Optical properties of rare-earth doped Sr3Al2O6

Optical Materials 32 (2010) 1341–1344

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Optical properties of rare-earth doped Sr3Al2O6 M.V.S. Rezende, R.M. Araújo, P.J.R. Montes *, M.E.G. Valerio Physics Department, Federal University of Sergipe, Campus Universitário, 491000-000 São Cristovão-SE, Brazil

a r t i c l e

i n f o

Article history: Received 4 November 2009 Received in revised form 5 April 2010 Accepted 22 April 2010 Available online 20 May 2010 Keywords: Rare-earth Sol–Gel Proteic Defect modeling Crystal field parameters

a b s t r a c t This paper describes a computational study of extrinsic defect of Sr3Al2O6 induced by trivalent rare-earth dopants. Solution energies for a range of possible doping mechanisms are calculated, and predictions made of doping sites and charge-compensation schemes. It is shown that there are definite trends going along the rare-earth series. Atomistic modeling is used to calculate the symmetry and detailed geometry of the dopant ion-host lattice system, and this information is then used to calculate the crystal field parameters, which are an important indicator in assessing the optical behaviour of the system. The transition levels are then calculated for the Eu3+-substituted material, and comparisons with the experimental results were done. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Phosphorescence (or afterglow) refers to the light emission of an insulator that persists at room temperature after stopping excitation (usually UV irradiation). The conventional phosphorescent materials such as ZnS:Cu did not exhibit both bright light emission and long afterglow time suitable for practical applications. In recent years, the strontium aluminates have attracted intense research, since they have excellent properties such as high quantum efficiency, long persistence of phosphorescence and good stability [1]. These properties have been observed in the following compounds: SrAl2O4:Eu2+,Dy3+,B3+ [1], Sr4Al14O25:Eu2+,Dy3+,B3+ [2], SrAl4O7:Eu2+,Dy3+ [3], SrAl12O19:Eu2+,Dy3+, Sr2Al6O11:Eu2+,Dy3+ [4]. In addition to a higher chemical stability, the intensity and the duration of the phosphorescence of strontium aluminates (in especially SrAl2O4:Eu2+,Dy3+,B3+) makes it possible to envisage a continuous light emission during a whole night (10 h), hence greatly renewing interests in the phosphorescence phenomenon. Potential applications of these compounds are numerous, especially in the areas of safety sign improvements and energy saving (e.g., traffic signs, emergency signs, safety clothes, advertising, etc.). In this work, a hybrid computer modeling method was employed, based on a combination of crystal field calculations, and energy minimization. The method uses energy minimization to predict the location of the dopant ion and the relaxed positions of the surrounding ions. This information is then input into a crystal field calculation which obtains the crystal field parameters, Bkq ,

* Corresponding author. E-mail addresses: [email protected] (P.J.R. Montes), mvalerio@fisica.ufs.br (M.E.G. Valerio). 0925-3467/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2010.04.015

which are then used to calculate the energies of the electronic transitions of the rare-earth ions. The combination of these modeling techniques is a very powerful tool to reveal some of the important features of the light emission processes in this material. 2. Methodology The methodology combines three different computer modeling techniques: defect calculations based on energy minimization, crystal field calculation via the simple overlap model and energy level calculations. For the first one, lattice energy minimization was used, where the interactions between the ions present on the material are parameterized via interatomic Buckingham potentials supplemented by electrostatic interaction terms. The three constants involved in the Buckingham potentials for each pair of ions were obtained using empirical fitting methods embodied in the GULP code [5]. The potential set used to model the Sr3Al2O6 were obtained by fitting the potential parameters to all available SrO–Al2O3 crystalline system as described previously [6]. Calculations of rare-earth doping were performed using the Mott–Littleton method [7] in which atoms in a spherical region immediately surrounding the defect are treated explicitly, and a continuum approach is used for more distant regions of the lattice. The interaction between the dopant ion and the ions in the host lattice were also described using the Buckingham potentials and they were obtained by fitting the parameters to the rare-earth oxide crystalline structure [8]. In the second step, the relaxed positions of the dopant and the surrounding ions are then input into a crystal field calculation which obtains the crystal field parameters, Bkq , using the simple overlap model (SOM) [9].

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In the third step, the Bkq ’s were used to calculate the energies of the electronic transitions via the modified crystal field theory based on the Judd–Ofelt theory [10,11]. In this theory, the interaction between the rare-earth ion and the surrounding (host crystal) ions are given by the Hamiltonian HCF:

HCF ¼

X

Bkq C kq

ð1Þ

k;q

where the C kq terms are the Racah spherical tensors and the Bkq terms are the crystal field parameters. The electronic structure of the dopant ions were evaluated using the following Hamiltonian [12–14]:

H ¼ H0 þ

X

F i fi þ n

2;4;6

þ

X

T i ti þ

2;3;4;6;7;8

X

X

sL þ aLðL þ 1Þ þ bGðG2 Þ þ cðG7 Þ

M i mi þ

0;2;4

X

Pi pi þ HCF

ð2Þ

2;4;6

H0 represents the spherical symmetric Hamiltonian due to the interactions of the 4f electrons with the nucleus. All the following terms up to HCF are free ion parameters (f, a, b, c, Ti, Mi and Pi) and were taken from Ref. [14]. The last term HCF is the crystal field Hamiltonian given by (1). The energy levels were then computed within this framework, using the SPECTRA code [15]. 3. Results and discussion In calculating the energy involved in doping Sr3Al2O6 with rareearth ions, the defect formation energy is firstly calculated (see Table 1). Kroger–Vink notation [16] is employed to identify the defects. The formation energy is defined as the energy difference between the defective and the perfect lattice and it is calculated in the limit of the infinite dilution approximation, i.e., just one defect is supposed to be formed in a infinite host lattice. The temperature effect was included in the modeling considering just the harmonic approximation. The formation energies themselves are not useful for comparison purposes because they do not include all steps in the incorporation of a particular dopant. Instead, the solution energy is more precise, which is defined as the total energy involved in the doping process, including charge compensation mechanisms if needed. The solution energies were based on a series of solid state reactions involving in doping the material. In Sr3Al2O4 lattice there are two possibilities for the location of the dopant metal ions and several possibilities for charge compen-

sation. Substitution might occur at either the Sr or Al site. When trivalent rare-earth ions substitute at the Al3+ site, no charge compensation is needed, but substitution at Sr2+ sites requires charge compensation. All solution reactions considered in this work are shown in Table 2. The solution energy is used to decide at which site, and with which form of charge compensation, a dopant ion may be accommodated with minimum energy. In all cases the rare-earth ion is supposed to be incorporated via dissociation of M2O3. The solution energy schemes that have been considered here are listed in Table 3. The solution energies were then computed based on the reactions shown in Table 2. As an example, the solution energy, ESol, for reaction (i), RE3+ substitution at the Al3+ site, is given by:

1 1 ESol ¼ EDef þ ElattðAl2 O3 Þ  ElattðM2 O3 Þ 2 2 where the Elatt terms are the lattice energies, defined as the amount of energy necessary to split a full molecular unit of the compound and they are shown in Table 4, and the EDef terms are defect energy. All terms were calculated within the Gulp code [5]. The incorporation of the rare-earth was considered at 0 K, at room temperature (300 K), and 873 K, temperature near the temperature used for preparing the samples (see Table 1). It should be noticed that there are six non-equivalent Sr2+ ions and two non-equivalent Al3+ ions in the Sr3Al2O6 structure. Defects involving Al3+ thus have to be modeled taking these two possible site symmetries into account and for defect involving Sr2+ several combinations are possible, mainly when the defect involves more than one cationic site. In this work all possibility sites were considered. As an example, the defect formed due to the incorporation of the RE3+ at the Sr2+ site with charge compensation by Sr2+ vacancy will have 11 different configurations obtained combining all possible pairs of neighboring Sr sites. Similar approaches were used for all the other defects showing more than one possible configuration. In Table 1 only the formation energies of the configuration that presented the lowest energy were shown. In Table 3, all corresponding solution energies are displayed and these values were obtained for each mechanism according to the reactions presented in Table 2. The lattice energies (energy associated to a single molecule of a compound) that are needed to compute the solution energies according to the reactions in Table 2 are presented in Table 4. From Table 2 it is clear that at 0 K the lowest solution energies for all dopants are for the substitution of the RE3+ dopant at the Sr2+

Table 1 Formation energies for all rare-earth at three different temperatures. Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

0K MAl 2M Sr  V 0Sr M Sr  Sr0Al 2M Sr  O00i 3M Sr  V 000 Al

17.76 24.64 17.26 57.47 7.46

16.84 25.20 16.91 57.50 8.74

16.84 25.05 16.97 57.25 8.11

16.09 26.06 16.38 58.41 9.65

15.59 26.73 15.98 59.14 10.27

15.10 27.34 15.62 59.53 11.32

14.76 27.44 15.50 59.84 11.61

14.18 28.44 14.95 60.92 13.23

13.81 28.88 14.68 61.36 13.99

13.30 29.51 14.30 62.04 15.09

12.69 30.18 13.89 62.76 16.27

12.55 30.38 13.77 62.98 16.61

12.26 30.73 13.56 63.36 17.23

300 K MAl 2M Sr  V 0Sr M Sr  Sr0Al 2M Sr  O00i 3M Sr  V 000 Al

12.99 29.61 12.54 61.65 12.42

12.08 30.16 12.07 61.80 13.46

12.08 29.93 12.13 62.16 11.57

11.34 31.00 12.44 64.13 13.05

10.84 31.67 11.16 64.78 15.12

10.37 32.27 10.80 64.74 16.15

10.03 32.36 10.69 64.36 16.44

9.46 33.36 10.15 64.17 18.05

9.09 33.78 9.89 65.87 18.80

9.05 34.41 9.51 66.54 19.87

7.98 35.06 9.11 67.25 21.05

7.85 35.26 8.99 67.47 21.38

7.56 35.61 8.78 67.84 21.99

873 K MAl 2M Sr  V 0Sr M Sr  Sr0Al 2M Sr  O00i 3M Sr  V 000 Al

5.10 40.91 3.82 70.87 19.66

4.34 37.88 3.44 72.02 21.68

3.30 39.56 2.63 70.98 21.17

3.66 38.33 2.94 71.63 22.42

1.95 39.00 1.56 72.15 22.42

1.88 39.71 2.31 73.13 23.25

1.45 39.69 2.19 73.68 24.50

1.09 40.65 1.63 75.31 25.06

1.05 44.38 1.43 75.07 27.32

0.55 44.24 0.98 74.85 26.56

0.12 44.91 0.85 78.07 30.92

0.90 43.52 1.26 76.72 29.49

0.97 45.03 1.83 77.83 29.24

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M.V.S. Rezende et al. / Optical Materials 32 (2010) 1341–1344 Table 2 Solid state reactions associated to the different schemes for incorporation of trivalent rare-earth ions in the Sr3Al2O4 lattice. (i) Substitution at the Al3+ site (no charge compensation needed)

1 2 M 2 O3

þ AlAl ! M Al þ 12 Al2 O3

(ii) Substitution at the Sr2+ site (charge compensation by Sr2+ vacancies) (iii) Substitution at the Sr2+ site (charge compensation by Sr2+/Al3+ substitution)

M2 O3 þ SrSr ! ð2M Sr  V 000 Sr Þ þ 3SrO

(iv) Substitution at the Sr2+ site (charge compensation by O2 interstitial) (v) Substitution at the Sr2+ site (charge compensation by Al3+ vacancies)

M2 O3 þ 2Sr Sr ! ð2MSr  O00i Þ þ 2SrO

1 2 M 2 O3 3 2 M 2 O3

þ Sr Sr þ AlAl ! ðM Sr  Sr 0Al Þ þ 12 Al2 O3 1 þ 3Sr Sr þ AlAl ! ð3M Sr  V 000 Al Þ þ 3SrO þ þ 2 Al2 O3

Table 3 Solution energies for all rare-earth at several temperatures. Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tin

Yb

Lu

0K MAl 2M Sr  V 0Sr M Sr  Sr0Al 2M Sr  O00i 3M Sr  V 000 Al

3.29 0.46 1.40 0.99 1.02

2.74 0.52 1.41 1.23 0.98

2.67 0.52 1.40 1.26 1.08

2.87 0.81 1.58 1.51 1.41

2.76 0.85 1.58 1.53 1.55

2.64 0.89 1.58 1.64 1.56

2.40 0.93 1.57 1.60 1.56

2.40 0.98 1.59 1.63 1.59

2.30 1.01 1.59 1.66 1.61

2.19 1.07 1.60 1.71 1.63

2.02 1.14 1.61 1.76 1.67

2.01 1.16 1.61 1.77 1.68

1.94 1.19 1.62 1.79 1.69

300 K MAl 2M Sr  V 0Sr M Sr  Sr0Al 2M Sr  O00i 3M Sr  V 000 Al

1.32 1.21 0.88 0.42 0.19

1.88 1.16 0.95 0.24 0.19

1.98 1.15 0.97 0.42 0.21

1.76 0.87 0.33 0.44 0.56

1.85 0.82 0.77 0.38 0.35

1.97 0.78 0.77 0.14 0.36

2.21 0.74 0.77 0.06 0.36

2.20 0.69 0.76 0.51 0.39

2.30 0.65 0.75 0.12 0.41

1.94 0.60 0.74 0.16 0.44

2.57 0.52 0.72 0.15 0.48

2.57 0.50 0.72 0.23 0.49

2.64 0.47 0.71 0.26 0.49

873 K MAl 2M Sr  V 0Sr M Sr  Sr0Al 2M Sr  O00i 3M Sr  V 000 Al

9.05 5.10 5.17 3.46 2.05

9.46 3.85 5.18 3.61 2.29

10.48 4.40 5.58 3.25 2.13

10.79 3.43 5.00 2.91 1.83

9.50 3.52 5.59 2.95 1.68

10.30 3.38 4.93 2.90 1.47

10.62 3.30 4.94 3.01 1.70

10.41 3.24 4.93 3.17 1.41

10.17 4.30 4.90 2.91 1.77

10.27 3.99 4.92 2.57 1.28

10.50 3.92 5.62 3.35 2.04

11.16 3.37 5.76 2.82 1.59

11.00 3.72 5.93 3.03 1.35

Table 4 Lattice energy for the relaxed oxide structures. System

Ce2O3 Pr2O3 Nd2O3 Sm2O3 Eu2O3 Gd2O3 Tb2O3 Dy2O3 Ho2O3 Er2O3 Tm2O3 Yb2O3 Lu2O3 SrO Al2O3

Table 5 Bkq values for the relaxed lattice.

Lattice energy 0K

300 K

873 K

129.30 130.04 129.89 131.79 132.58 133.31 133.51 134.67 135.21 136.02 136.89 137.15 137.60 34.43 158.23

129.21 129.91 129.71 131.63 132.45 133.15 133.36 134.51 135.06 135.85 136.73 136.99 137.44 34.41 157.83

130.22 130.93 130.96 132.65 133.04 134.17 134.39 135.53 136.08 136.88 137.76 138.01 138.47 34.87 158.52

site compensated by Sr vacancy. It is also noted that, with a few exceptions, there is a downward trend in solution energies going along the rare-earth series, which suggests that substitution should become progressively easier with increasing atomic number of the RE3+ ion. On the other hand, at room temperature and at 873 K, the RE3+ dopant clearly tends to substitute at the Al3+ site with the difference to solution energies of the 2M Sr  V 0Sr defect increasing progressively as the atomic number of the dopant increases. Table 6 gives the energy transition predicted for the RE3+ substituting at the Al3+ site at room temperature compared to the experimental, using the crystal field parameters (Bkq values) from Table 5. Previous works do not report a plausible number of experimental lines associated to Eu3+ transitions which could be used to compare with our modeling results. Page et al. [17] showed the

Bkq

Al2 site

B20 B21 B22 B40 B41 B42 B43 B44 B60 B61 B62 B63 B64 B65 B66

433.4026 358.275  465.9954i 581.1303 + 322.982i 581.3694 198.4094  607.0825i 86.0391 + 170.7273i 500.7718 + 597.0685i 92.1408  437.4521i 50.3284 468.4455  456.5843i 828.8022 + 197.1303i 5.77  252.0287i 301.4632 + 610.668i 344.2299 + 581.0394i 4.3194 + 402.6382i

emission spectra for different concentration of Eu3+, but the transitions were not clearly identified. Sharma et al. [18], did not show the emission spectral region associated to the 5D0 ? 7Fj (j = 3, 4) emission lines of the Eu3+ ions and Pan et al. [19] did not describe the 5D0 ? 7F4 in the emission spectra for Eu3+. Thus, all the experimental results available in the literature show a presented a certain degree of difficulty to compare with our results of the Eu3+ at Sr3Al2O6. Therefore, to obtain the better emission spectra, the Sr3Al2O6 doped with Eu3+ were produced in our laboratory through the Sol–Gel Proteic methodology, which is shown in details in previous works [20,21]. Samples doped with Eu3+ were produced at 600 °C for 4 h. Fig. 1 shows the emission spectrum of the Sr3Al2O6:Eu3+ nanopowders excited at 300 nm. The emission peaks correspond to the typical emissions of Eu3+. From that emission

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Table 6 Comparison between the predicted and experimental 4f–4f transitions of Eu3+ in the Sr3Al2O6 matrix at 300 K assuming that the substitution is at the Al2 site accompanied by lattice distortion (Bkq ’s values from Table 4). Experimental (nm)

Experimental– present work (nm)

Calculated (nm)

D%

Terms

579*, 579**

574.96 581.71 583.32 591.71

563.88 570.10 576.69 583.67

1.93 2.00 1.14 1.36

5

604.58 612.19 623.05

593.74 595.12 600.69 604.54 610.78

7.74

628.41 629.68 633.10 634.47 636.80 638.51 642.19

37.47

5

672.09 675.65 677.36 678.88 682.51 685.05 686.31 686.31 688.80

13.15

5

590*, 594**

612*, 612** 618**

* **

653** 659**

637.89 638.47 652.62 659.00

–

692.36 697.62 700.53 706.03 711.60

D0 ? 7F0

5

D0 ? 7F2

It is worth stressing that the calculated values are true predictions using the Bkq values obtained for the relaxed lattice surrounding the dopant (values from Table 3), without any necessary previous knowledge of the spectra of the real system. The percentage differences between the predicted to the experimental results for the 5 D0–7F0 and 5D0–7F1 transition energies of Eu3+ are around 2%. The total splitting of the 5D0 ? 7F2 transitions is predicted to be 18 nm, a value practically identical to the experimental splitting observed in Fig. 1. On the other hand, the total splitting of the 5 D0 ? 7F3 transitions is predicted to be 14 nm while the experimental value is 21 nm and for 5D0 ? 7F4 transition, the total splitting is predicted to be 17 nm while the experimental value is 19 nm. 4. Conclusions

D0 ? 7F3

D0 ? 7F4

Sharma et al. [16]. Pan et al. [17].

We successfully model the energetic of the defect induced by trivalent dopants in Sr3Al2O6. The energy minimization step indicates that at 0 K, the trivalent ions preferentially substitutes at the Sr site with the charge compensation mechanism clearly provided by Sr vacancies. At 300 K and 873 K, on the other hand, all rare-earth ions are likely to be incorporated at the Al sites. The crystal field parameters, Bkq , and transition energy were calculated for the case of Eu3+ substituting at the Al2 site, for which comparable experimental data was measured. It is found that an average agreement around of 7% is obtained between the predicted and experimental values of the transition energies, which gave confidence in the use of this combination of modeling technique for predicting the optical behaviour of rare-earth doped oxides. Acknowledgments The authors are grateful to CNPq CAPES and FINEP for financial support. References

Fig. 1. Emission spectrum of Sr3Al2O6 doped with 3% Eu3+ under UV 300 nm excitation. Experimental and calculated transitions.

spectrum, it is possible to identify all the 5D0 ? 7Fj (j = 0, 1, 2, 3, 4) emission lines. In Table 6 the difference between the predicted and the experimental energy transition of Eu3+ in the Sr3Al2O2 lattice are shown.

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