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Advances in OptoElectronics
Volume 2014, Article ID 674780, 9 pages
Research Article

Preparation of Compensation Ions Codoped SrTiO3:Pr3+ Red Phosphor with the Sol-Gel Method and Study of Its Luminescence Enhancement Mechanism

School of Information Science and Technology, Northwest University, Xi’an 710127, China

Received 30 June 2014; Revised 14 November 2014; Accepted 20 November 2014; Published 14 December 2014

Academic Editor: Jung Y. Huang

Copyright © 2014 Dan Guo et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


SrTiO3:Pr3+ is the most representative titanate matrix red phosphor for field emission display (FED). The red luminous efficiency of SrTiO3:Pr3+ will be greatly improved after the compensation ions codoping, so SrTiO3:Pr3+ red phosphor has been a research focus at home and abroad. SrTiO3:Pr3+, SrTiO3:Pr3+, Mg2+, and SrTiO3:Pr3+, Al3+ phosphors are synthesized by a new sol-gel method. Crystal structure, spectral characteristics, and luminescence enhancement mechanism of the sample were studied by XRD and PL spectra. The results showed that after co-doped, SrTiO3:Pr3+ phosphor is single SrTiO3 cubic phase, the main emission front is located at 614 nm, corresponding to Pr3+ ions 1D23H4 transition emission. SrTiO3:Pr3+, Mg2+ and SrTiO3:Pr3+, Al3+ phosphor luminescence intensity is enhanced, but the main luminescence mechanism is not changed. Acceptor impurity = Mg2+, Al3+ will replace Ti bit after being doped into the crystal lattice to form charge compensation corresponding defect centers to reduce the demand of Sr2+ or Ti3+ vacancy. While Sr-doped Pr will make lattice distortion and transition energy of 4f-5d is very sensitive to crystal electric field changes around Pr atom. Doping different impurities will make electric field distribution around the icon have a different change. It increases energy transfer of 4f-5d transition and improves the luminous intensity of SrTiO3:Pr3+ red phosphor.

1. Introduction

Field emission display (FED) is a new display technology, with its high quality, low cost, large area of ​​attractive advantages, achieving broad development prospects. Compared with cathode-ray tubes (CRT), FED has the high image quality. At the same time, FED has the thinness of liquid crystal display (LCD) and the large area characteristics of plasma display (PDP). FED has a considerable advantage in luminous efficiency, brightness, viewing angle, and power consumption way. In addition, FED also has a high resolution, fast response, high temperature resistant harsh, antivibration shock, weak electromagnetic radiation, and low production cost and is easy to implement digital display and so on.

Research found that SrTiO3:Pr3+ [13] is the most representative for FED display titanate matrix red phosphor in the perovskite structure. SrTiO3:Pr3+ phosphor generates red light when photoluminescence and cathode-ray excitation. And the red light’s coordinates is , , and it is very close to the American NTSC system providing an ideal red light. However, SrTiO3:Pr3+ material has a problem that Pr3+ ion luminous efficiency is low, limiting its application. Study found that compensation ion doping can greatly improve SrTiO3:Pr3+ material luminous intensity. For example, in SrTiO3:Pr3+ material synthesis process, the luminous intensity will increase nearly 200 times more when adding Al(OH)3 or Ga2O3. Domestic Zhang et al. [4] studied Al3+-doping of SrTiO3:0.2%Pr3+: Al ( = 0~0.35) to improve the performance of red phosphors [5] luminescence. When , its luminous intensity reaches the maximum; luminous intensity at this time is probably 20 times before. Yamamoto and Okamoto [6] used high temperature solid method to produce Al3+-doped SrTiO3:Pr3+ red phosphor and characterized and analyzed SrTiO3:Pr3+ red phosphor. They have found generated strontium aluminate in the experiment, as well as proposing charge compensation theory. SrO layer disappears in SrTiO3 lattice, degree of crystallinity is improved, and luminous enhancement has also been improved to some extent.

In this experiment, we use the sol-gel method to produce compensating ions Mg2+, Al3+ codoped SrTiO3:Pr3+ red phosphor. Sol-Gel method has the product’s high uniformity, high purity, low firing temperature and the reaction easy control and so forth. Therefore, the project produces compensating ions doped SrTiO3:Pr3+ red phosphor by the sol-gel method, improving the luminous intensity and persistence time by compensation ions doping method.

2. Experiment Details

In this experiment, Sr(NO3)2 and Ti(OC4H9)4 are used as the precursor; CH3CH2OH is the solvent and CH3COOH is the stabilizer. In the process of sol configuration, we select Pr3+, Al3+, and Mg2+ to be doping elements to make SrTiO3:Pr3+ red phosphor. Weigh moderate amount of Sr(NO3)2, Pr(NO3)3·6H2O, and the impurities (Mg(NO3)2·6H2O or Al(NO3)3·9H2O) to a beaker marked A, and then add 15 mL of pure water to dissolve them and stir for about 20 min to manufacture solution A. Then, corresponding molar ratio of Ti(OC4H9)4 and corresponding proportion of CH3COOH and 10 mL of CH3CH2OH are mixed into another beaker marked solution B. The solution A is added into solution B dropwise keeping churning up the solution B about 30 min until the liquid becomes yellowish sol. After keeping churning up the solution B for 40 min, SrTiO3:Pr3+ solution has been synthesized. (Sr) = (Ti) = 0.03 mol; the molar ratio of Ti(OC4H9)4 and H2O is 40 : 1; volume ratio of CH3COOH and H2O is 1.4 : 1; the amount of CH3CH2OH is 10 mL; aging time and temperature are 1.5 h/32°C, and the sol is turned into gel; dry the gel in 180°C for 8 h and the gel is turned into light yellow powder. The light yellow powder finally is annealed in air at 950°C for 1 hour and the SrTiO3:Pr3+ red phosphor has been made.

In this experiment, the crystal structure and lattice constant of the SrTiO3:Pr3+ red phosphor are investigated by powder X-ray diffractometer (XRD). Photoluminescence (PL) spectrum is taken by Hitachi F4500 luminescence spectrometer. And wavelength of the excitation light is 350 nm, the gap width is 5 nm, and scan range is from 525 nm to 650 nm.

The virtual crystal approximation (VCA) method is carried out to establish undoped SrTiO3 model and Al, Mg codoped SrTiO3:Pr3+ nanophosphors valence-bond model using the CASTEP software package [7]. For example, to SrTiO3:0.2%Pr3+, 25%Al3+, we set Sr and Ti atoms as mixture atoms in the crystal using the VCA method. When Pr3+ replaces Sr2+, the relative concentration of Pr3+ is 0.2% and Sr2+ is 99.8%. And when Al3+ replaces Ti4+, the relative concentration of Al3+ is 25% and Ti4+ is 75%. The interaction between nuclei and electrons is approximated with Vanderbilt ultrasoft pseudopotential [8] and the Perdew and Wang 91 parametrization [9] is taken as the exchange-correlation potential, which is the precise method used for the calculation of electronic structure at present [10]. From the electronic structure point of view (including the Band Structure, DOS, Mulliken population analysis) give a reasonable explanation for compensating ions Al, Mg, Li codoped SrTiO3:Pr3+ system enhancement mechanism of luminescence. We use Pm3m() as space group to establish the crystal cell; crystal cell parameters are  Å, . Plane wave basis with kinetic energy cutoff of 380 eV is used to represent wave functions. And the Brillouin Zone integration is approximated using the special k-points sampling scheme of Monkhorst-Pack [11] and 6 × 6 × 6 k-points grids are used.

3. Theoretical Calculations

After compensating ion Al codoping, SrTiO3:Pr3+ becomes a direct band gap semiconductor from an indirect band gap semiconductor, and the result is shown in Figure 1. Table 1 gives the calculated Mulliken population analysis for the prepared SrTiO3:Pr3+, SrTiO3:Pr3+, Mg2+, and SrTiO3:Pr3+, Al3+ samples. When Pr3+ is single doped, the bond lengths of Sr–O and Ti–O are 0.2760 and 0.1952 nm, and the corresponding bond populations are −0.03 and 0.62, which indicates that the Sr–O band is an ionicity and the Ti–O band is a high degree of covalency. As we have seen, the bond length of Pr–O is 0.2700 nm, which is 0.006 nm less than Sr–O, and a positive population value is 0.12, which obviously indicates that there exists strong interaction between Pr–O and the Pr–O’s ionicity is weaker than Sr–O. In the case of SrTiO3:Pr3+, Mg2+, considerable electron charge density near the Mg atom is redistributed. It is obvious that the calculated bond length of reduces to 0.1902 nm, the bond population of increases to 0.66, and the bond population of Ti–O also increases to 0.65. At the same time, the bond population of Pr–O in SrTiO3:Pr3+, Mg2+ is bigger than Pr–O band in SrTiO3:Pr3+. In the case of SrTiO3:Pr3+, Al3+, the bond length of decreases to 0.1893 nm, smaller than in SrTiO3:Pr3+, Al3+. Meanwhile, the bond population of increases to 0.70 and the bond population of Ti–O also increases to 0.68. As we all know, the calculated positive bond population in Mulliken population analysis indicates that there is strong attraction interaction between two atoms [12]. The larger the bond population is, the stronger the interaction is [12]. So, after codoping Mg2+ and Al3+ to SrTiO3:Pr3+, the interactions of Ti–O and Pr–O band are enhanced, and the interactions of Ti–O and Pr–O band in SrTiO3:Pr3+, Al3+ are stronger than those in SrTiO3:Pr3+, Mg2+.

Table 1: Calculated Mulliken population analysis for SrTiO3:Pr3+: SrTiO3:Pr3+, Mg2+, and SrTiO3:Pr3+, Al3+ samples.
Figure 1: The band structure of different concentrations of Al codoped SrTiO3:Pr3+, Al3+.

In prepared SrTiO3:Pr3+, SrTiO3:Pr3+, Mg2+, and SrTiO3:Pr3+, Al3+, SrTiO3 lattice absorbed ultraviolet leading to electron transition from O 2p occupied valence bands to Ti 3d unoccupied conduction bands. Based on the above discussion, the Ti–O and Pr–O band is enhanced and the corresponding bond lengths decrease by Al(Mg) codoping. So, the charge transfer efficiency is improved, which indirectly indicates that the energy transfer efficiency is improved. Therefore, the luminous efficiency of SrTiO3:Pr3+, Al3+ is improved.

The band gap increases from 3.2 eV to 4.25 eV after being 25%Al3+-doped. It introduced the impurity levels about −7 eV of the valence band and it is shown in Figure 1. As the doping concentration of Al increases, the energy band tends to degeneration and becomes more curved, the electronic effective mass becomes smaller, and it is more benefiting to transfer.

Figure 2 shows the DOS. It can be seen that the DOS peak is moved to right in the top of valence band. Fermi level enters into the valence band because the DOS peak of O p-stated electron is moved to left, and the contribution of O p electrons to the conduction band weakens or even disappears. The DOS peak is moved to left of Sr s-stated and d-stated electron, while p-stated is moved to right. The DOS peak of Ti d-stated electron is lessened and shifted to right. The contribution of Al, Mg of s-stated and p-stated electron is significant to the conduction band. Thus, the second impurity atoms will exclude the contribution of Ti d-stated electron to the conduction band, and the greater the doping concentration, the more apparent.

Figure 2: The DOS of different compensation ions codoped SrTiO3:Pr3+.

4. Results and Discussion

4.1. Luminescent Properties

Figure 3(a) shows the PL excitation spectra of the first group of samples Pr3+ single doped SrTiO3:Pr3+ red phosphor. As we can see from Figure 3(a), the luminescence peak appears at the wavelength of 610 nm; the luminescence peak can be attributed to 1D23H4 transition of Pr3+. Experimental results show that the sample of SrTiO3:0.002molPr3+ luminous intensity is the highest, which is consistent with other related experiments. Generally, samples’ luminescence intensity is not high enough; charge compensation principle described in the relevant literature can explain this phenomenon. Therefore, we introduce compensation ions codoping. Figure 3(b) shows the PL excitation spectra of the second group of samples Mg2+ codoped SrTiO3:Pr3+ red phosphor and Figure 3(c) shows, obviously, the third groups of samples Al3+ codoped SrTiO3:Pr3+ red phosphor. Experimental results show that samples have a luminescence peak which appears at the wavelengths of 611 nm and 614 nm, and the optimum ratio of the samples is Pr3+0.2%mol; Mg2+10%mol, Pr3+0.2%mol; Al3+25%mol, Pr3+0.2%mol.

Figure 3: (a) PL of samples Pr3+ single doped SrTiO3:Pr3+ (A: Pr3+0.1%mol; B: Pr3+0.2%mol; C: Pr3+0.5%mol; D: Pr3+1%mol; E: Pr3+3%mol). (b) PL of samples Mg2+ codoped SrTiO3:Pr3+ (A: Pr3+0.2%mol, Mg2+2%mol; B: Pr3+0.2%mol, Mg2+4%mol; C: Pr3+0.2%mol, Mg2+6%mol; D: Pr3+0.2%mol, Mg2+8%mol; E: Pr3+0.2%mol, Mg2+10%mol; F: Pr3+0.2%mol, Mg2+12%mol). (c) PL of samples Al3+ codoped SrTiO3:Pr3+ (A: Pr3+0.2%mol, Al3+0%mol; B: Pr3+0.2%mol, Al3+1%mol; C: Pr3+0.2%mol, Al3+5%mol; D: Pr3+0.2%mol, Al3+10%mol; E: Pr3+0.2%mol, Al3+15%mol; F: Pr3+0.2%mol, Al3+20%mol; G: Pr3+0.2%mol, Al3+25%mol; H: Pr3+0.2%mol, Al3+30%mol).

Next, we choose separately the maximum luminous intensity of samples in the first three sets of samples to compare with each other, and the result is shown in Figure 4. Experimental results show that different compensation ions codoping does not cause the change of the position of the luminescence peak. And the position of the luminescence peak ranges between 610 nm and 615 nm; however, different compensation ions codoping will improve luminous intensity of the luminescence peak of SrTiO3:Pr3+ red phosphor in varying degrees. Luminous intensity of Al3+ codoped SrTiO3:Pr3+ red phosphor at the luminescence peak is max and increases by appropriately 2 times compared with the sample of Pr3+ single doping. Thus, trivalent Al has a better luminous enhancement function. Finally, we have researched the annealing temperature of Al3+ codoped SrTiO3:Pr3+ red phosphor, and Figure 5 shows the experimental result. As can be seen from Figure 5, the best annealing temperature is 950°C.

Figure 4: PL of different samples (A: Pr3+0.2%mol; B: Pr3+0.2%mol, Mg2+10%mol; C: Pr3+0.2%mol, Al3+25%mol).
Figure 5: PL of different annealing temperature (A: Pr3+0.2%mol, Al3+25%mol, 890°C; B: Pr3+0.2%mol, Al3+25%mol, 950°C; C: Pr3+0.2%mol, Al3+25%mol, 1010°C).

When there is only one impurity Pr3+ in SrTiO3 crystal, Pr3+ replaces Sr2+ to generate positively charged impurity defect centers , which can deter electron transitions of the intra-4f from the 1D2→3H4 of Pr3+ ions [1, 13]. In the sample preparation process, it will produce some oxygen vacancies O2− to neutralize with the positive charge generated by . The amount of Ti4+ vacancies or Sr2+ which is possibly generated by O2− oxygen vacancies is rare, so excess positive charge compensation mechanism of induced by Pr3+ mostly is compensated by the oxygen vacancies O2−; the effect is not very obvious. Due to these reasons, luminous intensity of SrTiO3:Pr3+ materials without codoping is not high. Thus, we choose to introduce the second compensation ions codoping to mainly compensate the positive charge posted by . When the acceptor impurity X is mixed into the crystal lattice, it will replace Ti to form charge compensation pair corresponding to , which reduces the demand of Sr2+ or Ti3+ vacancy. Meanwhile, when Sr is doped to Pr, it will cause lattice distortion. The energy generated by the 4f-5d transition is very sensitive to the crystal electric field changes around the Pr ion. After the incorporation of different acceptor impurities, it will change the crystal electric field distribution, thereby weakening or promoting 4f-5d transition energy transfer.

Obviously, as we see in Figure 6, after Mg codoping, there is a certain influence on the electric field around Pr3+, but it is not obvious. When Al is codoped, there is a very distinct effect on the electric field around Pr3+. Figure 6 is the Electron Density Difference of samples.

Figure 6: The Electron Density Difference of samples.
4.2. XRD

Figures 7(a) and 7(b), respectively, show the two sets of samples SrTiO3:Pr3+: SrTiO3:Pr3+, Mg2+ and SrTiO3:Pr3+, Al3+ XRD patterns. It can be seen from the figure that the overall XRD diffraction peak indicates that SrTiO3 crystal grows along the (110) crystal orientation. Experimental diffraction peak coincides with the standard SrTiO3 sample card (pdf number: 35-0734). The diffraction angles of the samples increase after Al(Mg) codoping as shown in Table 2. However, there exists a impurity peaks. Based on JADE software, the impure peak is caused by TiO2, which shows that a bit of TiO2 exists in the crystal lattice and it causes the lattice changes. And the intensity of the diffraction peaks is different: the samples’ diffraction peak intensity of Al codoping is max, and the samples’ diffraction peak intensity of Mg codoped is min. Based on the JADE software, we get two parts of the sample data shown in Table 2.

Table 2: The first set of samples (110) diffraction peak characteristic values ​​and parameters.
Figure 7: (a) XRD patterns of the first set of samples (A: Pr3+0.2%mol; B: Pr3+0.2%mol, Mg2+10%mol; C: Pr3+0.2%mol, Al3+25%mol). (b) XRD patterns of the second set of samples (A: Pr3+0.2%mol, Al3+1%mol; B: Pr3+0.2%mol, Al3+15%mol; C: Pr3+0.2%mol, Al3+30%mol).

As we can see from Table 2, the diffraction angle has a slight increase from Pr3+ single-doped to Al3+ codoped. This phenomenon indicates that a part of Al and Mg at least is homogeneously incorporated into the SrTiO3 lattice.

5. Conclusions

In a word, Mg2+, Al3+-doped SrTiO3:Pr3+ red phosphors and different annealing temperatures SrTiO3:Pr3+:Al3+ are synthesized by the sol-gel method and the luminescence enhancement mechanism of compensation ions codoping is investigated. It can be seen from the experimental results that the compensation ions codoping can significantly improve SrTiO3:Pr3+ red phosphors’ luminous intensity. When the excitation light is 350 nm and the gap width is 5 nm, the position of the luminescence peak ranges between 610 nm and 615 nm. Trivalent Al luminescence enhancement is the best and divalent Mg also has enhancement, but the effect is not as Al3+. And the optimal annealing temperature is 950°C.

X-ray diffraction (XRD) patterns show that the impurity ions are incorporated into the SrTiO3 crystal lattice uniformly. Experimental diffraction peak coincides with the standard sample SrTiO3 card (pdf number: 35-0734). There exists some impure peaks in experimental results, based on JADE software; this impure peak produces TiO2. In comprehensive experimental results, the impurity ions are incorporated into the crystal lattice and cause some difference but did not cause changes in SrTiO3 main phase structure.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.


This work is supported by the Natural Science Basic Research Plan of China (Program no. 61306009). The authors are also thankful for the support of the Natural Science Basic Research Plan of Shaan Xi Province of China (Program no. 2013KJXX-24).


  1. S. Okamoto, H. Kobayashi, and H. Yamamoto, “Enhancement of characteristic red emission from SrTiO3:Pr3+ by Al addition,” Journal of Applied Physics, vol. 86, no. 10, pp. 5594–5597, 1999. View at Google Scholar
  2. L. Tian and S.-I. Mho, “Enhanced luminescence of SrTiO3:Pr3+ by incorporation of Li+ ion,” Solid State Communications, vol. 125, no. 11-12, pp. 647–651, 2003. View at Google Scholar
  3. B. Yan and K. Zhou, “In-situ sol-gel composition of hybrid precursors to synthesize SrTiO3: Pr3+ red ceramic phosphors,” Journal of Rare Earths, vol. 22, no. 3, pp. 272–274, 2004. View at Google Scholar
  4. P. Zhang, Z. Hong, H. Ge, L. Ren, X. Guo, and X. Fan, “Sol-gel synthesis and characterization of SrTiO3:0.002Pr3+ phosphor,” Rare Metal Materials and Engineering, no. S2, pp. 456–459, 2008. View at Google Scholar
  5. P. T. Dillo, P. Boutinaud, R. Mahiou, and J. C. Cousseins, “Red luminescence in Pr3+-doped calcium titanates,” Physica Status Solidi (a), vol. 160, no. 1, pp. 255–263, 1997. View at Publisher · View at Google Scholar
  6. H. Yamamoto and S. Okamoto, “Efficiency enhancement by aluminum addition to some oxide phosphors for field emission displays,” Displays, vol. 21, no. 2, pp. 93–98, 2000. View at Publisher · View at Google Scholar · View at Scopus
  7. M. D. Segall, P. J. D. Lindan, M. J. Probert et al., “First-principles simulation: ideas, illustrations and the CASTEP code,” Journal of Physics Condensed Matter, vol. 14, no. 11, 2002. View at Publisher · View at Google Scholar
  8. D. Vanderbilt, “Soft self-consistent pseudopotentials in a generalized eigenvalue formalism,” Physical Review B, vol. 41, no. 11, pp. 7892–7895, 1990. View at Google Scholar
  9. J. P. Perdew, J. A. Chevary, S. H. Vosko et al., “Atoms, molecules, solids, and surfaces: Applications of the generalized gradient approximation for exchange and correlation,” Physical Review B, vol. 46, no. 11, pp. 6671–6687, 1992. View at Publisher · View at Google Scholar · View at Scopus
  10. Z.-H. Deng, J.-F. Yan, F.-C. Zhang, X.-W. Wang, J.-P. Xu, and Z.-Y. Zhang, “First-principle calculation of effects of Sb doping on electrical conductivity of SnO2 transparent film,” Acta Photonica Sinica, vol. 36, pp. 110–115, 2007. View at Google Scholar
  11. H. J. Monkhorst and J. D. Pack, “Special points for Brillouin-zone integrations,” Physical Review B, vol. 13, no. 12, pp. 5188–5192, 1976. View at Publisher · View at Google Scholar
  12. J. Zhang, J. Yun, J. Yan, and Z. Deng, “Enhanced luminescence for SrTiO3: Pr3+ phosphors with the addition of Al3+ ions,” Rare Metal Materials and Engineering, vol. 37, pp. 304–307, 2008. View at Google Scholar
  13. S. Itoh, H. Toki, K. Tamura, and F. Kataoka, “A new red-emitting phosphor, SrTiO3:Pr3+, for low-voltage electron excitation,” Japanese Journal of Applied Physics, Part 1: Regular Papers and Short Notes and Review Papers, vol. 38, no. 11, pp. 6387–6391, 1999. View at Google Scholar