Luminescence of Ce3+-Doped MB2Si2O8 (M= Sr, Ba): A Deeper

Subscriber access provided by CMU Libraries - http://library.cmich.edu

Article 3+

2

2

8

Luminescence of Ce -Doped MBSiO (M = Sr, Ba): A Deeper Insight Into the Effects of Electronic Structure and Stokes Shift Qi Peng, Chunmeng Liu, Dejian Hou, Weijie Zhou, Chonggeng Ma, Guokui Liu, Mikhail G Brik, Ye Tao, and Hongbin Liang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10355 • Publication Date (Web): 18 Dec 2015 Downloaded from http://pubs.acs.org on December 24, 2015

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 41

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

Luminescence of Ce3+-Doped MB2Si2O8 (M = Sr, Ba): a Deeper Insight into the Effects of Electronic Structure and Stokes Shift Qi Peng†, Chunmeng Liu†, Dejian Hou†, Weijie Zhou†, Chong-Geng Ma‡,*, Guokui Liu§, Mikhail G. Brik‡,∥,,#, Ye Tao¶, Hongbin Liang†,* †

MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of Environment and

Energy Chemistry, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, P.R. China ‡

College of Sciences, Chongqing University of Posts and Telecommunications, Chongqing

400065, P.R. China §

Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois

60439, USA ∥



Institute of Physics, University of Tartu, Ravila 14C, Tartu 50411, Estonia Institute of Physics, Jan Dlugosz University, Armii Krajowej13/15, PL-42200 Czestochowa,

Poland #

Institute of Physics, Polish Academy of Sciences, al. Lotników 32/46, 02-668 Warsaw, Poland

ACS Paragon Plus Environment

1


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 2 of 41

Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of

Sciences, Beijing 100039, P.R. China

KEYWORDS: borosilicate; Ce3+; luminescence; electronic structure; electron-phonon coupling; crystal-field effect

ABSTRACT: Series of Sr1-2xCexNaxB2Si2O8 and Ba1-2xCexKxB2Si2O8 (x = 0.005, 0.01, 0.02, 0.04, 0.06, 0.08) samples were synthesized by a high-temperature solid state reaction. The low temperature excitation, emission and fluorescence decay spectra together demonstrated that all spectral bands arise from the Ce3+ ions located at only one kind of lattice site. The first-principles calculations of the structural and electronic properties of pure and Ce3+-doped MB2Si2O8 (M = Sr, Ba) were performed and the obtained results were used for understanding the structural changes after doping and identification of the observed position of the host absorption bands. The measured 4f-5d excitation and emission spectra of Ce3+ ions doped in MB2Si2O8 were analyzed and simulated in the framework of the crystal-field (CF) theory. The electron-phonon coupling effect generally ignored in most studies published so far was also taken into account by applying the configurational coordinate model. The validity of such a combined insight of the 5d CF energy level positions and the Stokes shift have been confirmed by the analyzing dependence of the Ce3+ spectroscopic properties on the dopants’ concentration. In addition, the influence of temperature on the luminescent properties of the studied samples was also explored and discussed in the present work.


2

Page 3 of 41

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 Ce3+ ions show 4f-5d transitions from the vacuum ultraviolet (VUV) to visible range of the spectrum. Since these are the parity allowed transitions, they are characterized by a fast decay. The phosphors doped with Ce3+ ions are used in important applications. For example, the wellknown phosphor Y3Al5O12: Ce3+ can act as a yellow-emitting component in the InGaN/GaNbased LEDs (light emitting diodes)1. The commercially available scintillator Lu2SiO5: Ce3+ can be used in medical imaging detectors for PET (positron emission tomography) system2. When Ce3+ ions enter a specific lattice site, their spectroscopic properties and the emission life time are to a large extent determined by the coordination environment. Understanding the relationship between structural and optical properties of phosphors can help us acquire insights into the methods of developing those materials with specific optical properties. As far as the Ce3+ doped phosphors are concerned, numerous research works can be mentioned3-5. Tuning of the 4f-5d transition energy is a key technological problem for the design and exploration of the Ce3+-doped optical materials for desired applications. From the theoretical point of view, such controlling technique can be realized by adjusting the combination of the 5d crystal-field (CF) energy level positions and the strength of the electron-phonon coupling effect (i.e. the Stokes shift). Most of the previous studies were focused on the former5,6, probably because the latter effect was not predominant in those studied cases. However, it is important to understand why the electronphonon coupling can be ignored (and for which systems) and whether there is a competition between both of them in some particular hosts. This is the primary motivation that prompted us to look into some isostructural compounds with different cation host components. For such compounds the electron-phonon coupling effect can be gradually enhanced with the cation host components becoming much heavier owing to the softening of the host lattices, and thus one can


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

Page 4 of 41

expect to see the competition of the Stokes shift variation with the change of the 5d CF energy levels.7,8 Series of danburite MB2Si2O8 (M = Ca, Sr and Ba) are naturally-occurring alkali borosilicate minerals that can be described by the orthorhombic crystallographic system with a Pnma space group (No. 62)9-11. Figure 1 shows a general view of the MB2Si2O8 crystal structure with one unit cell. It can be easily seen that the M2+ ions tenfold-coordinated by the oxygen ions are embedded in the corner shared BO45- and SiO44- tetrahedrons, which form the framework of four and eight membered rings interconnected layer, as reported by the previous researches 12-14. The Wyckoff position analysis for each ion in the crystal reveals that there are 8 general atom positions, i.e., M(4c), B(8d), Si(8d), O1(8d), O2(8d), O3(8d), O4(4c) and O5(4c) sites. Since the 4c and 8d Wyckoff positions correspond to the local site symmetries Cs and C1, respectively, the ten oxygen ligands surrounding M2+ ions can be symmetry-related by a mirror reflection operator and further classified into two groups O(8d) and O(4c) exhibiting two kinds of different electron cloud overlaps with M2+ ion. Such a local coordination environment of M2+ site can have an important influence on the luminescence properties of Ce3+-doped MB2Si2O8 if the doping Ce3+ ions can go into the M2+ site. The photoluminescence properties of the Ce3+-doped CaB2Si2O8 were reported in the last century15 and later referred to by several other researches16-18. It also should be pointed out that there are so many difficulties on the artificial synthesis of this danburite series12. Moreover, the relatively small Stokes shift for Ce3+-doped CaB2Si2O8 in those reports has left a deep impression upon the extended work in the future because it is known that the smaller Stokes shift indicates the stiffer host lattice and, as a result, less nonradiative relaxation after the doped ions are excited13.


4

Page 5 of 41

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


In this work, we continue this line of research and keep our research motivation mentioned before to investigate the VUV-vis photoluminescence properties of the Ce3+-doped MB2Si2O8 (M = Sr, Ba) for the first time. The luminescent properties were understood by the combined consideration of the 5d CF effect and the Stokes shift, and discussed in details under the experimental conditions with the different measurement temperatures and doping levels. The theoretical calculations employing the first-principles and CF models have provided a good support to our experimental analysis and clearly demonstrated the relationship between the local geometry structures around Ce3+ dopants and the observed 4f and 5d energy level structures of the Ce3+ ions. The present study gives an overall insight into understanding the shift of the 5d-4f emission of Ce3+ ions. We emphasize that the electron-phonon coupling effect very often ignored before is paid more attention in the present study.

Figure 1. Schematic representations for the unit cell of pure MB2Si2O8 (M = Ca, Sr and Ba) crystal and the local coordination structure around Ce3+ ion doped into M2+ site. Drawn with VESTA19.

2. EXPERIMENTAL SECTION


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

Page 6 of 41

2.1. Materials and synthesis. Series of polycrystalline samples with nominal chemical formulas Sr1-2xCexNaxB2Si2O8 (x = 0.005, 0.01, 0.02, 0.04, 0.06, 0.08; Sr series) and Ba12xCexKxB2Si2O8

(x = 0.005, 0.01, 0.02, 0.04, 0.06, 0.08; Ba series) were prepared by a solid-state

reaction route at high temperature. When substituting Ce3+ for Sr2+ or Ba2+, the alkali Na+ or K+ ions were added to maintain charge neutrality of whole compounds in consideration of their matching ionic radii20, respectively. The starting materials SrCO3 [analytical reagent (A.R.)], BaCO3 (A.R.), K2CO3 (A.R.), Na2CO3 (A.R.), H3BO3 (A.R.), SiO2 (A.R.) and CeO2 (99.99%) were weighed in stoichiometric amounts, and then the mixtures were adequately ground in an agate mortar by adding ethanol (A.R.). After pre-heated in air at 500 °C for 5 hours, the mixtures were spontaneously cooled to room temperature (RT) and thoroughly ground again. Finally, the powders were kept sintering for 10 hours in the thermal-carbon reducing atmosphere at 1000 °C for Sr series and 800 °C for Ba series, and were ground to fine powders for subsequent characterization after cooling to RT. 2.2. Characterization method. The X-ray diffraction (XRD) data were recorded on a BRUKER D8 ADVANCED model powder X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å) at RT. The vacuum ultraviolet (VUV) excitation and emission spectra were measured at the VUV spectroscopy experimental station on beamline 4B8 of Beijing Synchrotron Radiation Facility (BSRF) under a dedicated synchrotron mode (2.5 GeV, 150-60 mA)21. The UV-vis luminescence spectra and the luminescence decay curves were measured with an Edinburgh FSP920 combined fluorescence lifetime and steady state spectrometer, which was equipped with a CTI-Cryogenics temperature control system. A 450 W xenon lamp was used as the excitation source for the steady-state spectra, whilst a 150 W nF900 ns flash lamp with a


6

Page 7 of 41

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


pulse width of 1 ns and pulse repetition rate of 40 kHz was employed as the excitation source for the luminescence decay curves. 2.3. Computational method. 2.3.1. First-principles calculations. The first-principles calculations of the structural and electronic properties of pure and Ce3+-doped MB2Si2O8 (M = Sr, Ba) were performed by using the periodic ab initio CRYSTAL09 code based on the linear combination of atomic orbitals method22. The closed-shell and spin-polarized DFT calculation forms were applied for the pure and doped systems, respectively. The dopant effect was modeled by a 2×2×1 supercell containing 208 atoms, in which one Sr2+ or Ba2+ ion was replaced by a Ce3+ ion and then the nearest Sr2+ or Ba2+ site around the substituting Ce3+ ion was occupied by a Na+ or K+ ion due to the consideration of the local charge compensation. The hybrid exchangecorrelation functional WC1PBE consisting of a PBE correlation part and a Wu-Cohen exchange part with a fractional mixing (16%) of the nonlocal Hartree-Fock exchange was employed in this work to yield better estimation on the host band gaps23. The local Gaussian-type basis sets (BS’s) were chosen as follows: the all-electron BS’s in the form of 86-311d1G, 86-511G, 8-511G, 8411d1G and 6-21d1G were used for Si24, K25, Na25, O26 and B27 atoms, respectively; for Sr and Ba atoms, the Hay-Wadt small-core pseudo-potentials and the related valence BS’s from Ref.28 were adopted; for Ce atom, the inner electrons [Ar]3d10 were described by the scalar-relativistic effective-core pseudo-potential (ECP) developed by the Stuttgart/Cologne group29, whereas the other electrons 4s24p64d105s25p64f16s25d1 were explicitly treated by the valence BS (12s12p9d8f)/[8s7p4d4f]30 related to the ECP that we used. The Monkhorst-Pack schemes for 4×4×4 and 2×2×2 k-point meshes in the Brillouin zone were applied to the pure and doped cases, respectively. The truncation criteria for bielectronic integrals (Coulomb and HF exchange series) were set to 8, 8, 8, 8 and 16, and a predefined “extra


7


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 41

large” pruned DFT integration grid was adopted together with much higher DFT density and grid weight tolerances (values 8 and 16). The tolerance of the energy convergence on the selfconsistent field iterations was set to 10-8 Hartree. The Anderson mixing scheme with 60% mixing was employed to achieve the converged solutions for all the calculation cases and the total spin of the doped systems was locked to a value of 1/2 in the first five cycles due to the single electron occupation on the 4f orbits of Ce3+ ions. In the geometry optimization with the full structural relaxation, the convergence criteria on the root-mean-square of the gradient and the nuclear displacement were set to 0.00006 Hartree/bohr and 0.00012 bohr, respectively. 2.3.2. Crystal-field analysis. The CF analysis of the 4f-5d excitation and emission spectra of Ce3+ ions in MB2Si2O8 (M = Sr or Ba) was implemented by employing the combinational theoretical scheme of the parameterized CF Hamiltonian model for 4f1 and 5d1 electronic configurations and the exchange charge model (ECM). The details about the scheme itself can be found in our past work31, and the related calculation parameter setting for the present work was given in Supporting Information.

3. RESULTS AND DISCUSSION 3.1. Phase identification and structure analysis. The calculated crystal structural parameters of the pure hosts SrB2Si2O8 and BaB2Si2O8 were summarized in Table S1 (as shown in Supporting Information) together with the experimental ones. It is easily seen from the table that the agreement between the experimental and calculated structural characteristics is very good. These calculated structural parameters can be also used to successfully reproduce the following observations about the local ligand environments of alkaline-earth cation sites and various


8

Page 9 of 41

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


interionic distances: there is only one kind of ten-coordinated Sr2+ or Ba2+ site with point symmetry Cs; the average Sr2+-O2- and Ba2+-O2- bond distances are ~2.74 Å and ~2.86 Å, respectively; the nearest adjacent distance of Sr2+-Sr2+ is ~4.59 Å, while that of Ba2+-Ba2+ is ~5.06 Å.13,32 The representative XRD patterns of Sr1-2xCexNaxB2Si2O8 and Ba1-2xCexKxB2Si2O8 with x = 0.005, 0.01, 0.02, 0.04, 0.06 and 0.08 are illustrated in Figure 2. The XRD pattern simulations obtained by using the Crystallographic Information Framework (CIF) files of the two compounds10,11 are also presented for the sake of comparison. One can find that the diffraction patterns of Sr1-2xCexNaxB2Si2O8 and Ba1-2xCexKxB2Si2O8 with x = 0.01, 0.04, 0.08 match well with the simulated ones, respectively. Thus, it can be concluded that the defect substitution molarity up to 8% cannot induce the significant deformation of the orthorhombic crystallographic phase. There are some very tiny impurity diffraction peaks observed at about 2Ɵ = 21o for Sr0.98Ce0.01Na0.01B2Si2O8 and about 2Ɵ = 28o for Ba0.98Ce0.01K0.01B2Si2O8, respectively. Fortunately, those impurity phases cannot have very evident influence on the luminescent properties of the synthesized samples since there is only one kind of optical site found in our samples, as described in the end of Section 3.3. Herein, the Ce3+ ions can be assumed to be incorporated into the hosts by replacing the Sr2+ or Ba2+ ions based on the consideration of their similar ionic radii20. A more detailed inspection of Figure 2 shows that the diffraction peaks shift slightly towards higher angles with the increasing doping contents of Ce3+. This phenomenon is consistent with the substitutions of slightly smaller Ce3+ (1.25 Å) for Sr2+ (1.36 Å) or Ba2+ (1.52 Å) ions at 10-fold coordination. Furthermore, because of the ionic radii difference between the Ce3+ and Ba2+ ions being much larger than that one between the Ce3+ and Sr2+ ions, the shift of diffraction peaks in the Ba series is displayed


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

Page 10 of 41

more obviously than those in the Sr series as shown in Figures 2(c) and (d). The observation suggests that the shortening of the average (Ce, Ba)-O distance is more obvious than that of the average (Ce, Sr)-O distance. Such an experimental understanding has been supported by the geometry optimization results of the local coordination environments of Ce3+ ions doped in hosts given by our first-principles calculations: the calculated average Ce3+-O2- bond lengths are much smaller than those Sr2+-O2- and Ba2+-O2- ones, and the bond length contraction degree in BaB2Si2O8 is about three times than that in SrB2Si2O8 (as shown in Table 1). In addition, the introduction of the Na+ (1.39 Å) and K+ (1.59 Å) ions into the corresponding alkaline-earth cation sites can not result in a significant change on the host lattices due to almost negligible ionic radii difference between the substituted and substituting ions, which has been also confirmed by the calculated average Na-O and K-O bond lengths in Table 1. Table 1. Calculated distances (in Å) from the dopant site to ten nearest oxygen ligands before and after Ce3+- and Na+- (or K+-) codoping in MB2Si2O8 (M = Sr or Ba). Host

SrB2Si2O8

BaB2Si2O8

M

Sr

CeSr

NaSr

Ba

CeBa

KBa

Site

Cs

C1

C1

Cs

C1

C1

R(M-O)

Exp.

Calc.

Calc.

Exp.

Calc.

Calc.

O1

2.5092(4c) 2.4955 2.3980

2.4512

2.6232(4c) 2.6552 2.4197

2.6748

O2

2.5890(8d) 2.5882 2.5026

2.6128

2.7937(8d) 2.8051 2.6419

2.7452

O3

2.5890(8d) 2.5882 2.5028

2.6143

2.7937(8d) 2.8051 2.6421

2.7776

O4

2.5850(8d) 2.5788 2.5580

2.4767

2.7521(8d) 2.7815 2.6517

2.7786

O5

2.5850(8d) 2.5788 2.5581

2.4774

2.7521(8d) 2.7815 2.6519

2.7685

Calc.

Calc.


10

Page 11 of 41

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


O6

2.6081(8d) 2.6012 2.5614

2.6395

2.7628(8d) 2.7707 2.6620

2.7452

O7

2.6081(8d) 2.6012 2.5617

2.6411

2.7628(8d) 2.7707 2.6623

2.7460

O8

3.0817(8d) 3.0596 2.9625

3.1101

3.0602(4c) 3.0291 2.8543

2.8811

O9

3.0817(8d) 3.0596 2.9633

3.1117

3.1626(8d) 3.1689 2.9641

3.2083

O10

3.3700(4c) 3.4118 3.4495

3.3635

3.1626(8d) 3.1689 2.9646

3.2095

Ravg

2.7607

2.7498

2.8626

2.8535

2.7563 2.7018 (-1.98%)

(-0.24%)

2.8737 2.7115 (-5.64%)

(-0.70%)

Notes: The columns entitled as “Exp.” contain the experimental data of the M-O bond lengths in the pure hosts and the site symmetries of ten oxygen ligands are also labeled after those data. The defect-induced average bond length changes with respect to the calculated pure host cases are given in the last row.

Figure 2. XRD patterns of (a) Sr1-2xCexNaxB2Si2O8 and (b) Ba1-2xCexKxB2Si2O8 with x = 0.01, 0.04, 0.08; expanded XRD patterns in the region 31.0°-33.5°for (c) Sr1-2xCexNaxB2Si2O8 and (d)


11


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 41

Ba1-2xCexKxB2Si2O8. The simulated XRD patterns of SrB2Si2O8 and BaB2Si2O8 are for comparison.

3.2. Electronic properties of pure and Ce3+-doped MB2Si2O8. Figures 3(a) and (c) show the calculated band structures of pure SrB2Si2O8 and BaB2Si2O8 crystals, respectively. The common feature is that the valence bands are rather flat, which suggests a weak dispersion of holes in the reciprocal space, whereas the conduction bands are more dispersive (higher mobility of electrons than holes). The calculated band gaps are indirect, and equal to 7.71 eV (SrB2Si2O8) and 7.96 eV (BaB2Si2O8). Composition of the calculated electronic bands can be analyzed with the help of the density of states (DOS) and partial density of states (PDOS) diagrams shown in Figures 3(b) and (d). The valence bands in the two cases are formed by the O-2p states, whereas the conduction bands are basically composed of the d states of alkaline-earth metal atoms (the Sr-4d or Ba-5d states) and the Si-3p states with the admixture of the B-2p states. Introduction of Ce3+ and alkali-metal monovalent ions can considerably change the electronic properties of hosts. The calculated DOS and PDOS diagrams for Ce3+/Na+-doped SrB2Si2O8 and Ce3+/K+-doped BaB2Si2O8 are illustrated in Figure 4. The calculated band structure diagrams are also shown in Figure S1 as a reference in order to confirm our DOS and PDOS analysis. The 4f states of the Ce3+ ions are clearly seen in the band gap. The position of the Ce3+ 4f occupied state with respective to the top of the host’s valence band can be evaluated as 1.76 eV (SrB2Si2O8) and 1.19 eV (BaB2Si2O8). Such an energy separation can be related to the Coulomb interaction strength between the Ce3+ 4f electron and the O2- 2p electrons, and thus one can conclude that much shorter the Ce-O bond average length, much higher the Ce3+ 4f occupied energy level


12

Page 13 of 41

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


position. The calculated two Ce3+ 5d states appear below the bottom of the conduction band and their energy positions relative to the top of the host’s valence band can be evaluated as 7.05 eV and 7.53 eV for SrB2Si2O8, and 7.13 eV and 7.63 eV for BaB2Si2O8, whereas others are fully resonant in the conduction band. Since the 4f electron occupation can affect the 5d level positions due to the screening effect by the 4f electron, it should be kept in mind that the Ce3+ 5d energy level positions given by the present DFT ground state calculations are not the real 5d energy levels corresponding to the 4f-5d transitions because the 4f occupation is empty for the 5d excitation state of Ce3+ ions, as Du et al. regarded the Ce3+ 5d excitation state as a Ce4+ ion plus a free electron trapped into its 5d ground orbital33. Hence, the calculated 5d states here can be approximately thought to be the Ce 4f15d1 states if Ce3+ is turned Ce2+ by capturing a free electron into those empty 5d states. Inspection of the application diagram of the double zigzag model of Dorenbos to YPO4 supports such a hypothesis because the lowest 4f15d1 state of Ce2+ ion can lie in the band gap, whereas the lowest 4f2 state falls into the conduction band34. Moreover, there should be the hybridization of the Ce-5d states and the conduction band states and thus the strict border between two kinds of states is not in existence. Therefore, those socalled Ce 4f15d1 states can lead to a red shift of the host absorption appearing in the excitation spectra of the two doped systems. The energy levels of the doping Na+ or K+ ions are not found inside the band gap, and it seems that they only play a role of the charge compensator. However, the first-principles study of nearedge gap states in CaF2 with Na+ impurities reveals that the Na+ doping can slightly lift up the top of the valence band by changing the 2p electron cloud distribution of the nearest-neighbor Fions around Na+ ion due to the charge mismatch between the substituting and substituted ions35. Inspection of Figure 4 shows the position of the top of the valence band of Ce3+/Na+-codoped


13


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 41

SrB2Si2O8 is 0.13 eV higher than that of the pure host, whereas there is also an energy shift of 0.25 eV for the case of Ce3+/K+-codoped BaB2Si2O8. The dependence of such a slight shift of the top of the valence band on the supercell size was checked, and the same results were obtained in the case of the 222 supercell. The Mulliken population analysis was performed for individual ion in those local clusters composed of the dopant site and its nearest ten oxygen ligands before and after Ce3+- and Na+-(or K+-) codoping in MB2Si2O8. The calculation results given in Table 2 suggest that the O2- 2p electrons trend to go back to those alkaline-earth cation sites after Ce3+and Na+-(or K+-) codoping, and thus the 2p occupied orbitals in comparison with the case of the pure hosts are more dispersive so that their energies in the band structure diagrams will shift to the higher energy direction as shown by the rising of the top of the valence band in Figure 4. The energy level positions of Ce impurity ions with respect to the top of the valence band before and after doping were plotted in Figure 5 based on the calculation results above and the observed 4f-5d transition energies listed in Table 3. The energies of the host absorption onsets can be estimated as 6.92 eV and 6.88 eV for Ce3+/Na+-doped SrB2Si2O8 and Ce3+/K+-doped BaB2Si2O8, respectively. The theoretical predictions give a good agreement with the observation in the excitation spectra of the doped samples (as shown in Figures 6 and 7). Table 2. Effective Mulliken charge Q (e) of individual ion in those local clusters composed of the dopant site and its nearest ten oxygen ligands before and after Ce3+- and Na+-(or K+-) codoping in MB2Si2O8 (M = Sr or Ba). Host

SrB2Si2O8

BaB2Si2O8

M

Sr

CeSr

NaSr

Ba

CeBa

KBa

Q(M2+)

1.875

2.245

0.968

1.808

2.326

0.999


14

Page 15 of 41

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


Q(O12-)

-1.206

-1.169

-1.094

-1.218

-1.227

-1.147

Q(O22-)

-1.234

-1.123

-1.131

-1.114

-1.135

-1.070

Q(O32-)

-1.234

-1.123

-1.131

-1.114

-1.135

-1.069

Q(O42-)

-1.169

-1.154

-1.062

-1.119

-1.125

-1.035

Q(O52-)

-1.169

-1.154

-1.062

-1.119

-1.125

-1.035

Q(O62-)

-1.145

-1.121

-1.028

-1.195

-1.127

-1.116

Q(O72-)

-1.145

-1.121

-1.028

-1.195

-1.127

-1.116

Q(O82-)

-1.234

-1.131

-1.123

-1.218

-1.247

-1.194

Q(O92-)

-1.234

-1.131

-1.123

-1.195

-1.116

-1.127

Q(O102-)

-1.206

-1.204

-1.184

-1.195

-1.116

-1.127

Q(O)avg

-1.198

-1.143

-1.097

-1.168

-1.148

-1.104


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

Page 16 of 41

Figure 3. Calculated band structure (a, c) and DOS/PDOS (b, d) diagrams for pure MB2Si2O8 (M = Sr or Ba) crystals. The letters , Z, T, Y, S, X, U, R and LD stand for the high-symmetry k points (0,0,0), (0,0,1/2), (0,1/2,1/2), (0,1/2,0), (1/2,1/2,0), (1/2,0,0), (1/2,0,1/2), (1/2,1/2,1/2) and (0,0,z) (0

Luminescence of Ce3+-Doped MB2Si2O8 (M= Sr, Ba): A Deeper

Recommend Documents