EUROPIUM LUMINESCENCE IN CALCIUM, STRONTIUM, AND BARIUM THIOGALLATES

M.V. Nazarov1,2, D.Y. Noh1, and H.J. Kim3

1Department of Materials Science and Engineering, Gwangju Institute of Science and Technology 1Oryong-dong, Buk-gu, 500-712, Gwangju, Republic of Korea

2Institute of Electronic Engineering and Industrial Technologies, Academy of Sciences of Moldova, 3/3, Academiei str., MD - 2028, Chisinau, Republic of Moldova

3Samsung Electro-Mechanics Co, LTD, Metan 3-Dong, Yeogtong-Gu, Suwon, Republic of Korea

(Received 18 December 2006)

Abstract

The detailed XRD analysis, Raman spectroscopy, and PL study were carried out in or-der to enlarge the understanding of the radiative processes in Ca, Sr, and Ba thiogallate phos-phors prepared by solid state reaction with carbon as a reduction atmosphere. The crystal field splitting, Stokes shift, Red shift, and centroidal shift were estimated and they are in a good agreement with mathematical calculation and literature data. The nephelauxetic effect was first calculated in thiogallate phosphors.

1. Introduction

Luminescent properties of Barium, Strontium and Calcium thiogallates doped with Europium activator have been investigated since 1970 [1-7]. A new wave of intensive study of these materials began in recent years as they are very attractive photoluminescent, elect-roluminescent, and cathodoluminescent materials for the visible spectral range. Since the ab-sorption of the 4f → 5d transitions of the doping ions extends to the visible, they are appropriate phosphors for excitation by near UV or blue emitting diodes for solid state light-ing (SSL) applications [8].

Some attempts to combine the luminescent data of transition of Eu2+ in inorganic com-pounds (including thiogallate phosphors) were made by P. Dorenbos [9], but unfortunately, in the used references the raw materials and chemical methods of preparation phosphor, as well as activator concentration and temperature were different. The luminescent data also were various, because many properties like luminescence intensity and wave length peak strongly depend on activator concentration and preparation conditions.

Peters and Baglio synthesized thiogallate phosphors under H2S steam by CaCO3, Ga2O3 and Eu2O3 [1]. In other works on preparing thiogallates this gas was also used [10, 11]. How-ever, H2S is very toxic for a human body. Some interesting results were achieved recently without toxic gas using a reduction atmosphere of 5% H2 and 95% N2 [12, 13]. Other attempts were launched to prepare thiogallates without H2S. For example, F. Aidaev made the synthe-sis at low temperature from NH4CNS, but this process has long reaction times and it is com-plicated [14]. Nevertheless, the development of synthesis methods to improve the luminescent properties of thiogallate materials remains relevant.

In this paper we revisited the luminescence of the thiogallate phosphors. A systematic study of Ba, Sr, and Ca thiogallates under the same preparation and investigation conditions

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has been performed using carbon as a reduction atmosphere. The radiative mechanisms of Ba, Sr, and Ca thiogallates are not completely understood and it is necessary to enlarge the fun-damental knowledge of them. In this paper excitation and emission photoluminescence spec-tra with Raman spectroscopy and X-ray diffraction patterns were analyzed to estimate the nephelauxetic effect, crystal field splitting and Stokes shift in dependence on activator sur-rounding in different host lattices. These data will be useful to evaluate the quality of the powders prepared for different electronic devices.

2. Samples and experimental procedure

Polycrystalline BaGa2S4:Eu2+, SrGa2S4:Eu2+, and CaGa2S4:Eu2+ samples were synthe-sized by solid-state reaction correspondingly from BaS, SrS, CaS, and Ga2S3 sulphide pow-ders mixed in stoichiometric composition and annealed at 900 -1000 °C with a carbon reduction atmosphere for 4 h. The doping ions were introduced in the form of EuS. This preparation method differs from that reported by Peters et al. [1] as well as above-mentioned [10-14]. The method presented here provides powder samples with good crystalline properties as shown by X-ray diffraction measurements. Powder samples with 6 mol % Eu2+ concentra-tion were studied. The thiogallate samples exhibit a blue-green (BaGa2S4:Eu2+), deep green (SrGa2S4:Eu2+), and yellow (CaGa2S4:Eu2+) colors. Phosphor samples were characterized by crystalline structure and luminescence properties.

X-ray diffraction measurement – The crystal structures of the prepared samples were determined by X-ray diffraction measurement using goniometer (PANalytical, X’Pert pro MPD) with Cu- Kα (λ = 1.5418 Å) at 40 kV and 30 mA. The scan speed was 3 seconds per step (0.02o step – 2theta) and covered the range between 10o and 90o.

Photoluminescence (PL) measurement – Optical spectroscopy and PL characteristics were estimated on the basis of emission and excitation spectra registered at room temperature (Xe 500 W lamp) with DARSA PRO 5100 PL System (Professional Scientific Instrument Co, Korea). Excitation spectra were corrected for the energy distribution of the Xe-lamp. The ex-citation was performed with a 460 nm radiation, which is usually used in blue LED.

Raman spectroscopy - Raman scattering spectra of SrGa2S4, CaGa2S4, and BaGa2S4 were measured by a Renishaw 3000 spectrometer with a He-Ne laser (excitation wavelength of λ= 633 nm and λ= 785 nm) and a photomultiplier counter at room temperature in back scat-tering configuration. The spectral resolution of spectrometer is about 4 cm-1 at 633 nm and 1 cm-1 at 785 nm.

Morphology and size measurement – Particle sizes and morphologies of the investigated phosphors were determined by scanning electron microscope (SEM) Hitachi-S-3000N. In or-der to control the particle size and to find the size distribution, the Laser diffraction was car-ried out using HELOS particle size analysis system.

3. Results and discussion 3.1. XRD

Fig. 1 shows the X-ray diffraction patterns of the SrGa2S4:Eu2+ powder fired at 1000 °C

(1). Their positions are in a good agreement with fitting (2) and data given in ICSD-46019 (3).

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Fig. 1. X-ray diffraction patterns of SrGa2S4:Eu2+ and its fitting in comparison with ICSD-46019.

Solid curve (1) in Fig. 1 is the measured data and dash curve (2) means the calculation

result of Rietveld analysis, which was developed by H. M. Rietveld in 1967 for structure pro-file refinement of X-ray powder diffraction data [15]. The Rietveld calculation uses the least square method to get all the crystallographic information by comparing model profile and X-ray or neutron curves. The peak positions of X-ray diffraction curve are related to the unit cell lattice constants of the crystal structure and the peak intensities are affected by atomic position, atom species, atomic occupancy, and thermal parameters. The absence of peaks exp-lains the crystallographic symmetry. In addition, crystalline size and strain induce peak broadening in XRD pattern. All these important data could be obtained from the Rietveld analysis and, therefore, we have applied this analysis for X-ray pattern. In the case of multi-phase structures quantitative phase analysis is available by using the Rietveld method. The detailed crystallographic information from the Rietveld analysis is listed in Tables 1-3.

First of all we did Rietveld refinement to check the crystallographic structure of the samples. Fig. 1 shows that the structure of our synthesized samples is very similar to the structure of SrGa2S4 orthorhombic phase ICSD-46019 pattern (3). (ICSD: Inorganic Crystal-lographic Structure Database). Thereby, we could confirm that in realized synthesis condi-tions we obtained a single phase of SrGa2S4. The unit cell parameter of ICSD-46019 is a=20.932Å, b=20.549Å, and c=12.227Å, but the unit cell of our sample was a=20.8859(19)Å, b=20.5515(22)Å, and c=12.2138(12)Å. The atomic position was also slightly different from ICSD-46019. The positions of Sr atoms were the same to the reference, but the positions of Ga atoms and S atoms were a little different. The different atomic positions affected the unit cell parameter and the distance of Sr atoms and S atoms. Crystallographic data for synthe-sized SrGa2S4 and reference sample (original unit cell and atomic positions) are given in Ta-bles 1 and 1-1.

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Table 1. Crystallographic data for different ion positions in synthesized SrGa2S4.

wRp=0.12 orthorhombic x y z F Multiple factor

Rp=0.095 a=20.8558(19) Sr1 0.125 0.125 0.125 1 8 R(F2)=0.174 b=20.5115(22) Sr2 0.125 0.125 0.625 1 8 ICSD-46019 c=12.2138(12) Sr3 0.8726 0.125 0.125 1 16

Ga1 -0.0033(8) 0.8040(4) 0.1633(7) 1.05(2) 32 α=90 Ga2 -0.0052(8) 0.2370(4) 0.3752(8) 1.16(2) 32 β=90 S1 0.1672(10) 0.2472(13) 0.997(2) 1.15(4) 32 γ=90 S2 0.1725(14) 0.5078(17) 0.262(3) 0.83(4) 32

S3 0.999(2) 0.3438(8) 0.0089(15) 1.00(4) 32

SrGa2S4 Single phase

F d d d S4 -0.006(3) 0.4204(10) 0.2701(16) 1.00(3) 32

Table 1-1. Crystallographic data for ICSD-46019 SrGa2S4.

orthorhombic x y z F Multiple factor

a=20.932 Sr1 0.125 0.125 0.125 1 8 b=20.549 Sr2 0.125 0.125 0.625 1 8 c=12.227 Sr3 0.8726 0.125 0.125 1 16

Ga1 0.0005 0.8038 0.164 1 32 α=90 Ga2 0.0005 0.2374 0.373 1 32 β=90 S1 0.168 0.2483 0.998 1 32 γ=90 S2 0.1677 0.502 0.2539 1 32

S3 0.9995 0.3449 0.0062 1 32 F d d d

S4 0.0021 0.4222 0.2651 1 32

From the occupation factor F one can see that when F=1 or nearby, it means the site is occupied by an atom. When F<1 (S2 position in synthesized phosphor) the site could be sub-stituted by a lighter atom, in our case it may be oxygen, for example.

The thiogallate compounds of the type MIIGa2S4 (where MII = Ca, Sr) belong to the or-thorhombic crystal class with the space group D24 2h (Fddd) [16] and BaGa2S4 to cubic struc-ture with the space group Pa3(Th6) [5]. There are 32 formula-mass units per unit cell (z = 32) and, therefore, 56 atoms in a primitive cell: 8 MII, 16 Ga, 32 S. According to B. Eisenmann et al. [16] the MII atoms occupy square anti-prismatic sites formed by eight sulfur (symmetry group D4d) forming MII(S)8 units with C2 and D2 symmetry. In the SrGa2S4:Eu2+ and CaGa2S4:Eu2+ compound, the cation ions are substituted by the Eu2+ ions. From XRD data (Table 1) they are situated in three slightly different sites: 8a, 8b, and 16c, their multiplicity is not equal and exhibits 1:1:2 relative ratio dictated by symmetry. Gallium atoms are tetrahed-rally coordinated to four sulfur atoms forming Ga(S)4 units (symmetry group Td) and the sul-fur atoms are at the centre of deformed MII

2Ga2 tetrahedrons forming (S)MII2Ga2 units. The

assembly of the MII(S)8 anti-prismatic units with common edges forms chains parallel to the a axis of the unit cell. Each chain is linked to four chains by corner sharing. Gallium atoms are located between two consecutive chains. The unit cell consists of four layers along the c axis.

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Figure in Table 2 illustrates the symmetry of the S and MII environments. The site symmetry of each atom can be obtained from the correlation tables [17].

Table 2. Crystal structure of strontium surrounding in SrGa2S4 and short distance Sr-S in com-parison with reference data.

Short Distance

SrGa2S4

(Å) ICSD (ref)

(Å) Sr(1)-S(6) 4.778 4.778 Sr(1)-S(9) 3.131 3.134 Sr(2)-S(6) 3.451 3.452 Sr(2)-S(7) 3.108 3.110 Sr(2)-S(8) 3.119 3.122 Sr(3)-S(6) 2.878 2.881 Sr(3)-S(7) 3.115 3.117 Sr(3)-S(8) 3.095 3.098

Crystal structure of SrGa2S4

Sr(3)-S(9) 3.099 3.102

According to our data from Table 2, the shortest distance between Sr and S can be esti-

mated as 3.10 Å, which is in a good agreement with reference ICSD data. (The measurements were made for all lengths less than 5 Å). For CaGa2S4 this length Ca-S is shorter and it was found as 3.0 Å. For cubic BaGa2S4 the length Ba-S is longer and it is about 3.17 Å. The main data and crystallographic structure of BaGa2S4 are given in the Table 3.

Table 3. Crystal structure of BaGa2S4 and short distance Ba-S in comparison with reference data.

BaGa2S4-ICSD-35135 X Y Z Atomic distance (Å)

Ba(1) 0 0 0 Ba(1)-S(4) 3.533 Ba(2) 0.3722 0.3722 0.3722 Ba(1)-S(5) 3.590 Ga(3) 0.1959 0.3568 0.1263 Ba(2)-S(4) 3.271 S(4) 0.3627 0.2935 0.1268 Ba(2)-S(5) 3.172 S(5) 0.2291 0.5267 0.0773

a=12.685 b=12.685 c=12.685

Crystal structure of BaGa2S4

Cubic 90 90 90 Pa-3 symmetry

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3.2. Raman spectroscopy

The Stokes spectrum of the polycrystalline SrGa2S4, as well as comparison spectra of dominant lines for SrGa2S4, CaGa2S4, and BaGa2S4 is given from 50 cm-1 to 500 cm-1 in Figs. 2a and 2b. Stokes spectra for SrGa2S4 and CaGa2S4 consist of 19 vibration lines posi-tioned between 68 cm-1 and 408 cm-1 (the more intense lines are marked in Fig. 2a). The fac-tor-group method predicts that the total number of Raman-active phonons is 84 for this structure [18]. The reduced numbers of modes in the Raman spectra are in favour of a mo-lecular model to describe the vibrations of these compounds [19, 20]. Two dominant peaks at 279 cm-1 (34.5 meV) and at 357 cm-1 (44.2 meV) for SrGa2S4 and similar peaks at 284 cm-1

(35.2 meV) and 361 cm-1 (44.8 meV) for CaGa2S4 are registered. Only one intense vibration line at 302 cm-1 (37.5 meV) is found for BaGa2S4. (Fig. 2b). These Raman spectra are in good agreement with those reported in the literature [21, 22] and could be used in fits of the emis-sion bands.

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Fig. 2a. Raman spectrum of SrGa2S4, excited with the

785 nm laser line.

Fig. 2b. Raman spectra of dominant lines of CaGa2S4, SrGa2S4, and BaGa2S4 excited with the 633 nm laser line.

As shown for the orthorhombic chalcogallates [23], the Raman vibrations can be de-

scribed by a molecular model. The orthorhombic MIIGa2S4 spectra were interpreted by con-sidering the isolated GaS4 and MIIS8 vibrating units. The most intense vibration line was assigned to the totally symmetrical bond-stretching modes, where the S anions move together in the cation-anion direction in GaS4 or in MIIS8 units. The change of Ca or Sr cation leads to a slight translation in frequency of the overall spectrum. This shift is not caused by mass ef-fects but is ascribed to the differences in size of the cations. The Raman frequencies increase with decreasing ionic radius of the MII cation, i.e. with decreasing interatomic lengths and size of the unit cell (Table 2). Thus the phonon frequencies vary in the following order: νSr < νCa. If the vibrations of S around MII are described with MIIS8 molecular unit there is no contradiction with the fact that no motion of MII cation is observed: in the MIIS8 symmetry group (D4d) the frequencies of the Raman-active modes are independent of the MII mass [24]. At this point of the discussion, considering the vibrating structure as isolated GaS4 and MIIS8 units is in agreement with the Raman data.

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The spectrum of BaGa2S4 is very similar to the Raman spectra of orthorhombic MIIGa2S4 compounds with MII =Sr and Ca. The vibration lines are located in the same range of frequencies (between 50 and 410 cm−1), indicating that the mass of Ba affects very slightly the spectrum and that no vibration involving this cation directly is present in the Raman spect-rum. The most intense line of the BaGa2S4 spectrum (302 cm−1) is located at higher frequency than the most intense mode of the orthorhombic MIIGa2S4 spectra (between 278 and 285 cm−1). Considering that the Ga-S lengths (2.27 Å) are shorter and the Ba-S value lengths (3.17 Å) are longer in cubic BaGa2S4 than in orthorhombic MIIGa2S4 (Ga-S between 2.27 and 2.29 Å, MII-S between 3.00 and 3.13 Å) (Tables 2 and 3), this suggests that the unit responsi-ble for this mode is the GaS4 tetrahedrons and not the BaS6 or BaS8 units, their vibrations might lead to weaker frequency modes. So, the most intense vibration mode in BaGa2S4 can be ascribed to the totally symmetrical bond-stretching modes in GaS4 tetrahedrons.

3.3. Excitation and emission spectra

The emission and excitation spectra of the thiogallate compounds of the type MIIGa2S4 (where MII = Ca, Sr, Ba) doped with 6 mol % of Eu2+ at 300 K are very similar (Fig. 3). For all samples the emission spectra under excitation at 460 nm consist of a broad band in the visible range. The maximum wavelengths of the band are shifted from 504 nm for Ba, to 535 nm for Sr and to 558 nm for Ca. Full Widths at Half Maximum (FWHM) are equal, cor-respondingly, to 62, 49, and 50 nm. No additional band due to the emission of Eu2+ in investi-gated thiogallates is observed, which indicates that no secondary phase is present in our samples.

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Fig. 3. Emission (right) and excitation spectra (left) of the thiogallate compounds of the type MIIGa2S4 (where MII = Ba (1), Sr (2), Ca (3)) doped with 6 mol % of Eu2+ at 300 K.

The emission is ascribed to the dipole-allowed transition from the lower 4f6(7F)5d state to the 4f7(8S7/2) fundamental state of the Eu2+ ions [1]. The single configuration coordinate model (Fig. 4) was used to fit the emission band of considered thiogallates.

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Fig. 4. Configuration diagram.

In first approximation, we suppose that only the fundamental vibration level (m=0) of

the excited electronic state is occupied. Using this hypothesis, the intensity of the vibronic transition from the fundamental vibration level of the emitting electronic state to the n vibra-tion level of the ground electronic state, i.e. the m=0→n transition, is proportional to the ex-pression [25]

I~exp(-S)·Sn/n!, where S is the Huang-Rhys parameter and measures the interaction between the luminescent centre and the vibrating lattice. The energy gap between two vibronic peaks is equal to the quantity hνg, the mean lattice phonon energy for the 4f7(8S7/2) ground electronic state. The best fits of the emission band for SrGa2S4 were obtained with S=4 and hνg=34.5 meV (Fig. 5).

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ExperimentCalculationS=4hv=34.5 meV

Fig. 5. Experimen-

tal (broken curve) and calculated (circles) emis-sion spectrum of the SrGa2S4:Eu2+

(6 mol %) at 300 K under excitation at 460 nm.

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The same fits were made also for calcium and barium thiogallates and corresponding data are presented here: S=4; hνg=35.2 meV for CaGa2S4 and S=7; hνg=37.5 meV for BaGa2S4. The values of Huang-Rhys parameter correspond to a strong electron–phonon coup-ling and can be linked to the high symmetry of the emission bands for all samples. These va-lues are in a good agreement with those presented in literature for strontium and calcium thiogallates [11, 26] and a little different for barium [10]. This difference may be explained by different activator concentration and temperature.

By supposing that the ground state parabola of the configuration coordinate model pre-sents the same curvature as the excited state parabola, i.e., the phonon energy is the same for the 4f7(8S7/2) ground state as for the 4f6(7F)5d excited state (hνg =hνe =hν), we can determine the Stokes shift (ΔS). ΔS is related to the offset of the parabolas in the configuration coordi-nate diagram and ΔS is equal to (2S-1)hν. For SrGa2S4 powder ΔS=0.26 eV (1950 cm-1), for CaGa2S4 ΔS=0.26 eV (2000 cm-1) and for BaGa2S4 ΔS=0.5 eV (4113 cm-1). These values are also in a good agreement with literature [10, 11, 26].

The excitation spectrum of SrGa2S4:Eu2+ (6 mol%) at 300 K and its detailed analysis for 535 nm wavelength is presented in Fig. 6a.

Fig. 6a. Excitation spectrum of the SrGa2S4:Eu2+ (6 mol%) powder at 300 K for the emission at 535 nm.

Fig. 6b. Energetic scheme and electronic levels in SrGa2S4:Eu2+.

SrGa2S4:Eu2+ excitation spectrum is composed of three large bands in the UV–visible

range: The excitation band (A) (maximum at 254 nm) is ascribed to the transitions between the

valence band and the conduction band of the SrGa2S4 host matrix. The two excitation bands (B) and (C) are ascribed to the 4f7(8S7/2)-4f6 (7F)5d transitions

in Eu2+ electronic levels. In the SrGa2S4:Eu2+ compound, the Sr2+ ions are substituted by the Eu2+ ions. They oc-

cupy three different sites with C2 and D2 symmetry [5]. Since their shapes have minor differ-ences, the emission bands of Eu2+ in the three available sites are expected to lie near each other. The observed emission band (Fig. 6a) can be a superposition of these bands, because

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they are inhomogeneously broadened by the size difference between Eu2+ and Sr2+. As the 2 positions Sr1 and Sr2 (Table 1) are very close, the emission band could be decomposed in 2 spectra (Fig. 6a). In first approximation, they are located at the centre of square anti-prisms formed by the 8 S2- anions (Tables 1 and 2). In this D4d symmetry, the 5d orbitals are split in three levels [11] as illustrated in Fig. 7.

Fig. 7. Splitting of the five 5d orbitals in a square antiprismatic (D4d) symmetry.

The band (C) is ascribed to the 4f7 (8S7/2)→4f6 (7F)5d (a1) transitions and the band (B)

to the 4f7 (8S7/2) →4f6 (7F)5d (e2) transitions. By evaluating the energy gap between the (B) and (C) bands to 8000 cm-1 (1 eV), the crystal field strength, Δa-p=8.9Dq, can be estimated at about 16000 cm-1 (2 eV).

An almost mirror–image relationship seems to hold between the emission and the exci-tation spectra in the energy region between the two maxima (Fig. 6a). This relation is charac-teristic of a phonon-broadened emission and confirms the hypothesis we made previously; it suggests that the mean phonon energy hν is the same for the 4f7 (8S7/2) ground state as for the 4f6 (7F)5d (a1) excited state. We can determine the energy of the zero-phonon line E0 at the intersection of the emission and excitation spectra. From Fig. 6a, we found E0 = 2.44 eV, which corresponds to a wavelength of 508 nm (comparable with that given by Eichenauer et al. [27] and Chartier et al. [11]). Unfortunately, we can not distinguish at room temperature the fine structure due to the spin–orbit coupling in the 4f6 configuration leading to the split-ting of the 7F levels in seven levels: 7FJ with J = 0-6. The energy Eabs of the f → d transition from the ground state to the lowest level of the 4f65d (a1), corresponds to the 7F0 level. Since this level is not resolved, we can only give an estimation of Eabs. By using the mirror–image relationship between the emission and the excitation spectra, we have found Eabs = 2.57 eV (482 nm). The energy of the transition from the 4f7 (8S7/2) ground state to the 4f6 (7F0)5d ex-cited state is lowered from the free ion value when the Eu2+ ion is brought into a crystal envi-ronment. The energy of the f → d absorption and of the d → f emission can be written according to the formalism of P. Dorenbos [9]

Eabs = Efree - D and Eem= Efree – D-ΔS,

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where Efree is the energy difference between the lowest 4f7 level and the 4f6 (7F0)5d level for the free or gaseous ion, D is the energy lowering also called redshift and ΔS the Stokes shift. The redshift energy depends on the crystal environment and can be represented by a term, called the centroid shift or nephelauxetic (covalence) effect, concerning the shift of the aver-age of the five 5d levels relative to the free ion value, and a second term associated to the crystal field splitting. For Eu2+, Efree is equal to 4.19 eV [28]. The emission and excitation spectra provide the values of absorption and emission energies, Eabs = 2.57 eV and Eem = 2.31 eV. The Stokes shift is then ΔS = Eabs - Eem = 0.26 eV (2000 cm-1) and the redshift D = Efree - Eabs = 1.62 eV (13000 cm-1). The value of the Stokes shift is comparable with this pre-viously obtained by fitting the band emission: 0.26 eV (1950 cm-1). Thanks to the determina-tion of the crystal field strength and calculation of other parameters, we propose the energetic scheme given in Fig. 6b. The bottom of the first 4f65d levels (a1) consists of two near situated lines, corresponding to different Eu sites.

For CaGa2S4:Eu2+ we have very similar the emission and excitation spectra (Fig. 3, curves 3). It is not strange, because the crystallographic structure for calcium thiogallate is the same as for SrGa2S4:Eu2+. The position of the CaGa2S4:Eu emission peak depends on the acti-vator concentration in the compound. The peak position shifts from 553 nm (0% of Eu) to the longwave region 565 nm (at 10% Eu concentration) and approaches a constant [29]. There-fore, for comparison of all the parameters, we also doped CaGa2S4 with 6 mol% Eu. It should be noted that in the case of CaGa2S4, Eu2+ concentration can be increased over a wide range without the appearance of the concentration-quenching effect. This fact is verified by nume-rous experiments with CaxEu1-xGa2S4 (0≤x≤1) solutions and is explained by the closeness of the Eu2+ and Ca2+ ion radii. From the detailed analysis of Fig. 3 (curves 3), the crystal field splitting can be estimated at about 16800 cm-1 (2.09 eV). We repeated the experimental and calculation way and the emission and excitation spectra provide the values of absorption and emission energies, Eabs = 2.48 eV and Eem = 2.22 eV. The Stokes shift is then ΔS = Eabs - Eem = 0.26 eV (2079 cm-1) and the redshift D = Efree - Eabs = 1.71 eV (13700 cm-1). The value of the Stokes shift is comparable with this previously obtained by fitting the band emission: 0.26 eV (2000 cm-1).

For BaGa2S4:Eu2+ (6 mol%) there is a little difference in spectra, crystallographic struc-ture and explication of the results in comparison with strontium and calcium thiogallates. The excitation spectrum (Fig. 3, curve 1) consists of 3 broad bands in visible range being not very well resolved at room temperature. The excitation spectrum reported by Peters et al. [1] for BaGa2S4:Eu2+ (2%) is similar. The spectra presented at 77 K by Jabbarov et al. [10] for (1%) Eu are better resolved but, principally, the main peaks correspond to our results.

In the BaGa2S4 crystal, the Ba2+ ions occupy two lattice sites with different symmetry and coordination number: 8 Ba2+ ions are 6-fold coordinated in the special position c with point symmetry C3(3), whereas 4 Ba2+ ions are in the special position a or b with point sym-metry S6(3) and coordination number 12 (Table 3). These data correspond to Eisenmann et al. [5]. Since Eu2+ ions replace Ba2+ ions in BaGa2S4:Eu2+, they are also distributed into these two different sites. In first approximation following [10] we can consider that the 6-fold coor-dinated Eu2+ ions have an octahedral (Oh) environment and that the 12-fold coordinated Eu2+ ions lie in icosahedral (Ih) sites. In Oh symmetry the 5d orbitals are split in two levels t2g and eg, whereas they are still degenerated in Ih symmetry (hg level). Thus, three f→d transitions are expected: the 4f7(8S7/2) →4f6(7F)5d (t2g) and the more energetic 4f7(8S7/2) →4f6(7F)5d (eg) transitions for the 6-fold coordinated Eu2+ ions, and the 4f7(8S7/2) →4f6(7F)5d (hg) transition for the 12-fold coordinated ions. Three bands are observed in the excitation spectrum. Fig. 3, spectrum 1, provides a crystal-field splitting equal to about 12100 cm-1(1.5 eV).

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The energy of the zero-phonon line, E0, at the intersection of the emission and excita-tion spectra for BaGa2S4 was estimated to 2.67 eV (464 nm). By using the mirror-image rela-tionship between the emission and the excitation spectra, we have found Eabs=2.9 eV (424 nm). The energy of the Stokes shift ΔS can be evaluated from excitation and emission spectra: ΔS=Eabs-Eemi= 2.9-2.4=0.5 eV (4000 cm-1). This value is consistent with the Stokes shift found previously by fitting the emission band (4113 cm-1). The knowledge of Eabs allows us to determine the redshift D. We found D=Efree-Eabs=4.19-2.9= 1.3 eV (10,484 cm-1).

We observe the increase of the S parameter and Stokes shift with increasing size of the divalent cation MII in thiogallate MIIGa2S4:Eu2+ phosphors, i.e. in the following order: Ca, Sr, Ba.

The crystal field splitting is expected to increase with decreasing size of the divalent cation MII. The rule is confirmed for MII=Ba, Sr, and Ca: the strength of the crystal-field was estimated at 12100 cm-1 for BaGa2S4: Eu2+, at 16000 cm-1 for SrGa2S4:Eu2+, and at 16700 cm-1 for CaGa2S4: Eu2+.

The lower redshift in BaGa2S4:Eu2+ (D=10000 cm-1) compared with SrGa2S4:Eu2+ (13000 cm-1) and CaGa2S4: Eu2+ (13700 cm-1) is mainly explained by the lower crystal field splitting but also by the lower nephelauxetic effect or centroid shift. This shift εc was first cal-culated for all three considered thiogallates and schematically shown in Fig. 8. The Ba ele-ment is less electronegative than the Sr element and Ca. The MII–S lengths are larger in BaGa2S4:Eu2+ (3.17 Å) than in SrGa2S4:Eu2+ (3.10 Å) and CaGa2S4:Eu2+ (3.0 Å) (Tables 2 and 3). The Ba–S bonds are then less covalent than the Sr–S and Ca-S ones and the nephaulexetic effect is weaker in BaGa2S4 than in SrGa2S4 and CaGa2S4.

Fig. 8. Nephelauxetic effect in Ca, Sr, and Ba thiogallates.

Summarized luminescence and crystallographic data for calcium, strontium, and barium

thiogallates doped with 6 mol % Eu2+ concentration are given in Tables 4 and 5.

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Table 4. Luminescence data for calcium, strontium, and barium thiogallates doped with 6 mol% Eu2+ concentration.

Formula λ(abs) nm/(eV)

Eo nm/(eV)

λ(em) nm/(eV)

FWHM (nm) S hν

(meV)ΔS

cm-1/(eV)D

cm-1/(eV) ε (cfs)

cm-1/(eV) ε(centr)

cm-1/(eV)

BaGa2S4 424 2.9

464 2.67

504 2.4 62 7 30 4000

0.5 10500

1.3 12100

1.5 3200 0.40

SrGa2S4 482 2.57

508 2.44

535 2.31 49 4 34.5 2000

0.26 13000 1.62

16000 2.0

3400 0.42

CaGa2S4 500 2.48

529 2.3

558 2.22 50 4 35.2 2079

0.26 13700 1.71

16800 2.09

3600 0.45

Table 5. Crystallographic data for calcium, strontium, and barium thiogallates.

Formula Structure Space group a (Å) b (Å) c (Å) z Shortest M-S(Å)

Coord. number

Sym-metry

BaGa2S4 cubic

α=β=γ=90 Tetrahedral

Th6-Pa3 12.685 12.685 12.685 12 3.172 6, 12 Pa-3

SrGa2S4 Orthorhombic D2h24-Fddd 20.855 20.511 12.213 32 3.10 8 D2, C2

CaGa2S4 Orthorhombic D2h24-Fddd 20.122 20.09 12.133 32 3.0 8 D2, C2

One of the important questions for practical application of thiogallates in LED, displays,

and other devices is radiant efficiency and stability. Usually these materials are very bright and have very high efficiency, but sometimes their stability toward hydrolysis and tempera-ture is not sufficient. Ternary compounds are more stable than binary ones. Thiogallates are much less sensitive to the action of moisture than binary chalcogenides. The radiant efficiency of the Eu2+ activated thiogallates decreases in the order Ca>Sr>Ba and the stability decreases in the order Ba>Sr>Ca [1, 7]. Schematically this tendency in correlation with crystal field splitting (Table 4) can be shown in Fig. 9. Less split powders show more stable properties.

Fig. 9. Stability (qualitative) and crystal field splitting on M sites (M=Ba, Sr or Ca) in binary and ternary compounds.

SrS17300cm-1 CaS

18750cm-1

CaGa2S4 16700cm-1

SrGa2S4 16000cm-1

BaGa2S4 12100cm-1

Stability

BaS 16000cm-1

Ba Sr Ca Cation M

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4. Conclusions

In this paper we revised the luminescence properties in barium, strontium, and calcium thiogallate phosphors. This study allows the comparison of the properties of ternary sulfide compounds prepared under the same conditions with 6 mol % Eu2+ concentration. The lumi-nescence and crystallographic data are summarized and listed in Tables 4 and 5. These data will be useful to evaluate the quality of the powders prepared for different electronic devices.

The detailed XRD analysis, Raman spectroscopy, and PL study were carried out in or-der to enlarge the understanding of the radiative processes. The crystal field splitting, Stokes shift, Red shift, and centroidal shift were estimated and they are in a good agreement with mathematical calculation and literature data. This work provides deeper knowledge of the Eu luminescence in barium, strontium, and calcium thiogallates.

The nephelauxetic effect was first calculated in thiogallate phosphors. This study confirms that barium, strontium, and calcium thiogallates are suitable phos-

phors in lighting and display devices.

References [1] T.E. Peters and J.A. Baglio, J. Electrochem. Soc., 119, 230, (1972). [2] T.E. Peters, J. Electrochem. Soc., 119, 1720, (1972). [3] P.C. Donohue and J.E. Hanlon, J. Electrochem. Soc., 121, 137, (1974). [4] C. Donohue and J.E. Hanlon, J. Electrochem. Soc.: Solid State Sci. Technol., 121, 137,

(1974). [5] B. Eisenmann, M. Jakowski, and H. Schafer, Mat. Res. Bull., 17, 1169, (1982). [6] A. Garcia, C. Fouassier, and P. Dougier, J. Electrochem. Soc., 129, 2063, (1982). [7] M.R. Davolos, A. Garcia, C. Fouassier, and P. Haggenmuller, J. Solid State Chem., 83,

316, (1989). [8] G.O. Mueller and R. Mueller-Mach, IDW’00, Proceeding, 821, Nov. 29 – Dec. 1, Kobe,

Japan, (2000). [9] P. Dorenbos, Journal of luminescence, 104, 239, (2003). [10] R.B. Jabbarov, C. Chartier, B.G. Tagiev, O.B. Tagiev, N.N. Musayeva, C. Barthou, and

P. Benalloul, Journal of Physics and Chemistry of Solids, 66, 1049, (2005). [11] C. Chartier, C. Barthou, P. Benalloul, and J. Frigerio, Journal of Luminescence, 111,

147-158, (2005). [12] J.M. Kim, K.N. Kim, J.K. Park, C.H. Kim, and H.G. Jang, Journal of the Korean Chemi-

cal Society, 48, 4, 371, (2004). [13] J.M. Kim, K.N. Kim, J.K. Park, C.H. Kim, H.G. Jang, Proc. of IMID-2005, Digest, 1623,

(2005). [14] F.Sh. Aidaev, Inorg. Mater., 39, 96, (2003). [15] H.M. Rietveld, Acta Cryst., 22, 151, (1967). [16] B. Eisenmann, M. Jakowski, W. Klee, and H. Schaefer, Rev. Chim. Miner., 20, 255,

(1983). [17] W.G. Fateley, N.T. McDewitt, and F.F. Bentley, Appl. Spectrosc., 25, 155, (1970). [18] R. Halford, J. Chem. Phys., 14, 8, (1946). [19] N. Mamedov, S. Iida, T. Aoki-Matsumoto, A. Kato, M. Yamazaki, N. Yamamoto,

T. Shishido, and K. Iwai, Japan. J. Appl. Phys., 39, Suppl. 1, 307, (2000). [20] S. Iida, N. Mamedov, A. Aoki-Matsumoto, N. Yamamoto, M. Yamakasi, A. Kato and

K. Iwai, Japan. J. Appl. Phys., 39, Suppl. 1, 315, (2000).

M.V. Nazarov, D.Y. Noh, and H.J. Kim

345

[21] C. Chartier, R. Jabbarov, M. Jouanne, J. Morhange, P. Benalloul, C. Barthou, J. Frigerio, and B. Tagiev, Proc. of EL2002, Ghent, (2002).

[22] C. Barthou, R. Jabbarov, P. Benalloul, C. Chartier, M. Musayeva, B. Tagiev, and O. Tagiev, J. Electrochem. Soc., 153, 3, G253, (2006).

[23] C. Chartier, R. Jabbarov, M. Jouanne, J.F. Morhange, P. Benalloul, C. Barthou, J.M. Frigerio, B. Tagiev, and E. Gambarov, J. Phys.: Condens. Matter, 14, 13693, (2002).

[24] H. Schlafer and H. Wasgestian, Theoret. chim. Acta, 1, 369, (1963). [25] S. Shionoya and W.M. Yen (Eds.), Phosphor Handbook, CRC Press, New York,

921,1998. [26] S. Iida, T. Matsumoto, N.T. Mamedov, G. An, Y. Maruyama, A.I. Bairamov,

B.G. Tagiev, O.B. Tagiev, and R.B. Dzhabbarov, Jpn. J. Appl. Phys., 36, L857, (1997). [27] L. Eichenauer, B. Jarofke, H.-C. Mertins, J. Dreyhsig, W. Busse, H.-E. Gumlich,

P. Benalloul, C. Barthou, J. Venoit, C. Fouassier, and A. Garcia, Phys. Stat. Sol. A, 153, 515, (1996).

[28] J. Sugar and N. Spector, J. Opt. Soc. Am., 64, 11, 1484, (1974). [29] B.G. Tagiev, M.G. Shakhtakhtinskii, V.A. Dzhalilov, T.A. Gyul’malyev, B.M. Izzatov,

G.K. Aslanov, O.B. Tagiev, and Ya.G. Talybov, Inorg. Mater., 29, 1238, (1993).