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Preparation, characterization and luminescence of La2TeO6 phosphor doped with Eu3+
Materials Research Bulletin 43 (2008) 2763–2768 www.elsevier.com/locate/matresbu
Preparation, characterization and luminescence of La2TeO6 phosphor doped with Eu3+ Jaime Llanos *, Rodrigo Cortes Departamento de Química, Universidad Católica del Norte, Avda. Angamos 0610, Casilla 1280, Antofagasta, Chile Received 5 July 2007; received in revised form 27 September 2007; accepted 18 October 2007 Available online 30 October 2007
Abstract Phosphors of La2TeO6 doped with Eu3+ ions have been synthesized by the oxidation of the corresponding rare-earths oxytellurides of formula La2xEuxO2Te (x = 0.02, 0.06, and 0.1) at 1050 K. Powder X-ray diffraction confirms that the as prepared materials consist of the orthorhombic La2TeO6 as main phase. The photoluminescence (PL) of red-emitting La2xEuxTeO6 powder phosphors is reported. The emission spectrum, exhibits an intense emission peak due to 5D0 ! 7F2 transition at 616 nm, which indicates that the Eu3+ ion occupies a non-centrosymmetric site in the host lattice. These materials could find application for use as lamp phosphors in the red region. # 2007 Elsevier Ltd. All rights reserved. Keywords: A. Optical materials; A. Inorganic compounds; B. Chemical synthesis; D. Luminescence
1. Introduction One of the great challenges in solid-state chemistry today is the design and synthesis of novel inorganic solids possessing desirables structures and properties, in this sense the developing of rare-earth-ion doped compounds has attracted considerable interest because of its excellent luminescent properties and consequent applications in lasers, wide gap electroluminescent devices, scintillator detectors and phosphors [1–4]. Classical inorganic phosphors typically consist of an inert host lattice that is doped with activator ions, usually transition (3d) or rare-earth (4f) metals. The host lattice is transparent to the incident radiation and the activator is excited to emit photons [5]. Various type of excitation source may be used and in the last years the research is well organized according to the excitation source. Luminescent materials in plasma display panels (PDP) and field emission display (FED) use the high-energy side of the ultraviolet spectrum and even low voltage electrons, respectively. Phosphors on light emitting diodes (LED) and in fluorescent lamps are excited by near UV or blue light [6]. Because LEDs have important environmental features, including no use of mercury, it is expected that white LEDs will replace traditional fluorescent lamps in the near future [7]. These white LEDs are constructed on the combination of a blue LED and a yellow phosphor and according to Hirosaki and co-workers are lacks of green and red components [8]. Interesting candidates for red phosphors are the Eu(III) doped Ln materials. The colorimetric characteristic of the phosphors depends on the host lattice, since it is determined by the symmetry of Eu(III) in the host lattice. When
* Corresponding author. Tel.: +56 55 355624: fax: +56 55 355632. E-mail address: [email protected] (J. Llanos). 0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2007.10.023
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Eu(III) ion occupies a site with inversion symmetry, the emission corresponds to an orange-red light is predominantly observed. When the ion is present in a non-centrosymmetric site, the transition corresponding to a red emission is dominant [9]. In a previous paper we report about the chemical and physical properties of the oxytellurides of general formula Ln2TeO2 [10]. This approach has led us to explore the stability against oxidation of these phases and to examine the possible way to obtain doped rare-earth tellurates (Ln2TeO6) suitable for use as inorganic phosphors. In the present paper, we report on the preparation and the characterization of La2TeO:Eu3+ phosphor and its photoluminescence properties. 2. Experimental All phosphors were synthesized starting from the respective oxide (La2O3 Aldrich 99.99% pure, Eu2O3 Aldrich 99.99%) and elemental tellurium (Aldrich 99.99% pure). The syntheses of the samples were carried out in three steps; the first step consists in the dissolution of La2O3 and Eu2O3 in stoichiometric amounts in 50 ml of HNO3 (0.5 M). After complete dissolution of the oxides, NH4OH solution was added under continuous stirring until the pH of the solution reached 9 [11]. White precipitates were immediately produced, washed with distillated water and separated by centrifugation. The powders obtained were thermally annealed at 400 K in order to obtain europium doped lanthanum oxides of various compositions (La2O3:Eu3+(1%), La2O3:Eu3+(3%), and La2O3:Eu3+(5%)). The second step consists the preparation of europium doped lanthanum oxytellurides. This procedure was previously described in literature [10–12]. Each of these precipitates was placed in an alumina boat and inserted in a quartz tube. An excess of elemental tellurium was loaded in other alumina boat and located adjacent to the boat containing the europium doped lanthanum oxide. The reactants were heated at 950 K for 5 h under a hydrogen flow. Hydrogen was used not only as a carrier gas, but also as a reducing agent in accordance with the equation La2 O3 : Eu3þ ðsÞ þ TeðgÞ þ H2ðgÞ ! La2 O2 Te : Eu3þ ðsÞ þ H2 OðgÞ According to the results of the thermogravimetric analysis (vide infra), the third step consisted the synthesis of La1.9Eu0.1TeO6, La1.94Eu0.06TeO6 and La1.98Eu0.02TeO6 by means of a simple heating of the europium doped oxytellurides at 1073 K under oxygen atmosphere for 3 h. All the samples were white powders. The nature of these phases and composition were established by energy-dispersive X-Ray (EDS) analyses. All analyses were obtained using a JEOL JSM-6400 scanning electron microscopy (SEM) equipped with an Oxford Link INCA detector. To identify the compounds, powder X-ray diffraction data were collected with a Siemens D-5000 diffractometer fitted with a graphite monochromator, using Cu Ka radiation (l = 154.057 pm). Thermogravimetric analysis was performed using a Perkin-Elmer HTG-7 thermo-balance; all the measurements were carried out under oxygen atmosphere using heating rates of 10 K/min. The photoluminescence (PL) spectra (emission and excitation) were measured using a JASCO FP-6500 spectrofluorometer. All spectra were registered at room temperature. 3. Results and discussion 3.1. Thermal analysis The TG data of La2O2Te are shown in Fig. 1. When the temperature is increased, the probe was oxidized in three steps until a stable form is attained. The gain in weight in the sample heated to 1050 K proves the formation of the phase of formula La2TeO6. 3.2. XRD patterns La2TeO6 crystallizes with a orthorhombic unit cell a = 5.510(1) Å, b = 9.441(2) Å, c = 10.387(3) Å, Z = 4, space group P212121 (N819) [13]. In this structure, the rare-earth-ions are seven-coordinated by O2. The trivalent europium ion is expected to occupy the lanthanum site in La2TeO6:Eu3+ since the ionic radius of Eu3+ (106.6 pm) is slightly smaller than that of La3+ (110 pm) [14]. The powder X-ray diffraction patterns of La2TeO6, La1.9Eu0.1TeO6, La1.94Eu0.06TeO6 and La1.98Eu0.02TeO6 are shown in Fig. 2. With the Eu3+ incorporation into the La2TeO6 lattice, two
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Fig. 1. Thermogravimetric curve for the oxytelluride La2xO2TeEux.
Fig. 2. Powder X-ray diffraction patterns of La2xEuxTeO6 where (a) x = 0; (b) x = 0.02; (c) x = 0.06 and (d) x = 0.1. The powder pattern for La2O6Te was calculated from the data tabulated in ICSD [16].
phenomena are observed; the first is the apparent loss of crystallinity with increasing Eu3+ content, this is especially evident for the 5% doped sample. The second one is the inversion of intensity among the diffraction peak; the intensity of the 1 1 2 peak is higher in the doped samples than the 1 1 2 main peak in the un-doped material. This could indicate that the particles of the as-prepared phases have preferred orientation. The same results have been observed by Kim and Kang [11], when the compounds are prepared under conditions of high pH. Nevertheless, all the compounds could be indexed in the space group P212121. The indexing, observed and calculated d-spacing and relative intensity are given in Table 1. On the other hand, the variation of the unit cell volume with the Eu3+ content could confirm the existence of a range of solid solutions, represented by the formula La2xEuxTeO6 (x = 0, 0.02, 0.06, and 0.1), see Fig. 3. The values of the cell parameters for all compounds are given in Table 2, together with their standard deviations. Indexing, cell volume, and cell parameters determination were performed using the Unit Cell program [15].
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Table 1 Powder diffraction data of La2xEuxTeO6 (x = 0.02, 0.06, 0.1, and 0) La1.98Eu0.02TeO6
La1.94Eu0.06TeO6
hkl
dobs
dcalc
D(d)
Intensity
hkl
dobs
dcalc
D(d)
Intensity
120 112 121 013 031 122 113 200 032 123 213 231 232
3.587 3.513 3.394 3.202 3.017 2.953 2.799 2.761 2.692 2.490 2.105 2.033 1.926
3.593 3.503 3.393 3.232 3.014 2.948 2.790 2.764 2.689 2.485 2.101 2.037 1.928
0.005 0.010 0.001 0.030 0.004 0.005 0.008 0.003 0.002 0.006 0.004 0.005 0.001
19 100 39 26 13 12 21 30 14 8 13 21 23
120 112 121 013 031 122 113 200 032 123 213 231 232
3.582 3.508 3.388 3.192 3.013 2.948 2.795 2.754 2.689 2.486 2.100 2.031 1.925
3.589 3.497 3.389 3.225 3.012 2.944 2.785 2.759 2.687 2.480 2.097 2.035 1.925
0.007 0.011 0.000 0.034 0.001 0.004 0.011 0.006 0.003 0.006 0.004 0.004 0.000
42 100 91 28 26 18 50 34 25 27 13 26 24
La1.9Eu0.1TeO6
La2TeO6
hkl
dobs
dcalc
D(d)
Intensity
hkl
dobs
dcalc
D(d)
Intensity
120 112 121 013 031 122 113 200 032 123 213 231 232
3.572 3.505 3.386 3.191 3.009 2.944 2.791 2.749 2.685 2.487 2.099 2.027 1.922
3.583 3.494 3.384 3.225 3.007 2.941 2.783 2.755 2.683 2.478 2.095 2.031 1.922
0.011 0.011 0.002 0.035 0.002 0.003 0.008 0.005 0.002 0.009 0.004 0.004 0.000
30 100 66 27 20 18 42 32 20 19 23 30 23
120 112 121 013 031 122 113 200 032 123 213 231 232
3.584 3.509 3.399 3.251 3.012 2.950 2.800 2.755 2.692 2.491 2.102 2.033 1.925
3.586 3.509 3.389 3.251 3.013 2.951 2.800 2.755 2.692 2.491 2.102 2.033 1.926
0.001 0.000 0.009 0.001 0.001 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000
40 79 100 27 24 11 44 30 24 21 20 23 21
Fig. 3. Variation of the unit cell volume with Eu3+ content x in the system La2xEuxTeO6 (x = 0, 0.02, 0.06, and 0.1).
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Table 2 Lattice parameter (in Å) and cell volume (in Å3) of La2xEuxTeO6 (x = 0.02, 0.06, 0.1, and 0) Compound
a
b
c
Cell volume
Ref.
La1.98Eu0.02TeO6 La1.94Eu0.06TeO6 La1.9Eu0.1TeO6 La2TeO6
5.5284 (6) 5.5189 (6) 5.5098 (6) 5.510 (1)
9.4535 (11) 9.4486 (11) 9.4311 (11) 9.441 (2)
10.3192 (13) 10.2949 (13) 10.2968 (13) 10.387 (3)
539.31 536.83 535.05 540.33
This work This work This work [13]
(7) (7) (7) (2)
Space group P212121. Standard deviations are given in parentheses.
3.3. Luminescent properties The emission and excitation photoluminescence spectra of La2TeO6, La1.9Eu0.1TeO6, La1.94Eu0.06TeO6 and La1.98Eu0.02TeO6 are shown in Figs. 3 and 4, respectively. The emission spectra for all the phosphors were registered when the 254 and 395 nm wavelengths were used for excitation. The emission spectra of La1.9Eu0.1TeO6, La1.94Eu0.06TeO6 and La1.98Eu0.02TeO6 under both excitation wavelengths expose that the intense peak emission is located at 616 nm. The emission spectrum is dominated by the red peak at 616 nm due to 5D0 ! 7F2 transition, which indicates that Eu3+ ions occupy a non-centrosymmetric site in the host lattice of La2TeO6. The orange peak at 593 nm corresponding to the transition 5D0 ! 7F1 is weak. The color purity of these phases is satisfied and could be components of the tri-color phosphors. One also observe from Fig. 3 that as the Eu3+ concentration augments from 1 to 5% the intensity of the lines at 593 and 616 nm also goes on increasing. Under the excitation of 254 nm UV light, the most intensity emission corresponds to La1.9Eu0.1TeO6 phosphor, thus this compound could be a good alternative for use in fluorescent lamps. On the other hand, the un-doped La2TeO6 material does not show any luminescence (Fig. 4). The excitation spectrum of 616 nm emission for Eu3+ is dominated by the presence of three bands centered at 395.6, 466, and 534.6 nm. These sharp excitation peaks are assigned to the typical 4fn ! 4fn transitions of Eu3+. According to previous works, the excitation around 395 nm corresponds to 7F0 ! 5L6 transition, the excitation at 466 nm corresponds to 7F0 ! 5D2 transition and finally the excitation around 534 nm corresponds to 7F0 ! 5D2 transition. In the excitation spectrum, an intense peak at 250 nm is also observed this result from the charge-transfer transition (CT) from the oxygen ions to Eu3+. This CT band is closely related to the covalency between O2 and Eu3+ in the structure [17] (Fig. 5).
Fig. 4. Emission spectra of La2xEuxTeO6 (x = 0; 0.02; 0.06 x = 0.1) powder at an excitation wavelength of 254 nm, measured at room temperature (293 K).
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Fig. 5. Excitation spectrum of La1.9Eu0.1TeO6 powder at an emission wavelength of 616 nm, measured at room temperature (293 K).
4. Conclusions The La2TeO6 and La2TeO6 phosphors doped with Eu3+ have been successfully synthesized using a three-step preparation method and its photoluminescent (PL) properties were investigated. The emission and excitation spectra were discussed. The excitation spectrum is dominated by three bands corresponding to the excitations of electrons from Eu3+ 4f ground state to different excited 4f levels of Eu3+. The emission spectrum is characterized by an intense peak centered at 616 nm due to 5D0 ! 7F2 transition of Eu3+ ions, whereas the peak corresponds to 5D0 ! 7F1 transition is weak. As it is known, that the 5D0 ! 7F2/5D0 ! 7F1 intensity ratio is a good measure of the Eu3+ ion site symmetry, we conclude that Eu3+ occupy a non-centrosymmetric site in the lattice and satisfied the color purity of the phosphors. Acknowledgment This work was supported by FONDECYT-CHILE (Grant 1050994). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
M.M. Lage, R.L. Moreira, F.M. Matinaga, J.-Y. Gesland, J. Chem. Mater. 17 (2005) 4523. Y.Q. Quang, A.J. Steckle, Appl. Phys. Lett. 82 (2003) 502. A.J. Steckle, J.M. Zavada, MRS Bull. 24 (1999) 16. X.-J. Liu, H.-L. Li, R.-J. Xie, N. Hirosaki, X. Xu, L.-P. Huang, J. Lumin. 127 (2007) 469. A.R. West, Solid State Chemistry and its Applications, John Wiley and Sons, New York, 1992, pp. 583. T. Jüstel, H. Nikol, C. Ronda, Angew. Chem. Int. Ed. 37 (1998) 3084. N. Hirosaki, R.-J. Xie, NIMS Now Intern. 4 (5) (2006) 1. K. Sakuma, N. Hirosaki, R.-J. Xie, J. Lumin. 126 (2007) 843. G. Blasse, B.C. Grabmaier, Luminiscent Materials, Springer-Verlag, New York, 1994, pp. 42. J. Llanos, et al. Mater. Res. Bull. 43 (2008) 312. T. Kim, S. Kang, Mater. Res. Bull. 40 (2005) 1945. R.A. Kent, H.A. Eick, Inorg. Chem. 1 (1962) 956. M. Trömel, F.W. Huetzler, H.G. Burckhardt, C. Platte, E. Muench, Z. Anorg. Allg. Chem. 551 (1987) 95. R.D. Shannon, Acta Cryst. A32 (1976) 751. T.J.B. Holland, S.A.T. Redfern, Miner. Mag. 61 (1997) 65. Inorganic Crystal Structure Database, ICSD, Fachinformationzentrum Karlsruhe, Gmelin Institut, Release 1/2005 (Coll. Code # 62134). B.R. Judd, Phys. Rev. 127 (1962) 750.
Preparation, characterization and luminescence of La2TeO6 phosphor doped with Eu3+ Jaime Llanos *, Rodrigo Cortes Departamento de Química, Universidad Católica del Norte, Avda. Angamos 0610, Casilla 1280, Antofagasta, Chile Received 5 July 2007; received in revised form 27 September 2007; accepted 18 October 2007 Available online 30 October 2007
Abstract Phosphors of La2TeO6 doped with Eu3+ ions have been synthesized by the oxidation of the corresponding rare-earths oxytellurides of formula La2xEuxO2Te (x = 0.02, 0.06, and 0.1) at 1050 K. Powder X-ray diffraction confirms that the as prepared materials consist of the orthorhombic La2TeO6 as main phase. The photoluminescence (PL) of red-emitting La2xEuxTeO6 powder phosphors is reported. The emission spectrum, exhibits an intense emission peak due to 5D0 ! 7F2 transition at 616 nm, which indicates that the Eu3+ ion occupies a non-centrosymmetric site in the host lattice. These materials could find application for use as lamp phosphors in the red region. # 2007 Elsevier Ltd. All rights reserved. Keywords: A. Optical materials; A. Inorganic compounds; B. Chemical synthesis; D. Luminescence
1. Introduction One of the great challenges in solid-state chemistry today is the design and synthesis of novel inorganic solids possessing desirables structures and properties, in this sense the developing of rare-earth-ion doped compounds has attracted considerable interest because of its excellent luminescent properties and consequent applications in lasers, wide gap electroluminescent devices, scintillator detectors and phosphors [1–4]. Classical inorganic phosphors typically consist of an inert host lattice that is doped with activator ions, usually transition (3d) or rare-earth (4f) metals. The host lattice is transparent to the incident radiation and the activator is excited to emit photons [5]. Various type of excitation source may be used and in the last years the research is well organized according to the excitation source. Luminescent materials in plasma display panels (PDP) and field emission display (FED) use the high-energy side of the ultraviolet spectrum and even low voltage electrons, respectively. Phosphors on light emitting diodes (LED) and in fluorescent lamps are excited by near UV or blue light [6]. Because LEDs have important environmental features, including no use of mercury, it is expected that white LEDs will replace traditional fluorescent lamps in the near future [7]. These white LEDs are constructed on the combination of a blue LED and a yellow phosphor and according to Hirosaki and co-workers are lacks of green and red components [8]. Interesting candidates for red phosphors are the Eu(III) doped Ln materials. The colorimetric characteristic of the phosphors depends on the host lattice, since it is determined by the symmetry of Eu(III) in the host lattice. When
* Corresponding author. Tel.: +56 55 355624: fax: +56 55 355632. E-mail address: [email protected] (J. Llanos). 0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2007.10.023
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Eu(III) ion occupies a site with inversion symmetry, the emission corresponds to an orange-red light is predominantly observed. When the ion is present in a non-centrosymmetric site, the transition corresponding to a red emission is dominant [9]. In a previous paper we report about the chemical and physical properties of the oxytellurides of general formula Ln2TeO2 [10]. This approach has led us to explore the stability against oxidation of these phases and to examine the possible way to obtain doped rare-earth tellurates (Ln2TeO6) suitable for use as inorganic phosphors. In the present paper, we report on the preparation and the characterization of La2TeO:Eu3+ phosphor and its photoluminescence properties. 2. Experimental All phosphors were synthesized starting from the respective oxide (La2O3 Aldrich 99.99% pure, Eu2O3 Aldrich 99.99%) and elemental tellurium (Aldrich 99.99% pure). The syntheses of the samples were carried out in three steps; the first step consists in the dissolution of La2O3 and Eu2O3 in stoichiometric amounts in 50 ml of HNO3 (0.5 M). After complete dissolution of the oxides, NH4OH solution was added under continuous stirring until the pH of the solution reached 9 [11]. White precipitates were immediately produced, washed with distillated water and separated by centrifugation. The powders obtained were thermally annealed at 400 K in order to obtain europium doped lanthanum oxides of various compositions (La2O3:Eu3+(1%), La2O3:Eu3+(3%), and La2O3:Eu3+(5%)). The second step consists the preparation of europium doped lanthanum oxytellurides. This procedure was previously described in literature [10–12]. Each of these precipitates was placed in an alumina boat and inserted in a quartz tube. An excess of elemental tellurium was loaded in other alumina boat and located adjacent to the boat containing the europium doped lanthanum oxide. The reactants were heated at 950 K for 5 h under a hydrogen flow. Hydrogen was used not only as a carrier gas, but also as a reducing agent in accordance with the equation La2 O3 : Eu3þ ðsÞ þ TeðgÞ þ H2ðgÞ ! La2 O2 Te : Eu3þ ðsÞ þ H2 OðgÞ According to the results of the thermogravimetric analysis (vide infra), the third step consisted the synthesis of La1.9Eu0.1TeO6, La1.94Eu0.06TeO6 and La1.98Eu0.02TeO6 by means of a simple heating of the europium doped oxytellurides at 1073 K under oxygen atmosphere for 3 h. All the samples were white powders. The nature of these phases and composition were established by energy-dispersive X-Ray (EDS) analyses. All analyses were obtained using a JEOL JSM-6400 scanning electron microscopy (SEM) equipped with an Oxford Link INCA detector. To identify the compounds, powder X-ray diffraction data were collected with a Siemens D-5000 diffractometer fitted with a graphite monochromator, using Cu Ka radiation (l = 154.057 pm). Thermogravimetric analysis was performed using a Perkin-Elmer HTG-7 thermo-balance; all the measurements were carried out under oxygen atmosphere using heating rates of 10 K/min. The photoluminescence (PL) spectra (emission and excitation) were measured using a JASCO FP-6500 spectrofluorometer. All spectra were registered at room temperature. 3. Results and discussion 3.1. Thermal analysis The TG data of La2O2Te are shown in Fig. 1. When the temperature is increased, the probe was oxidized in three steps until a stable form is attained. The gain in weight in the sample heated to 1050 K proves the formation of the phase of formula La2TeO6. 3.2. XRD patterns La2TeO6 crystallizes with a orthorhombic unit cell a = 5.510(1) Å, b = 9.441(2) Å, c = 10.387(3) Å, Z = 4, space group P212121 (N819) [13]. In this structure, the rare-earth-ions are seven-coordinated by O2. The trivalent europium ion is expected to occupy the lanthanum site in La2TeO6:Eu3+ since the ionic radius of Eu3+ (106.6 pm) is slightly smaller than that of La3+ (110 pm) [14]. The powder X-ray diffraction patterns of La2TeO6, La1.9Eu0.1TeO6, La1.94Eu0.06TeO6 and La1.98Eu0.02TeO6 are shown in Fig. 2. With the Eu3+ incorporation into the La2TeO6 lattice, two
J. Llanos, R. Cortes / Materials Research Bulletin 43 (2008) 2763–2768
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Fig. 1. Thermogravimetric curve for the oxytelluride La2xO2TeEux.
Fig. 2. Powder X-ray diffraction patterns of La2xEuxTeO6 where (a) x = 0; (b) x = 0.02; (c) x = 0.06 and (d) x = 0.1. The powder pattern for La2O6Te was calculated from the data tabulated in ICSD [16].
phenomena are observed; the first is the apparent loss of crystallinity with increasing Eu3+ content, this is especially evident for the 5% doped sample. The second one is the inversion of intensity among the diffraction peak; the intensity of the 1 1 2 peak is higher in the doped samples than the 1 1 2 main peak in the un-doped material. This could indicate that the particles of the as-prepared phases have preferred orientation. The same results have been observed by Kim and Kang [11], when the compounds are prepared under conditions of high pH. Nevertheless, all the compounds could be indexed in the space group P212121. The indexing, observed and calculated d-spacing and relative intensity are given in Table 1. On the other hand, the variation of the unit cell volume with the Eu3+ content could confirm the existence of a range of solid solutions, represented by the formula La2xEuxTeO6 (x = 0, 0.02, 0.06, and 0.1), see Fig. 3. The values of the cell parameters for all compounds are given in Table 2, together with their standard deviations. Indexing, cell volume, and cell parameters determination were performed using the Unit Cell program [15].
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Table 1 Powder diffraction data of La2xEuxTeO6 (x = 0.02, 0.06, 0.1, and 0) La1.98Eu0.02TeO6
La1.94Eu0.06TeO6
hkl
dobs
dcalc
D(d)
Intensity
hkl
dobs
dcalc
D(d)
Intensity
120 112 121 013 031 122 113 200 032 123 213 231 232
3.587 3.513 3.394 3.202 3.017 2.953 2.799 2.761 2.692 2.490 2.105 2.033 1.926
3.593 3.503 3.393 3.232 3.014 2.948 2.790 2.764 2.689 2.485 2.101 2.037 1.928
0.005 0.010 0.001 0.030 0.004 0.005 0.008 0.003 0.002 0.006 0.004 0.005 0.001
19 100 39 26 13 12 21 30 14 8 13 21 23
120 112 121 013 031 122 113 200 032 123 213 231 232
3.582 3.508 3.388 3.192 3.013 2.948 2.795 2.754 2.689 2.486 2.100 2.031 1.925
3.589 3.497 3.389 3.225 3.012 2.944 2.785 2.759 2.687 2.480 2.097 2.035 1.925
0.007 0.011 0.000 0.034 0.001 0.004 0.011 0.006 0.003 0.006 0.004 0.004 0.000
42 100 91 28 26 18 50 34 25 27 13 26 24
La1.9Eu0.1TeO6
La2TeO6
hkl
dobs
dcalc
D(d)
Intensity
hkl
dobs
dcalc
D(d)
Intensity
120 112 121 013 031 122 113 200 032 123 213 231 232
3.572 3.505 3.386 3.191 3.009 2.944 2.791 2.749 2.685 2.487 2.099 2.027 1.922
3.583 3.494 3.384 3.225 3.007 2.941 2.783 2.755 2.683 2.478 2.095 2.031 1.922
0.011 0.011 0.002 0.035 0.002 0.003 0.008 0.005 0.002 0.009 0.004 0.004 0.000
30 100 66 27 20 18 42 32 20 19 23 30 23
120 112 121 013 031 122 113 200 032 123 213 231 232
3.584 3.509 3.399 3.251 3.012 2.950 2.800 2.755 2.692 2.491 2.102 2.033 1.925
3.586 3.509 3.389 3.251 3.013 2.951 2.800 2.755 2.692 2.491 2.102 2.033 1.926
0.001 0.000 0.009 0.001 0.001 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000
40 79 100 27 24 11 44 30 24 21 20 23 21
Fig. 3. Variation of the unit cell volume with Eu3+ content x in the system La2xEuxTeO6 (x = 0, 0.02, 0.06, and 0.1).
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Table 2 Lattice parameter (in Å) and cell volume (in Å3) of La2xEuxTeO6 (x = 0.02, 0.06, 0.1, and 0) Compound
a
b
c
Cell volume
Ref.
La1.98Eu0.02TeO6 La1.94Eu0.06TeO6 La1.9Eu0.1TeO6 La2TeO6
5.5284 (6) 5.5189 (6) 5.5098 (6) 5.510 (1)
9.4535 (11) 9.4486 (11) 9.4311 (11) 9.441 (2)
10.3192 (13) 10.2949 (13) 10.2968 (13) 10.387 (3)
539.31 536.83 535.05 540.33
This work This work This work [13]
(7) (7) (7) (2)
Space group P212121. Standard deviations are given in parentheses.
3.3. Luminescent properties The emission and excitation photoluminescence spectra of La2TeO6, La1.9Eu0.1TeO6, La1.94Eu0.06TeO6 and La1.98Eu0.02TeO6 are shown in Figs. 3 and 4, respectively. The emission spectra for all the phosphors were registered when the 254 and 395 nm wavelengths were used for excitation. The emission spectra of La1.9Eu0.1TeO6, La1.94Eu0.06TeO6 and La1.98Eu0.02TeO6 under both excitation wavelengths expose that the intense peak emission is located at 616 nm. The emission spectrum is dominated by the red peak at 616 nm due to 5D0 ! 7F2 transition, which indicates that Eu3+ ions occupy a non-centrosymmetric site in the host lattice of La2TeO6. The orange peak at 593 nm corresponding to the transition 5D0 ! 7F1 is weak. The color purity of these phases is satisfied and could be components of the tri-color phosphors. One also observe from Fig. 3 that as the Eu3+ concentration augments from 1 to 5% the intensity of the lines at 593 and 616 nm also goes on increasing. Under the excitation of 254 nm UV light, the most intensity emission corresponds to La1.9Eu0.1TeO6 phosphor, thus this compound could be a good alternative for use in fluorescent lamps. On the other hand, the un-doped La2TeO6 material does not show any luminescence (Fig. 4). The excitation spectrum of 616 nm emission for Eu3+ is dominated by the presence of three bands centered at 395.6, 466, and 534.6 nm. These sharp excitation peaks are assigned to the typical 4fn ! 4fn transitions of Eu3+. According to previous works, the excitation around 395 nm corresponds to 7F0 ! 5L6 transition, the excitation at 466 nm corresponds to 7F0 ! 5D2 transition and finally the excitation around 534 nm corresponds to 7F0 ! 5D2 transition. In the excitation spectrum, an intense peak at 250 nm is also observed this result from the charge-transfer transition (CT) from the oxygen ions to Eu3+. This CT band is closely related to the covalency between O2 and Eu3+ in the structure [17] (Fig. 5).
Fig. 4. Emission spectra of La2xEuxTeO6 (x = 0; 0.02; 0.06 x = 0.1) powder at an excitation wavelength of 254 nm, measured at room temperature (293 K).
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Fig. 5. Excitation spectrum of La1.9Eu0.1TeO6 powder at an emission wavelength of 616 nm, measured at room temperature (293 K).
4. Conclusions The La2TeO6 and La2TeO6 phosphors doped with Eu3+ have been successfully synthesized using a three-step preparation method and its photoluminescent (PL) properties were investigated. The emission and excitation spectra were discussed. The excitation spectrum is dominated by three bands corresponding to the excitations of electrons from Eu3+ 4f ground state to different excited 4f levels of Eu3+. The emission spectrum is characterized by an intense peak centered at 616 nm due to 5D0 ! 7F2 transition of Eu3+ ions, whereas the peak corresponds to 5D0 ! 7F1 transition is weak. As it is known, that the 5D0 ! 7F2/5D0 ! 7F1 intensity ratio is a good measure of the Eu3+ ion site symmetry, we conclude that Eu3+ occupy a non-centrosymmetric site in the lattice and satisfied the color purity of the phosphors. Acknowledgment This work was supported by FONDECYT-CHILE (Grant 1050994). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
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