ca6BaP4O17, and in particular the valence band edge is formed by 2p states of the 7



Oiso atoms that do not belong to any PO4 tetrahedra. The bottom of the conduction band is mainly composed of Ca 3d states with small contributions from Ba 5d and O 2p states. The conduction band edge is constituted by a small peak at 6.59 eV above the valence band edge, which is mainly composed of s-character states of Ca and O atoms with smaller contribution from Ca-d states (Figure 2, inset).

60

Total O p Ca d Ba d Ca s O s P s Ba s

DOS (States/eV)

80

DOS (States/eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 25

1.5 1.0 0.5 0.0 6.4

6.5

6.6

6.7

6.8

6.9

Energy (eV)

40

20

Eg = 6.59 eV 0 -10

-8

-6

-4

-2

0

2

4

6

8

10

Energy (eV)

Figure 2. Total and orbital-projected DOS for the Ca6BaP4O17 unit cell calculated with the DFT-PBE0 method and a 3×5×3 k-point grid to sample the Brillouin zone. An enlarged view of the DOS at the bottom of the conduction band is shown in the inset. The Fermi level is set at zero energy.

3.2. Structural and Electronic Properties of Ce, Si-codoped Ca6BaP4O17. For Ce, Si-codoped Ca6BaP4O17 supercells, the optimized lattice parameters are presented in Table S2 of the SI, in which the data for undoped supercell are also listed for comparison. It indicates that the incorporation of CeCa−SiP double substitutions induces small increases (by 0.475−0.713%) of the supercell volume, and slightly distorts the monoclinic phase into triclinic structures, with maximum deviations of 0.041 Å and 0.086º in the length and angle, respectively. Thus, the DFT-PBE0 calculations predict a negligible deformation of the 8


Page 9 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


crystallographic phase when a small amount of Ce is substituted into the Ca sites together with Si located at the P sites for charge compensation, in consistence with experimental observations.11,12 Table 1. Calculated Total Energies for Ce, Si-Codoped Ca6BaP4O17 Supercells. The Ce−Si Distances (d in Å) in the Unrelaxed and Optimized Supercells are also Included. substitution center 1 center 2 center 3 center 4 center 5 center 6 center 7 center 8 center 9 center 10 center 11

CeCa1−SiP1 CeCa1−SiP1 CeCa2−SiP1 CeCa2−SiP1 CeCa2−SiP1 CeCa1−SiP2 CeCa1−SiP2 CeCa2−SiP2 CeCa2−SiP2 CeCa1−SiP1 CeCa2−SiP2

dCe−Si (Å) unrelaxed optimized 3.279 3.178 3.632 3.583 3.288 3.188 3.630 3.593 3.636 3.604 3.083 3.118 3.551 3.620 3.081 3.121 3.543 3.646 9.331 9.346 7.932 7.988

total energy (eV) −1171.164404 −1171.084964 −1171.148478 −1171.110289 −1171.077130 −1170.844687 −1170.710427 −1170.784064 −1170.709716 −1170.872756 −1170.571008

relative total energy (meV) 0 79 16 54 87 320 454 380 455 292 593

Table 1 presents the calculated DFT total energies for Ce, Si-codoped Ca6BaP4O17 supercells. The results show that, no matter whether Ce is substituted on the Ca1 or Ca2 site, the five double substitutions with Si located at a NN P1 site (centers 1−5) have comparable stabilities, with the total energy differences to within 87 meV. The same is true for the four double substitutions with Si located at a NN P2 site (centers 6−9) with the total energy differences to within 135 meV. That is, when the charge-compensating Si is located at a given NN P1 or P2 site, the occupations of Ce on the Ca1 and Ca2 sites are approximately equally favored. On the other hand, the former five CeCa−SiP1 substitutions are predicted to be more stable than the latter four CeCa−SiP2 by 233−455 meV. This means that the charge compensator prefers to occupy the NN P1 site over the NN P2 site, which determines the greater stability of CeCa−SiP1 than CeCa−SiP2 with similar Ce−Si distances, regardless of Ce 9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

substitution on the Ca1 or Ca2 site. This site preference could be correlated with a difference in the coordination environments of oxygen atoms in the P1O4 and P2O4 tetrahedra, which is that the distances from the O atoms in P2O4 to the NN Ba atoms are much shorter than those from the O atoms in P1O4 (see Figure S1 in the SI). As a result, the SiP2 substitution would create more stress in the lattice, making it less stable than the SiP1 substitution. Table 1 also shows that, for a given type of combination, the two single defects prefer to be close to each other, as expected from their opposite effective charges, which is especially manifested by a comparison of the results for centers 10 and 11 with their NN counterparts (for example, centers 1 and 7, respectively).

Figure 3. DFT-optimized local coordination structures of Ce at the Ca1 (center 1) and Ca2 site (center 3). The relaxed (unrelaxed) distances (in Å) from Ce to the nearby O and Si atoms are indicated.

Figure 3 depicts two representative local structures of Ce located at the Ca1 and Ca2 sites (centers 1 and 3 ), with the values of relaxed (and unrelaxed) bond lengths indicated. The local structures of Ce in the other centers are plotted in Figure S2 of the SI. The results show that the CeCa substitutions cause anisotropic distortions of the coordination polyhedra of the dopant sites (M). For CeCa1 (CeCa2) substitutions, the distances from M to the isolated Oiso and 10


Page 10 of 25

Page 11 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the farthest O13 or O12 atoms are decreased by 0.178−0.237 (0.137−0.261) and 0.249−0.400 (0.282−0.428) Å, respectively, whereas the other M−O distances are mostly increased by 0.036−0.153 (0.055−0.177) Å. Notably, the CeCa2 substitutions induce a shortening of the distances to the long-ranged O12 atom, from 3.021 Å to 2.593−2.739 Å, substantiating the 8-fold coordination of Ce at the Ca2 site. Despite these distortions, the average M−O bond lengths are almost unchanged by the substitutions, with decreases of only 0.004−0.011 Å and 0.002−0.010 Å for CeCa1 and CeCa2, respectively. This makes physical sense since, upon CeCa substitution, the coordination polyhedron will expand due to the larger ionic radius (1.143 Å) of Ce3+ than that of Ca2+(1.12 Å) in the 8-fold coordination,27 but meanwhile it will contract due to the additional electrostatic attraction of Ce3+ with O2− compared to that of Ca2+ with O2−. Moreover, when comparing the local structures of CeCa1 and CeCa2, the average M−O bond lengths are very close to each other (within the range of 2.498−2.508 Å), similar to that observed for the local structures of Ca1 and Ca2 in undoped Ca6BaP4O17. This could be correlated with the comparable stabilities of Ce substitutions on the Ca1 and Ca2 sites, when the charge compensator located at a given NN P1 or P2 site. Finally, we mention that the SiP substitutions lead to expansions by 0.345−0.361% of the volume of the coordination tetrahedron and increases by 0.092−0.097 Å of the average bond lengths, consistent with the larger size and smaller positive charge of Si4+ than P5+. Figure 4 shows calculated total and partial DOSs of a representative supercell containing the most stable CeCa1−SiP1 center (center 1). The incorporation of the double defect into the host leads to formation of an occupied 4f state deep inside the band gap (indicated by the dashed line), located at ~1.66 eV above the host VBM (Figure 4a). It corresponds to a lone Ce 4f electron, and hence indicates a 3+ oxidation state of the Ce cation. The unoccupied Ce 4f states and Ce 5d states are resonant in the conduction band, and are broadened due to hybridization with other empty states (Figure 4b).

11



100

Total

(a)

50

Spin up

∆E4f = 1.66 eV

0

DOS (States/eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 25

-50 -100 20

Spin down

(b)

Spin up

Ce d Ce f

10 0 -10

Spin down

-20 -10

-8

-6

-4

-2

0

2

4

6

8

10

Energy (eV)

Figure 4. (a) Total DOS of the supercell containing the most stable CeCa1−SiP1 center (center 1) and (b) partial DOS of the 4f and 5d states of Ce3+ calculated with the DFT-PBE0 method. The occupied Ce3+ 4f state is indicated by the dashed line.

3.3. 4f → 5d transitions of Ce3+ in Ca6BaP4O17. On the basis of DFT-optimized structures for Ce, Si-codoped Ca6BaP4O17 supercells, Ce-centered embedded clusters were constructed with their environments represented by AIMPs and point charges at lattice sites. Wave function-based CASSCF/CASPT2 calculations with the spin−orbit coupling were then conducted to obtain 4f1 and 5d1 energy levels of Ce3+. For clarity, only the results for the five most stable centers (centers 1−5) with a NN SiP1 charge compensator and the two centers (centers 10, 11) with a remote SiP1 or SiP2 compensator are listed in Table 2, while those for the other less stable centers (centers 6−9) with a NN SiP2 compensator are given in Table S3 of the SI. From Table 2, one sees that, without the presence of a NN Si4+, the lowest 5d1 level of Ce3+ at the Ca1 site (center 10) is higher by 847 cm−1 than that at the Ca2 site (center 11). They are both lower than the 5d1 levels of Ce3+ in the five most stable NN CeCa−SiP1 centers (centers 1−5) by average values of 779 and 1428 cm−1 for CeCa1 and CeCa2 types, respectively. 12


Page 13 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Calculated Energy Levels (in cm‒1) of 4f1 and 5d1 Configurations for Ce3+ with a NN SiP1 Compensator (Center 1−5) and a Distant SiP1 or SiP2 Compensator (Center 10,11) in Ca6BaP4O17, Using the CASSCF/CASPT2 Method with Spin-Orbit Coupling. The Values in Parentheses Indicate Unrelaxed Ce−Si Distances. ∆Eced and ∆Ecfs Denote the Centroid Energy and the Crystal-Field Splitting of 5d1 Configuration, Respectively.

center 1

center 2

center 3

center 4

center 5

center 10

center 11

CeCa1−SiP1 CeCa1−SiP1 CeCa2−SiP1 CeCa2−SiP1 CeCa2−SiP1 CeCa1−SiP1 CeCa2−SiP2 (3.279 Å) (3.632 Å) (3.288 Å) (3.630 Å) (3.636 Å) (9.331 Å) (7.932 Å) 0 0 0 0 0 0 0 683 588 836 632 562 611 757 1356 1364 1256 1382 1343 1414 1417 2240 2292 2350 2297 2289 2293 2340 2641 2655 2888 2681 2635 2691 2747 3618 3305 3507 3316 3283 3365 3499 3821 4360 4138 4398 4341 4349 4612

4f1 4f2 4f3 4f4 4f5 4f6 4f7 5d1 5d2 5d3 5d4 5d5

25900 37511 38624 45724 47761

25193 34233 39135 45158 51736

25516 37669 38882 46253 48076

25258 34538 39387 45087 51635

25271 34395 39246 45596 51508

24767 35302 40353 45297 49877

23920 35521 40304 45383 50117

∆Eced ∆Ecfs

39104 21861

39091 26543

39279 22560

39181 26377

39203 26237

39119 25110

39049 26197

This means that the presence of a Si4+ at a NN P1 site leads to a blue shift of the 4f1 → 5d1 transition of Ce3+ at both the Ca1 and Ca2 sites, which is consistent with experimental observations.11 When the Si4+ is located at a NN P2 site (centers 6−9), which is less stable, the calculated Ce3+ 5d1 levels (Table S3 of the SI) are lower by an average value of ~1260 cm−1 than those with the Si4+ at a NN P1 sites. We have also calculated the 4f1 and 5d1 energy levels of Ce3+ substituted at the Ba site, and found that the 5d1 level is higher by 8445−10452 cm−1 than those of Ce3+ at the Ca1 or Ca2 site. A schematic representation of the calculated 4f1 → 5di (i = 1−5) transitions of Ce3+ in the nine NN CeCa−SiP centers (centers 1−9) is given in Figure 5, where the relative intensities 13



Experimental

A

(a)

B Relative intensities (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 25

(b)

Calculated CeCa-SiP1

(c)

Calculated CeCa-SiP2

20

25

30

35

center 1 center 2 center 3 center 4 center 5

center 6 center 7 center 8 center 9

40 3

45

50

-1

Energy (10 cm )

Figure 5. Schematic diagram for the calculated energies and relative oscillator strengths of 4f1 → 5di (i = 1−5) transitions (in order of increasing energy) of Ce3+ associated with NN CeCa−SiP1 double substitutions in Ca6BaP4O17. The experimental excitation spectrum11,12 is also included for comparison.

within each center were calculated using the wave functions and energies at the spin-orbit level. From the figure one sees that the experimentally observed lowest-energy excitation band11,12 (band A in Figure 5a) can be assigned to the 4f1 → 5d1 transitions of Ce3+ in the NN CeCa−SiP1 centers (Figure 5b), with its low-energy tail containing contributions from 4f1 → 5d1 transitions of Ce3+ in the NN CeCa−SiP2 centers (Figure 5c), the numer of which is much less than the former centers due to their lower formation probabilities. It could be expected that increasing Ce3+ and Si4+ doping concentrations would hinder lattice relaxation and thus raise the relative formation probability of the NN CeCa−SiP2 centers with respect to that of the more stable NN CeCa−SiP1 centers, enhancing the relative intensity of the low-energy tail, as observed experimentally.11 The present calculations show that the calculated 4f1 → 5d1 transition energies of Ce3+ substituted at the Ca sites in Ca6BaP4O17 lie in the range of 23893−25900 cm−1, which are 14


Page 15 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


lower than the free ion value (49340 cm−1) by 23440−25447 cm−1. This redshift is quite large in comparison with those (less than 22000 cm−1) for other Ce-doped phosphates.13 Here the redshift is analyzed in terms of the changes in 5d centroid shift and crystal field splitting. The 4f1 → 5d1 transition energy can be decomposed as ∆E(4f1 → 5d1) = ∆Eced(4f1 → 5d1) − ∆Ecfs(5d1),

(1)

where ∆Eced(4f1 → 5d1) is the energy difference between 4f1 ground level and 5d centroid, and ∆Ecfs(5d1) is the energy of 5d1 level relative to that of 5d centroid (i.e., the crystal field stabilization energy). From the data listed in Table 2 and Table S3 of the SI, one sees that the values of ∆Eced(4f1 → 5d1) lie in a narrow range of 38817−39279 cm−1 (average 39091 cm−1), downward shifted from the free ion value (51230 cm−1) by 11951−12413 cm−1 (average 12139 cm−1), which are larger by at least 2000 cm−1 than those for other Ce-doped phosphates.28 On the other hand, the ∆Ecfs(5d1) values are predicted to be in the range of 13204−15206 cm−1, which are larger by at least 2500 cm−1 than those of Ce3+ in other phosphates.28 Therefore, the increases in 5d centroid shift and crystal-field splitting contribute comparably to the large redshift of the first 4f1 → 5d1 transitions of Ce3+ in Ca6BaP4O17 when compared other Ce-doped phosphates. It has been shown above that the charge-compensating Si4+ ion prefers to occupy a NN P1 site around Ce3+, and induces a blue shift of 426−1596 cm−1 of the lowest-energy 4f1 → 5d1 transition when compared to the results obtained with a remote charge-compensating Si4+ ion (Table 2). It is interesting to analyze the structural and/or electronic reasons behind this blue shift. In order to achieve this, we performed energy-level calculations on five additional CeCa centers, for which the atomic coordinates of the embedded clusters and their surroundings are respectively identical to those of the five most stable CeCa centers (centers 1−5), except that the NN Si4+ ions were replaced by P5+ ions. With the results for these additional centers, the electronic effects as caused by the NN SiP1 substitutions can be evaluated by comparing them with the results of the five most stable centers, and the structural effects can be obtained by 15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

comparing them with the two centers (centers 10 and 11) without a NN Si4+ ion. These effects are analyzed in terms of the values of the quantities in eq 1, which are listed in Table S4 of the SI. From the last column (C → A) of the table, we can see that the increases of ∆E(4f1 → 5d1) values (by 426−1596 cm−1) are predominantly caused by the decreases of ∆Ecfs(5d1) values (by 454−1366 cm−1), which are mainly a consequence of the electronic effects (column B → A, 995−1289 cm−1) as induced by the NN SiP1 substitutions. This result is surprising since ∆Ecfs(5d1) is usually considered to be determined by the shape and size of the coordination polyhedron around Ce3+ and hence the structural effects would be expected to play an more important role.28 Similar observations have been made for the effects of La codoping on the 4f1 → 5d1 transition energy of Ce3+ in Y3Al5O12.29 For ∆Eced(4f1 → 5d1), the increases of their values (B → A) as expected from the NN SiP substitutions are largely compensated by the decreases (C → B) caused by structural distortions, resulting in small changes in 5d centroid energy. In Ce-doped Ca6BaP4O17, an Oiso is present in the coordination polyhedra of Ce3+ with the optimized Ce−Oiso distances in the range of 2.236−2.360 Å (Figure 3 and Figure S2 of the SI). According to the analysis based on ligand polarization model,28,30 the Oiso is expected to be more polarizable by the Ce3+ 5d electron and thus contributes more to 5d centroid shift than any other coordinating oxygen that belongs to PO4 tetrahedra. If we assume that the contributions from the coordinating oxygen atoms to the 5d centroid shift are additive,28 we may evaluate them quantitatively by replacing the coordinating O atom successively with a rigid (nonpolarizable) point charge (q = −2e) and then comparing the calculated results with those obtained before the replacement. We chose the most stable center 1 for this evaluation. The results shows that substituting the point charge for Oiso results in an increase of ∆E(4f1 → 5d1) by 4479 cm−1, much larger than the increasing values of 914−2141 cm−1 caused by the other substitutions. Then, by decomposing ∆E(4f1 → 5d1) via eq (1), we found that ∆Eced(4f1 → 5d1) is increased by 3286 cm−1 upon the former substitution, whereas it is only increased 16


Page 16 of 25

Page 17 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


by 226−830 cm−1 for the latter ones, as compared to the value for the reference center 1. Thus the presence of the Oiso in the coordination structure of Ce3+ plays a much more important role in determining 5d centroid shift than any other coordinating O atom and, as a result, contributes significantly to the larger redshift of the first 4f1 → 5d1 transitions of Ce3+ in Ca6BaP4O17 in comparison with other Ce-doped phosphates. 3.4. Energy-level diagram for Ln3+ and Ln2+ in Ca6BaP4O17. It has been shown above that 5d centroid shift (∆Eced) of Ce3+ located at the Ca1 or Ca2 site of Ca6BaP4O17 are predicted to be within 462 cm−1 of one another. Its average value of 12139 cm−1 (1.51 eV) may provide a good approximation to the experimental 5d centroid shift in view of the good agreement between theoretical and experimental 4f → 5d transition energies. By using the relationship, U (6 ) = 5.44 + 2.834 e− ∆Ec / 2.2 eV,18 we first obtain the energy difference between 4f ground states of Eu2+ and Eu3+ with a value of U(6) = 6.84 eV, from which the 4f energy level of Eu3+ can be derived from that of Eu2+ or vice versa. Secondly, spectral measurements of Eu3+ in Ca6BaP4O17 at 8 K (Figure S3 of the SI) reveals a broad band with maximum at 331 nm (ECT = 3.75 eV) in the excitation spectrum, which is assigned to the charge transfer (CT) transition of Eu3+. The value of ECT can be taken as the energy position of 4f7 ground state of Eu2+ above the host VBM.32 With available values of U(6) and ECT, the Eu3+ 4f ground state is placed at 3.09 eV below the host VBM. Given the energy positions of 4f ground states of Eu3+ and Eu2+, we may derive such energy positions for the other Ln3+ and Ln2+ ions located at the same site of the host, by utilizing the characteristic energy differences between Ln3+ or Ln2+ ions which are largely independent with the host material.33 The results are schematically represented by the double zigzag curves 1 and 2 in Figure 6, where the host VBM was set as zero energy and the conduction band minimum (CBM) was drawn at 6.63 eV as estimated experimentally. We note that the Ce3+ 4f ground state is located at 2.15 eV above the host VBM, which is discriminated from the position of the occupied 4f state in Figure 4, because 17



the latter was calculated by hybrid DFT in the single particle approximation. The Ho2+ 4f ground state is predicted to be around 0.48 eV below the host CBM and thus can act as an electron trapping level, consistent with the experimental observation34 that the persistent luminescence of Ca6BaP4O17:Eu2+ was greatly enhanced by the introduction of Ho3+ in the phosphor.

10 8

Conduction band 2+

Ln (5d1) (4)

6 HRBE (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 25

3+

Ln (5d1)

4

(3)

2+

Ln (4f) (2)

2

U(6)=6.84 eV

2.15 eV

0 3+

Ln (4f) (1)

-2 -4

Valence band

-6 La Ce Pr 0 1 2

Nd Pm Sm Eu Gd Tb

3

Dy Ho Er Tm Yb Lu

4 5 6 7 8 9 10 11 12 13 14 3+ Number of 4f electrons in Ln

Figure 6. Host referred binding energies (HRBE) for the 4f ground states and the lowest energy 5d1 states of Ln3+ and Ln2+ ions located at the Ca site of Ca6BaP4O17, with the values of selected energy differences indicated. The energy positions for the lowest 5d1 levels of Ln3+ and Ln2+ ions can be further derived by employing the redshift model by Dorenbos,33 which states that the lowering of the first 4f1 → 5d1 transition energy with respect to that of the free ion is the same for each Ln3+ (or Ln2+) ion situated at the same site of the same host. From Table 2, one sees that the calculated 4f1 → 5d1 transition energies of Ce3+ in the five most stable CeCa1−SiP1 centers are within a narrow range (25193−25900 cm−1) with an average value of 25428 cm−1 (3.15 eV), from which the 5d1 level positions of Ce3+ and the other Ln3+ ions are obtained and plotted as curve 3 of Figure 6. Without the presence of a NN charge-compensating Si4+ ion, the curve would be 18


Page 19 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


displaced downward by about 0.08 and 0.18 eV for Ce3+ located at Ca1 and Ca2 sites, respectively. Then, by using the relationship between the redshifts of the 4f1 → 5d1 transitions of Ln3+ and Ln2+ ions,35 the 5d1 levels positions of Ln2+ can be estimated. Since the substitution of a Ln2+ on the Ca site does not need charge compensation, the 5d1 energy-level data of Ce3+ without a NN Si4+ ion were used. The resultant 5d1 levels for Ln2+ located at the Ca2 site are lower than those at the Ca1 site by 0.06 eV, and the average values over the two sites are depicted as curves 4 of Figure 6. The 5d1 levels of Eu2+ situated at the Ca1 and Ca2 sites are located at about 0.38 and 0.44 eV below the host CBM, enabling the observation of Eu2+ 5d → 4f emission as shown in experiments.10,34

4. CONCLUSIONS First-principles calculations have been performed on structural properties and 4f → 5d transitions of Ce3+ located at the two crytallographically different Ca sites of Ca6BaP4O17 with the charge imbalances compensated by codoping Si4+ at the P sites. The hybrid DFT-PBE0 method with the supercell model was first employed to optimize the local structures of Ce3+, based on which Ce-centered embedded clusters were constructed and wave function-based CASSCF/CASPT2 calculations with the spin-orbit coupling were then carried out to obtain the energies of 4f1 and 5d1 levels of Ce3+. It was found from DFT total energy calculations that the dopant Ce3+ has approximately equal preference for the two distinct Ca sites, while the codopant Si4+ exhibits a strong preference for the NN P1 site over the NN P2 site. These results, combined with those obtained from the comparison between calculated and experimental 4f → 5d transition energies, revealed that the lowest-energy excitation band as observed experimentally for Ce, Si-coped Ca6BaP4O17 arises mainly from the 4f1 → 5d1 transitions of Ce3+ located at the two Ca sites with the charge-compensating Si4+ at a NN P1 site, while the 4f1 → 5d1 transitions of Ce3+ with Si4+ at a NN P2 site contribute to the 19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

low-energy tail of the excitation band. This preference of Si4+ for the NN P1 site results in a blueshift of the Ce3+ 4f1 → 5d1 transition, in agreement with experimental observations. It is further shown that the blueshift is a result of the decrease in 5d crystal field splitting of Ce3+ associated mainly with electronic effects of the NN SiP1 substitution, with a negligible contribution from the change in 5d centroid energy. The role of the coordinating oxygen that does not belong to any PO4 tetrahedon in the Ce3+ 4f1 → 5d1 transition energy was also addressed, with the expected finding that it contributes significantly to 5d centroid shift than the other coordinating O atoms. Finally, on the basis of theoretical and experimental results, the energy-level diagram for the 4f ground states and the lowest 5d states of all Ln3+ and Ln2+ ions on the Ca2+ sites of Ca6BaP4O17 is constructed and discussed in connection with experimental observations. The present work demonstrates the usefulness of first-principles calculations in elucidating luminescence properties of Ce3+ in complex local environments, which are usually the case in practical Ce-doped luminescent materials.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:. Calculated atomic coordinates for Ca6BaP4O17 (Table S1), lattice parameters for Ce, Si-codoped Ca6BaP4O17 (Table S2), 4f1 and 5d1 levels of Ce3+ with a NN SiP2 compensator (Table S3), analysis of structural and electronic effects on the blueshift induced by the presence of a NN SiP1 compensator (Table S4), local coordination structure of Ba in Ca6BaP4O17 (Figure S1), and experimental excitation and emission spectra of Eu3+-doped Ca6BaP4O17 (Figure S2).

AUTHOR INFORMATION Corresponding Authors *Phone: +86 553 3869748. E-mail: [email protected]. *Phone: +86 10 82377955. E-mail: [email protected].

Notes 20


Page 20 of 25

Page 21 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work has been supported by the National Natural Science Foundation of China (Grant Nos. 11174005, 51272242, 11574003, 51572023). L.N. acknowledges support from the Special and Excellent Research Fund of Anhui Normal University.

REFERENCES (1) Blasse, G.; Grabmaier, B. C. Luminescent Materials; Springer: Berlin, 1994. (2) Yen, W. M.; Shionoya, A.; Yamamoto, H. Phosphor Handbook, 2nd ed.; CRC Press: Boca Raton, FL, 2007; Chapter 4−13. (3) Zhou, L.; Liang, H.; Tanner, P. A.; Zhang, S.; Hou, D.; Liu, C.; Tao, Y.; Huang, Y.; Li, L. Luminescence, Cathodoluminescence and Ce3+/Eu2+ Energy Transfer and Emission Enhancement in the Sr5(PO4)3Cl:Ce3+, Eu2+ phosphor. J. Mater. Chem. C, 2013, 1, 7155−7165. (4) Li, K.; Shang, M.; Geng, F.; Lian, H.; Zhang, Y.; Fan, J.; Lin, J. Synthesis, Luminescence, and Energy-Transfer Properties of β‑Na2Ca4(PO4)2(SiO4):A (A = Eu2+, Dy3+, Ce3+/Tb3+) Phosphors. Inorg. Chem. 2014, 53, 6743−6751. (5) Dong, X.; Zhang, J.; Zhang, X.; Hao, Z.; Luo, Y. Luminescence and Energy Transfer Mechanism in Sr9Sc(PO4)7:Ce3+, Mn2+ Phosphor. J. Lumin. 2014, 148, 60−63. (6) Li, K.; Shang, M.; Zhang, Y.; Fan, J.; Lian, H.; Lin, J. Photoluminescence Properties of Single-Component White-emitting Ca9Bi(PO4)7:Ce3+, Tb3+, Mn2+ Phosphors for UV LEDs. J. Mater. Chem. C 2015, 3, 7096−7104. (7) Ji, H.; Huang, H.; Xia, Z.; Molokeev, M. S.; Atuchin, V. V.; Fang, M.; Liu, Y. Discovery of New Solid Solution Phosphors via Cation Substitution Dependent Phase Transition in M3(PO4)2:Eu2+(M = Ca/Sr/Ba) Quasi Binary Sets. J. Phys. Chem. C 2015, 119, 2038−2045. (8) Zhou, J.; Zhuang, L.; Li, F. Upconversion Nanophosphors for Small-Animal Imaging. Chem. Soc. Rev. 2012, 41, 1323−1349. (9) Rabouw, F. T.; den Hartog, S. A.; Senden, T.; Meijerink, A. Photonic Effects on the Förster 21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 25

Resonance Energy Transfer Efficiency. Nat. Commun. 2014, 5, 3610. (10) Komuro, N.; Mikami, M.; Shimomura, Y.; Bithellc, E. G.; Cheethamd, A. K. Synthesis, Structure and Optical Properties of Europium Doped Calcium Barium Phosphate – a Novel Phosphor for Solid-State Lighting. J. Mater. Chem. C 2014, 2, 6084−6089. (11) Komuro, N.; Mikami, M.; Shimomura, Y.; Bithellc, E. G.; Cheethamd, A. K. Synthesis, Structure and Optical Properties of Cerium-Doped Calcium Barium Phosphate - a Novel Blue-Green Phosphor for Solid-State Lighting. J. Mater. Chem. C 2015, 3, 204−210. (12) Chen, M.; Xia, Z.; Liu, Q. Luminescence Properties and Energy Transfer of Ce3+/Tb3+ Co-doped Ca6Ba(PO4)4O Phosphor for Near-UV Pumped Light-Emitting Diodes. J. Mater. Chem. C 2015, 3, 4197−4204. (13) Dorenbos, P. The 5d Level Positions of the Trivalent Lanthanides in Inorganic Compounds. J. Lumin. 2000, 91, 155−176. (14) Muñoz-García, A. B.; Seijo, L. Structural, Electronic, and Spectroscopic Effects of Ga Codoping on Ce-Doped Yttrium Aluminum Garnet: First-Principles Study. Phys. Rev. B

2010, 82, 184118. (15) Ning, L.; Lin, L.; Li, L.; Wu, C.; Duan, C.; Zhang, Y.; Seijo, L. Electronic Properties and 4f → 5d Transitions in Ce-Doped Lu2SiO5: A Theoretical Investigation. J. Mater. Chem. 2012, 22, 13723−13731 (16) Ning, L.; Wang, Z.; Wang, Y.; Liu, J.; Huang, S.; Duan, C.; Zhang, Y.; Liang, H. First-Principles Study on Electronic Properties and Optical Spectra of Ce-Doped La2CaB10O19 Crystal. J. Phys. Chem. C 2013, 117, 15241−15246. (17) Perdew, J. P.; Ernzerhof, M.; Burke, K. Rationale for Mixing Exact Exchange with Density Functional Approximations. J. Chem. Phys. 1996, 105, 9982. (18) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169−11186. (19) Kresse,

G.;

Joubert,

D.

From

Ultrasoft

Pseudopotentials

to

the

Projector

Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758−1775. (20) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953−17979.

22


Page 23 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(21) Barandiarán, Z.; Seijo, L. The Abinitio Model Potential Representation of the Crystalline Environment. Theoretical Study of the Local Distortion on NaCl:Cu+. J. Chem. Phys.

1988, 89, 5739−5748. (22) Gellé, A.; Lepetit, M. Fast Calculation of the Electrostatic Potential in Ionic Crystals by Direct Summation Method. J. Chem. Phys. 2008, 128, 244716. (23) Karlström, G.; Lindh, R.; Malmqvist, P.-Å.; Roos, B. O.; Ryde, U.; Veryazov, V.; Widmark, P. O.; Cossi, M.; Schimmelpfennig, B.; Neogrady, P.; Seijo, L. Molcas: a Program Package for Computational Chemistry. Comput. Mater. Sci. 2003, 28, 222−239. (24) Seijo, L.; Barandiarán, Z.; Ordejón, B. Transferability of Core Potentials to f and d States of Lanthanide and Actinide Ions. Mol. Phys. 2003, 101, 73−80. (25) Barandiarán, Z.; Seijo, L. The Ab initio Model Potential Method. Cowan−Griffin Relativistic Core Potentials and Valence Basis Sets from Li (Z=3) to La (Z=57). Can. J. Chem. 1992, 70, 409−415. (26) Dorenbos, P. The Eu3+ Charge Transfer Energy and the Relation with the Band Gap of Compounds. J. Lumin. 2005, 111,89−104. (27) Shannon, R. D. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallogr. A 1976, 32, 751−767. (28) Dorenbos, P. 5d-Level Energies of Ce3+ and the Crystalline Environment. III. Oxides Containing Ionic Complexes. Phys. Rev. B 2001, 64, 125117. (29) Muñoz-García, A. B.; Pascual, J. L.; Barandiarán, Z.; Seijo, L. Structural Effects and 4f−5d Transition Shifts Induced by La Codoping in Ce-Doped Yttrium Aluminum Garnet: First-Principles Study. Phys. Rev. B 2010, 82, 064114. (30) Morrison, C. A. Host Dependence of the Rare-Earth Ion Energy Separation 4fN−4fN−1nl. J. Chem. Phys. 1980, 72, 1001−1002. (31) Dorenbos, P. Lanthanide 4f-Electron Binding Energies and the Nephelauxetic Effect in Wide Band Gap Compounds. J. Lumin. 2013, 136, 122−129. (32) Dorenbos, P. Systematic Behaviour in Trivalent Lanthanide Charge Transfer Energies. J. Phys.: Condens. Matter 2003, 15, 8417−8434. (33) Dorenbos, P. Ce3+ 5d-Centroid Shift and Vacuum Referred 4f-Electron Binding Energies of All Lanthanide Impurities in 150 Different Compounds. J. Lumin. 2013, 135, 93−104. 23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(34) Guo, H.; Chen, W.; Zeng, W.; Li, G.; Wang, Y.; Li, Y.; Li, Y.; Ding, X. Structure and Luminescence Properties of a Novel Yellow Super Long-Lasting Phosphate Phosphor Ca6BaP4O17:Eu2+, Ho3+. J. Mater. Chem. C 2015, 3, 5844−5850. (35) Dorenbos, P. Relation between Eu2+ and Ce3+ f↔d Transition Energies in Inorganic Compounds. J. Phys.: Condens. Matter 2003, 15, 4797−4807.

24


Page 24 of 25

Page 25 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Graphic

25


Effects of Si Codoping on Optical Properties of Ce-Doped

Recommend Documents