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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 21 — Oct. 21, 2013
  • pp: 24742–24752
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Doping concentration induced phase transition in Eu3+-doped β-PbF2 nano-particles

Hui Guo, Hua Yu, Xinxing Zhang, Lifen Chang, Zijian Lan, Yiming Li, and Lijuan Zhao  »View Author Affiliations


Optics Express, Vol. 21, Issue 21, pp. 24742-24752 (2013)
http://dx.doi.org/10.1364/OE.21.024742


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Abstract

Europium doped β-PbF2 nano-particles with different doping concentration are prepared to investigate the site structure of Eu3+ dopants. It is concluded that the site symmetry of Eu3+ dopants in β-PbF2 nano-particles lowers from Oh to D4h with the increase of doping concentration. By X-ray diffraction analysis and photoluminescence spectroscopy study, a doping concentration induced phase transition from lowly doped cubic Pb3EuF9 to highly doped tetragonal PbEuF5 is detected. The intermediate phase of moderately doped nano-particles, which contains both phases mentioned above, is observed for the first time. Moreover, the temperature-dependent intermediate phase analysis suggests that the tetragonal phase is more stable than the cubic phase, which is also confirmed by the first-principle calculations. Our results suggest that the doping concentration induced phase transition in β-PbF2 nano-particles can be used for understanding other Lanthanide-doped nano-particle systems.

© 2013 Optical Society of America

1. Introduction

In recent decades, trivalent Lanthanide ions (Ln3+) doped materials are well-known due to their luminescent properties and have been utilized widely in display field and fluorescent lights. Compared to organic fluorophores and semi-conducting nanocrystals, Ln3+-doped inorganic nano-particles have high photochemical stability, sharp emission bandwidths and large anti-Stokes shifts, therefore can be applied as excellent luminescence materials [1

1. D. K. Chatterjee, M. K. Gnanasammandhan, and Y. Zhang, “Small upconverting fluorescent nanoparticles for biomedical applications,” Small 6(24), 2781–2795 (2010). [CrossRef] [PubMed]

,2

2. J. Chen, C. R. Guo, M. Wang, L. Huang, L. P. Wang, C. C. Mi, J. Li, X. X. Fang, C. B. Mao, and S. K. Xu, “Controllable synthesis of NaYF(4) : Yb,Er upconversion nanophosphors and their application to in vivo imaging of Caenorhabditis elegans,” J. Mater. Chem. 21(8), 2632–2638 (2011). [CrossRef] [PubMed]

]. More recently, they also attract great interests as a new class of bioprobes, because their long-lived and intense emissions offer promising applications to biosensing, biological labeling and imaging technology [3

3. F. Auzel, “Upconversion and anti-Stokes processes with f and d ions in solids,” Chem. Rev. 104(1), 139–174 (2004). [CrossRef] [PubMed]

9

9. M. Haase and H. Schäfer, “Nanopartikel für die Aufwärtskonversion,” Angew. Chem. 123(26), 5928–5950 (2011). [CrossRef]

]. In general, the optical properties of Ln3+ ions are very sensitive to the local environment, and especially the emission intensity of Ln3+-doped materials are closely correlated with the surrounding crystal-field and the crystal structure [10

10. O. Lehmann, K. Kömpe, and M. Haase, “Synthesis of Eu3+-doped core and core/shell nanoparticles and direct spectroscopic identification of dopant sites at the surface and in the interior of the particles,” J. Am. Chem. Soc. 126(45), 14935–14942 (2004). [CrossRef] [PubMed]

,11

11. S. W. Hao, L. Sun, G. Y. Chen, H. L. Qiu, C. Xu, T. N. Soitah, Y. Sun, and C. H. Yang, “Synthesis of monoclinic Na3ScF6:1 mol% Er3+/2 mol% Yb3+ microcrystals by a facile hydrothermal approach,” J. Alloy. Comp. 522, 74–77 (2012). [CrossRef]

]. Therefore, Ln3+ ions are normally used as probes to survey the local structure in luminescent materials [12

12. M. H. V. Werts, R. T. F. Jukes, and J. W. Verhoeven, “The emission spectrum and the radiative lifetime of Eu3+ in luminescent Lanthanide complexes,” Phys. Chem. Chem. Phys. 4(9), 1542–1548 (2002). [CrossRef]

16

16. A. Kar and A. Patra, “Impacts of core-shell structures on properties of lanthanide-based nanocrystals: crystal phase, lattice strain, downconversion, upconversion and energy transfer,” Nanoscale 4(12), 3608–3619 (2012). [CrossRef] [PubMed]

].

The family of Ln3+-doped inorganic nano-particles with fluorite structure, such as PbF2, CaF2, CdF2, SrF2, are widely applied in display devices, lasers and bioassays due to the low phonon frequencies [17

17. Y. Wang and J. Ohwaki, “New transparent vitroceramics codoped with Er3+ and Yb3+ for efficient frequency upconversion,” Appl. Phys. Lett. 63(24), 3268–3270 (1993). [CrossRef]

20

20. V. K. Tikhomirov, K. Driesen, C. Görller-Walrand, and M. Mortier, “Broadband telecommunication wavelength emission in Yb(3+)-Er(3+)-Tm(3+) co-doped nano-glassceramics,” Opt. Express 15(15), 9535–9540 (2007). [CrossRef] [PubMed]

]. As luminescence center, fluorescence properties of Ln3+ dopants are determined by their site symmetries in fluorite structures. However, there is no common agreement on the site symmetry of the dopant Ln3+ ions in fluorite nano-particles has been reached, except the fact that the doping concentration can strongly affect the local environment of Ln3+ dopants. Different doping concentration can induce different site symmetries of Ln3+ ions in nano-particles. The X-ray diffraction (XRD) studies of β-PbF2:Er3+ nano-particles indicate that the segregation of Ln3+ ions induces the lattice parameters modification, i.e. the structural alteration of cubic fluorite phase PbF2 [21

21. M. Mortier, P. Goldner, C. Chateau, and M. Genotelle, “Erbium doped glass-ceramics: concentration effect on crystal structure and energy transfer between active ions ,” J. Alloys Compd. 323&324, 245–249 (2001).

,22

22. M. Beggiora, I. M. Reaney, and M. S. Islam, “Structure of the nanocrystals in oxyfluoride glass ceramics,” Appl. Phys. Lett. 83(3), 467–469 (2003). [CrossRef]

]. By using the upconversion spectroscopy, Wright et al. and Bouffard et al. have intensively studied the effect of doping concentration in system of MF2:Ln3+ (M = Sr, Ca, Cd, Pb, Ba; Ln = Eu, Tm), and concluded that cubic and single-pair are the main site types and the Ln3+-occupied site symmetry is correlated with the concentration of dopants [23

23. R. J. Hamers, J. R. Wietfeld, and J. C. Wright, “Defect chemistry in CaF2:Eu3+,” J. Chem. Phys. 77(2), 683–692 (1982). [CrossRef]

26

26. M. Bouffard, J. P. Jouart, and M. F. Joubert, “Red-to-blue up-conversion spectroscopy of Tm3+ in SrF2, CaF2, BaF2 and CdF2,” Opt. Mater. 14(1), 73–79 (2000). [CrossRef]

]. Méndez-Ramos et al. [27

27. J. Méndez-Ramos, V. Lavín, I. R. Martín, U. R. Rodríguez-Mendoza, V. D. Rodríguez, A. D. Lozano-Gorrín, and P. Núñez, “Site selective study of Eu3+-doped transparent oxyfluoride glass ceramics,” J. Appl. Phys. 94(4), 2295–2301 (2003). [CrossRef]

] have also found that the diluted Eu3+ ions occupy two different sites with high crystalline phase symmetry in moderately doped samples. Driesen et al. [28

28. K. Driesen, V. K. Tikhomirov, and C. Görller-Walrand, “Eu3+ as a probe for rare-earth dopant site structure in nano-glass-ceramics,” J. Appl. Phys. 102(2), 024312 (2007). [CrossRef]

] reported that Eu3+ ions substitutes Pb2+ ions and the Eu3+ ions doping induces an orthorhombic distortion to D4h for the face-centered β-PbF2 structure. However, the experimental studies rely on fluorescence characterization techniques which can only obtain the structural information indirectly. The detailed site symmetry of Ln3+ ions or even the structure of nano-particles at different doping level are rarely studied. Only very recently, a tetragonal structure of PbREF5 in highly Er3+-Yb3+ co-doped β-PbF2 nanocrystals has been proposed by Hu et al. by using the energy dispersive X-ray spectroscopy (EDS) and XRD analysis [29

29. N. Hu, H. Yu, M. Zhang, P. Zhang, Y. Z. Wang, and L. J. Zhao, “The tetragonal structure of nanocrystals in rare-earth doped oxyfluoride glass ceramics,” Phys. Chem. Chem. Phys. 13(4), 1499–1505 (2011). [CrossRef] [PubMed]

]. However the site symmetry of Ln3+ ions at low doping level is still an open question.

By employing Eu3+ ions as an outstanding fluorescent probe [15

15. Y. H. Wang, Y. S. Liu, Q. B. Xiao, H. M. Zhu, R. F. Li, and X. Y. Chen, “Eu3+ doped KYF4 nanocrystals: synthesis, electronic structure, and optical properties,” Nanoscale 3(8), 3164–3169 (2011). [CrossRef] [PubMed]

], we have prepared fluoride nano-particles with different doping concentration to investigate the structure of crystal-field. Site symmetry of Eu3+ ions is identified with photoluminescence spectroscopy and XRD, and the phenomenon of doping concentration induced phase transition is described. Particularly the intermediate phase is observed at moderately doping level for the first time. Furthermore the direct and quantitative XRD characterization and the Rietveld full-pattern fitting algorithm [29

29. N. Hu, H. Yu, M. Zhang, P. Zhang, Y. Z. Wang, and L. J. Zhao, “The tetragonal structure of nanocrystals in rare-earth doped oxyfluoride glass ceramics,” Phys. Chem. Chem. Phys. 13(4), 1499–1505 (2011). [CrossRef] [PubMed]

,30

30. H. Yu, H. Guo, M. Zhang, Y. Liu, M. Liu, and L. J. Zhao, “Distribution of Nd3+ ions in oxyfluoride glass ceramics,” Nanoscale Res. Lett. 7(1), 275 (2012). [CrossRef] [PubMed]

] are employed to confirm our structure models. The detailed local structure and site symmetry are finally obtained, which help to perform the fluorescence modification of nano-particles and provide possibilities of further applications in optical field.

2. Experimental

Precursor oxyfluoride glasses with the composition (50-x)SiO2-40PbF2-10CdF2-xEu2O3 (mole fraction x = 0.05, 0.1, 0.5, 1.5), were prepared by traditional melting-quenching method. With the prepared precursor glasses, glass ceramics (GCs) were obtained by thermal treatment for 48 hours at certain temperature which is determined by differential thermal analysis (DTA). GCs are labeled with the mole fraction x such as 0.05GC. The thermal treatment temperature for 0.5GC was 400 °C, 405 °C and 410 °C, while it was 410 °C treated for the rest. With hydrofluoric acid etching, the corresponding fluoride nano-particles were obtained afterwards [31

31. H. Yu, N. Hu, Y. N. Wang, Z. L. Wang, Z. S. Gan, and L. J. Zhao, “The fabrication of nano-particles in aqueous solution from oxyfluoride glass ceramics by thermal induction and corrosion treatment,” Nanoscale Res. Lett. 3(12), 516–520 (2008). [CrossRef] [PubMed]

].

All the XRD measurements were performed with a Rigaku D/Max-2500 diffractometer (Rigaku Corporation, Tokyo, Japan) using CuKα as the radiation. Rietveld analysis of XRD patterns were carried out with the Fullprof program based on the profile function of pseudo-Voigt with axial divergence asymmetry [32

32. J. Rodriguez-Carvajal, “FULLPROF program for Rietveld, Profile Matching and Integrated Intensities Refinement of X-ray and/or Neutron Data, Satellite Meeting on Powder Diffraction of the XVth Congress of IUCr,” Toulouse, France, p. 127 (1990).

,33

33. T. Roisnel, and J. Rodriguez-Carvajal, “WinPLOTR: A Windows tool for powder diffraction patterns analysis,” in Epdic 7: European Powder Diffraction Pts 1 and 2, 378, 118 (2001).

]. The XRD data in the range from 10° to 135° were collected in step-scan mode with the step width 0.02° (2θ) at a counting time of 1 s per step. High-resolution transmission electron microscope (HRTEM) analysis was performed to observe the morphology of samples on a Philips TECNAI TEM (FEI Co., Netherlands) operating at 200 kV. The emission and site-selective excitation spectra and photoluminescence decays of nano-particles were recorded on an Edinburgh Instruments FLS920 spectrofluoremeter equipped with both continuous (450 W) and pulsed xenon lamps. All measurements were performed at room temperature.

3. Results and discussion

3.1 XRD analysis and concentration induced phase transition

Leśniak suggested a tetragonal structure model with C4v (4mm) point group symmetry, in which the Ln3+ ions are surrounded by eight ligand F- ions and another interstitial F- ion for charge compensation [38

38. K. Leśniak, “Model simulation of the tetragonal symmetry centre of a rare-earth ion in a fluorite lattice,” J. Phys. C Solid State Phys. 19(15), 2721–2727 (1986). [CrossRef]

,39

39. K. Leśniak, “Crystal fields and dopant-ligand separations in cubic centres of rare-earth ions in fluorites,” J. Phys. Condens. Matter 2(25), 5563–5574 (1990). [CrossRef]

]. Beggiora et al. have proved that F- interstitial mechanism is more favorable than Pb vacancy compensation mechanism with computer simulations [22

22. M. Beggiora, I. M. Reaney, and M. S. Islam, “Structure of the nanocrystals in oxyfluoride glass ceramics,” Appl. Phys. Lett. 83(3), 467–469 (2003). [CrossRef]

]. The interstitial F- neighboring with Ln3+ dopant can form a tetragonal crystal-field symmetry, while the site symmetry becomes cubic when F- and Ln3+ are next-neighbor [24

24. S. Mho and J. C. Wright, “Site selective spectroscopy of defect chemistry in CdF2:Eu,” J. Chem. Phys. 77(3), 1183–1192 (1982). [CrossRef]

,40

40. J. M. Baker, W. Hayes, and D. A. Jones, “Paramagnetic resonance of impurities in CaF2,” Proc. Phys. Soc. 73(6), 942–945 (1959). [CrossRef]

42

42. M. J. Weber and R. W. Bierig, “Paramagnetic resonance and relaxation of trivalent rare-earth ions in calcium fluoride. I. resonance spectra and crystal fields,” Phys. Rev. 134(6A), A1492–A1503 (1964). [CrossRef]

]. Our group proposed that two Ln3+ ions are substituted for two Pb2+ sites in a β-PbF2 face-centered cell of highly doped nano-particles [29

29. N. Hu, H. Yu, M. Zhang, P. Zhang, Y. Z. Wang, and L. J. Zhao, “The tetragonal structure of nanocrystals in rare-earth doped oxyfluoride glass ceramics,” Phys. Chem. Chem. Phys. 13(4), 1499–1505 (2011). [CrossRef] [PubMed]

]. By XRD and TEM analysis, we present a concentration induced phase transition from cubic to tetragonal in Fig. 2
Fig. 2 A complete description of the phase transition: the phase transformation from cubic to tetragonal phase in lowly and highly doped nano-particles, respectively.
.

3.2 Photoluminescence spectroscopy analysis

To probe the local structure around Eu3+ dopants in fluorite cell, four samples (0.05GC, 0.1GC, 0.5GC and 1.5GC) were studied with photoluminescence spectroscopy. Figure 3
Fig. 3 Emission spectra of the 5D07Fj (j = 0, 1, 2) transitions with the excitation at 393 nm. The inset shows the photoluminescence decays of the 5D07F1 transitions in 0.05GC, 0.5GC and 1.5GC.
displays the transitions of 5D0 to the 7Fj (j = 0, 1, 2) under 393 nm excitation. The small intensity ratio of electric-dipole (5D07F2) to magnetic-dipole (5D07F1) transition demonstrates that the local site symmetry of Eu3+ ions in crystalline phase is nearly centrosymmetric. Moreover, the magnetic-dipole transition contains a number of Stark components. As shown in Fig. 3, it is found that 1.5GC exhibits two strong Stark splits at 586.5 nm and 591.7 nm. With the decrease of doping concentration, another Stark component at 589.0 nm appears and its intensity increases rapidly. Relatively, the intensities of Stark components at 586.5 nm and 591.7 nm decrease gradually and they can be only observed as two small shoulders of the main 589.0 nm emission until decreasing Eu3+ doping content to be 0.05GC. The inset shows the photoluminescence decays of 5D07F1 transitions (By excitation at 393 nm and monitoring at 589.0 nm). The decays of 0.05GC and 1.5GC can be well fitted with a single exponential function (I(t)=I0exp(t/τ)), which indicates the nearly homogeneous crystal-field environment around Eu3+ ions in a single lattice site [43

43. Q. Ju, Y. S. Liu, R. F. Li, L. Q. Liu, W. Q. Luo, and X. Y. Chen, “Optical spectroscopy of Eu3+-doped BaFCl nanocrystals,” J. Phys. Chem. C 113(6), 2309–2315 (2009). [CrossRef]

]. The photoluminescence lifetimes of 5D0 levels in 0.05GC and 1.5GC are 1.78 ms and 4.41 ms, respectively, suggesting that the distance between Eu3+ ions in 0.05GC nano-particles is much shorter than that in 1.5GC nano-particles. The decay of 0.5GC shows a double exponential behavior with two lifetimesτ1andτ2corresponding to 1.76 ms and 4.63 ms, respectively. τ1has the similar value with the lifetime in 0.05GC, whileτ2and the lifetime in 1.5GC are nearly the same. This result shows that Eu3+ ions occupy the both sites in 0.5GC nano-particles.

In order to reveal the multiple sites of Eu3+ ions in nano-particles, site-selective excitation spectra of 7F05D1 transition of all the three Stark emission peaks (5D07F1) are presented in Fig. 4
Fig. 4 Site-selective excitation spectra of the 7F05D1 transition monitoring at 586.5 nm, 589.0 nm and 591.7 nm.
. According to branching rules and transition selection rules of the 32 point groups [43

43. Q. Ju, Y. S. Liu, R. F. Li, L. Q. Liu, W. Q. Luo, and X. Y. Chen, “Optical spectroscopy of Eu3+-doped BaFCl nanocrystals,” J. Phys. Chem. C 113(6), 2309–2315 (2009). [CrossRef]

], no split of the magnetic-dipole moment bands denotes that the Eu3+ ions reside in the perfect cubic Oh symmetry field. When there are two strong Stark components of this transition appear, the site symmetry of Eu3+ ions lowers to the tetragonal D4h. In 0.05GC, the excitation spectra are nearly identical with only one peak at 523.8 nm implying single-site occupation by Eu3+ ions with Oh point group symmetry. The split peaks (at 523.8 nm and 524.6 nm) in 1.5GC suggest that the site symmetry of Eu3+ alters to tetragonal D4h. The cubic and tetragonal site symmetries are labeled with ‘C site’ and ‘T site’ in figures, respectively. In 0.1GC and 0.5GC, the excitation spectra include all features of 0.05GC and 1.5GC, indicating that both cubic and tetragonal structures exist in these moderately doped nano-particles. These results agree with the XRD analysis.

3.3 Theoretical simulations and quantitative XRD analysis

To confirm our proposed nano-particles structure models, Rietveld method with Fullprof program [32

32. J. Rodriguez-Carvajal, “FULLPROF program for Rietveld, Profile Matching and Integrated Intensities Refinement of X-ray and/or Neutron Data, Satellite Meeting on Powder Diffraction of the XVth Congress of IUCr,” Toulouse, France, p. 127 (1990).

,33

33. T. Roisnel, and J. Rodriguez-Carvajal, “WinPLOTR: A Windows tool for powder diffraction patterns analysis,” in Epdic 7: European Powder Diffraction Pts 1 and 2, 378, 118 (2001).

] was employed for XRD data refinement. The theoretical simulated line spectrum of each existed phase has been displayed for comparisons.

Figure 5(a)
Fig. 5 XRD Rietveld refinements and the corresponding error curve of 0.05GC (a), 1.5GC (b) and 0.5GC (c). The positions of the Bragg reflections are represented by vertical bars (|).The simulated diffraction line spectra of the oxide phase PbSiO3 (d), the cubic phase Pb3EuF9 (e) and tetragonal phase PbEuF5 (f) are presented for comparisons.
shows the observed diffraction patterns of 0.05GC. The XRD patterns of the Pb3EuF9 and PbSiO3 are distinguished and indexed separately. To exclude the effect of PbSiO3, only XRD data of the Pb3EuF9 are refined by the Rietveld method. The corresponding refinement factors Rp (= 6.64%) and Rwp (= 8.81%) indicate that the cubic Pb3EuF9 model is reasonable. The cell parameters of Pb3EuF9 (a = b = c = 5.9517 Å) are obtained from the refinement. The simulation results of 1.5GC are displayed in Fig. 5(b). The corresponding factors Rp (= 5.04%) and Rwp (= 6.49%) indicate a relatively good agreement between the experimental and calculated data. The cell parameters of tetragonal PbEuF5 are a = b = 4.1467 Å and c = 5.8625 Å. Moreover, the value of c/a is close to2, which indicates that the tetragonal PbEuF5 is taken out from a ‘pseudo-cubic’ cell [29

29. N. Hu, H. Yu, M. Zhang, P. Zhang, Y. Z. Wang, and L. J. Zhao, “The tetragonal structure of nanocrystals in rare-earth doped oxyfluoride glass ceramics,” Phys. Chem. Chem. Phys. 13(4), 1499–1505 (2011). [CrossRef] [PubMed]

]. Due to the fact that coexistence of cubic Pb3EuF9 and tetragonal PbEuF5 in 0.5GC, both structure models are adopted in the XRD refinements and their results are shown in Fig. 5(c). The corresponding Rietveld factors are Rp (= 5.17%) and Rwp (= 6.57%), Rb (= 7.35%) and Rf (= 5.13%) of cubic Pb3EuF9 as well as Rb (= 3.38%) and Rf (= 2.14%) of tetragonal PbEuF5, indicating a good agreement between the experimental data and calculated values by our models. The cell parameters of cubic Pb3EuF9 (a = b = c = 5.9461 Å) and that of tetragonal PbEuF5 (a = b = 4.1507 Å, c = 5.8601 Å) are obtained from the refinement of 0.5GC, which are close to those of 0.05GC and 1.5GC. The simulated diffraction line spectra of the oxide phase PbSiO3, the cubic Pb3EuF9 and tetragonal PbEuF5 are shown in Figs. 5(d)-5(f) for comparisons with the experimental data respectively. In 0.05GC, the average deviations of 2θ and relative intensity between theoretical and experimental values are only 4.2*10−3 degree and 2.5*10−2, respectively, and the corresponding values are 2.5*10−3 degree and 2.8*10−2 in 1.5GC. These results quantitatively support our proposed models. In conclusion, our proposed models are precise for analyzing the Eu3+ doped nano-particles and confirm the doping concentration induced phase transition from cubic to tetragonal.

3.4 Thermal stability of proposed structures

The XRD patterns of 0.5GC thermal treated at 400 °C, 405 °C, and 410 °C are shown in Fig. 6
Fig. 6 XRD patterns of 0.5GC with thermal treatment temperature at 400 °C, 405 °C, and 410 °C. The left inset shows the enlarged diffraction peaks and the right inset presents the crystallized fraction of the cubic and tetragonal phase at different thermal treatment temperature.
, with the enlarged diffraction peaks represented in its left inset. The diffraction intensity of the cubic phase increases with the thermal treatment temperature. The parameter R, the area of the cubic or tetragonal crystalline phase contrasting the total area of the XRD diagram calculated by fitting XRD peaks with a Gaussian line, could roughly evaluate the amount of crystalline phase in GCs [44

44. G. Dantelle, M. Mortier, D. Vivien, and G. Patriarche, “Effect of CeF3 addition on the nucleation and up-conversion luminescence in transparent oxyfluoride glass-ceramics,” Chem. Mater. 17(8), 2216–2222 (2005). [CrossRef]

]. As shown in the right inset, the tetragonal phase fraction RT grows slowly while the cubic phase fraction RC increases sharply from about 7% to 16% when the thermal treatment temperature varies from 400 °C to 410 °C. This indicates that tetragonal phase is more favorable at elevated temperatures other than cubic phase.

Density functional theory (DFT) [45

45. B. Delley, “An all-electron numerical method for solving the local density functional for polyatomic molecules,” J. Chem. Phys. 92(1), 508–517 (1990). [CrossRef]

,46

46. B. Delley, “From molecules to solids with the DMol3 approach,” J. Chem. Phys. 113(18), 7756–7764 (2000). [CrossRef]

] calculations were performed using DMol3 program (Accelyrs Inc.). The local density approximation (LDA) using formula of Perdew and Wang (PWC) exchange-correlation functional was employed. Double numeric basis sets supplemented with d-polarization functions (DND) and all-electron calculations were used [45

45. B. Delley, “An all-electron numerical method for solving the local density functional for polyatomic molecules,” J. Chem. Phys. 92(1), 508–517 (1990). [CrossRef]

,47

47. J. P. Perdew and Y. Wang, “Accurate and simple analytic representation of the electron-gas correlation energy,” Phys. Rev. B Condens. Matter 45(23), 13244–13249 (1992). [CrossRef] [PubMed]

]. The binding energy of the tetragonal phase (selecting the ‘pseudo-cubic’ cell for calculation) is 2.327 eV lower than that of the cubic phase, which means that the tetragonal phase is more stable and relatively easier to form at higher doping concentration. This theoretical calculation results are consistent with the experimental phenomenon and also give the reason of no cubic phase structures existing in highly doped nano-particles.

4. Conclusions

By using Eu3+ as a fluorescence probe in Lanthanide-doped β-PbF2 nano-particles, Ln3+ ions are substituted for Pb2+ sites and the doping concentration induces a site symmetry distortion from Oh to D4h. By photoluminescence and XRD study, we conclude that the structure of lowly doped nano-particles is cubic Pb3EuF9 (Oh (m-3m), Pm-3m (NO. 221)). With the increase of doping concentration, the cubic Pb3EuF9 transforms to tetragonal PbEuF5 (D4h (4/mmm), P4/mmm (NO. 123)). Particularly, the coexistence of both structures in moderately doped nano-particles is proposed and confirmed for the first time. The binding energy of the C and T structures differs with about 2.327 eV, which means that the T structure is more stable and easier to form in highly doped materials. Our work represents a significant advance towards a more comprehensive understanding of the site symmetry of Ln3+ ions in fluoride nano-particles, which would benefit the further research on the optical properties, such as fluorescence regulation and control of Ln3+ ions, and have great importance in the applications of this material in optical fields.

Acknowledgments

This work is supported by National Science Fund for Talent Training in Basic Sciences (No. J1103208). The authors are also grateful for the fruitful discussions with Bing Yang and Nan Hu.

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O. Lehmann, K. Kömpe, and M. Haase, “Synthesis of Eu3+-doped core and core/shell nanoparticles and direct spectroscopic identification of dopant sites at the surface and in the interior of the particles,” J. Am. Chem. Soc. 126(45), 14935–14942 (2004). [CrossRef] [PubMed]

11.

S. W. Hao, L. Sun, G. Y. Chen, H. L. Qiu, C. Xu, T. N. Soitah, Y. Sun, and C. H. Yang, “Synthesis of monoclinic Na3ScF6:1 mol% Er3+/2 mol% Yb3+ microcrystals by a facile hydrothermal approach,” J. Alloy. Comp. 522, 74–77 (2012). [CrossRef]

12.

M. H. V. Werts, R. T. F. Jukes, and J. W. Verhoeven, “The emission spectrum and the radiative lifetime of Eu3+ in luminescent Lanthanide complexes,” Phys. Chem. Chem. Phys. 4(9), 1542–1548 (2002). [CrossRef]

13.

J. C. Boyer, F. Vetrone, J. A. Capobianco, A. Speghini, and M. Bettinelli, “Variation of fluorescence lifetimes and Judd-Ofelt parameters between Eu3+ doped bulk and nanocrystalline cubic Lu2O3,” J. Phys. Chem. B 108(52), 20137–20143 (2004). [CrossRef]

14.

A. M. Cross, P. S. May, F. C. J. M. van Veggel, and M. T. Berry, “Dipicolinate sensitization of europium luminescence in dispersible 5%Eu:LaF3 nanoparticles,” J. Phys. Chem. C 114(35), 14740–14747 (2010). [CrossRef]

15.

Y. H. Wang, Y. S. Liu, Q. B. Xiao, H. M. Zhu, R. F. Li, and X. Y. Chen, “Eu3+ doped KYF4 nanocrystals: synthesis, electronic structure, and optical properties,” Nanoscale 3(8), 3164–3169 (2011). [CrossRef] [PubMed]

16.

A. Kar and A. Patra, “Impacts of core-shell structures on properties of lanthanide-based nanocrystals: crystal phase, lattice strain, downconversion, upconversion and energy transfer,” Nanoscale 4(12), 3608–3619 (2012). [CrossRef] [PubMed]

17.

Y. Wang and J. Ohwaki, “New transparent vitroceramics codoped with Er3+ and Yb3+ for efficient frequency upconversion,” Appl. Phys. Lett. 63(24), 3268–3270 (1993). [CrossRef]

18.

P. A. Tick, N. F. Borrellia, L. K. Cornelius, and M. A. Newhouse, “Transparent glass ceramics for 1300 nm amplifier applications,” J. Appl. Phys. 78(11), 6367–6374 (1995). [CrossRef]

19.

K. Driesen, V. K. Tikhomirov, C. Görller-Walrand, V. D. Rodríguez, and A. B. Seddon, “Transparent Ho3+-doped nano-glass-ceramics for efficient infrared emission,” Appl. Phys. Lett. 88(7), 073111 (2006). [CrossRef]

20.

V. K. Tikhomirov, K. Driesen, C. Görller-Walrand, and M. Mortier, “Broadband telecommunication wavelength emission in Yb(3+)-Er(3+)-Tm(3+) co-doped nano-glassceramics,” Opt. Express 15(15), 9535–9540 (2007). [CrossRef] [PubMed]

21.

M. Mortier, P. Goldner, C. Chateau, and M. Genotelle, “Erbium doped glass-ceramics: concentration effect on crystal structure and energy transfer between active ions ,” J. Alloys Compd. 323&324, 245–249 (2001).

22.

M. Beggiora, I. M. Reaney, and M. S. Islam, “Structure of the nanocrystals in oxyfluoride glass ceramics,” Appl. Phys. Lett. 83(3), 467–469 (2003). [CrossRef]

23.

R. J. Hamers, J. R. Wietfeld, and J. C. Wright, “Defect chemistry in CaF2:Eu3+,” J. Chem. Phys. 77(2), 683–692 (1982). [CrossRef]

24.

S. Mho and J. C. Wright, “Site selective spectroscopy of defect chemistry in CdF2:Eu,” J. Chem. Phys. 77(3), 1183–1192 (1982). [CrossRef]

25.

F. J. Weesner, J. C. Wright, and J. J. Fontanella, “Laser spectroscopy of ion-size effects on point-defect equilibria in PbF2:Eu3+,” Phys. Rev. B Condens. Matter 33(2), 1372–1380 (1986). [CrossRef] [PubMed]

26.

M. Bouffard, J. P. Jouart, and M. F. Joubert, “Red-to-blue up-conversion spectroscopy of Tm3+ in SrF2, CaF2, BaF2 and CdF2,” Opt. Mater. 14(1), 73–79 (2000). [CrossRef]

27.

J. Méndez-Ramos, V. Lavín, I. R. Martín, U. R. Rodríguez-Mendoza, V. D. Rodríguez, A. D. Lozano-Gorrín, and P. Núñez, “Site selective study of Eu3+-doped transparent oxyfluoride glass ceramics,” J. Appl. Phys. 94(4), 2295–2301 (2003). [CrossRef]

28.

K. Driesen, V. K. Tikhomirov, and C. Görller-Walrand, “Eu3+ as a probe for rare-earth dopant site structure in nano-glass-ceramics,” J. Appl. Phys. 102(2), 024312 (2007). [CrossRef]

29.

N. Hu, H. Yu, M. Zhang, P. Zhang, Y. Z. Wang, and L. J. Zhao, “The tetragonal structure of nanocrystals in rare-earth doped oxyfluoride glass ceramics,” Phys. Chem. Chem. Phys. 13(4), 1499–1505 (2011). [CrossRef] [PubMed]

30.

H. Yu, H. Guo, M. Zhang, Y. Liu, M. Liu, and L. J. Zhao, “Distribution of Nd3+ ions in oxyfluoride glass ceramics,” Nanoscale Res. Lett. 7(1), 275 (2012). [CrossRef] [PubMed]

31.

H. Yu, N. Hu, Y. N. Wang, Z. L. Wang, Z. S. Gan, and L. J. Zhao, “The fabrication of nano-particles in aqueous solution from oxyfluoride glass ceramics by thermal induction and corrosion treatment,” Nanoscale Res. Lett. 3(12), 516–520 (2008). [CrossRef] [PubMed]

32.

J. Rodriguez-Carvajal, “FULLPROF program for Rietveld, Profile Matching and Integrated Intensities Refinement of X-ray and/or Neutron Data, Satellite Meeting on Powder Diffraction of the XVth Congress of IUCr,” Toulouse, France, p. 127 (1990).

33.

T. Roisnel, and J. Rodriguez-Carvajal, “WinPLOTR: A Windows tool for powder diffraction patterns analysis,” in Epdic 7: European Powder Diffraction Pts 1 and 2, 378, 118 (2001).

34.

C. Bensalem, M. Mortier, D. Vivien, and M. Diaf, “Thermal and optical investigation of EuF3-doped lead fluorogermanate glasses,” J. Non-Cryst. Solids 356(1), 56–64 (2010). [CrossRef]

35.

M. Mortier and F. Auzel, “Rare-earth doped transparent glass-ceramics with high cross-sections ,” J. Non-Cryst. Solids 256&257, 361–365 (1999).

36.

C. Liu and J. Heo, “Electron energy loss spectroscopy analysis on the preferential incorporation of Er3+ ions into fluoride nanocrystals in oxyfluoride glass-ceramics,” J. Am. Ceram. Soc. 95(7), 2100–2102 (2012). [CrossRef]

37.

C. Liu, X. J. Zhao, and J. Heo, “Direct imaging of inhomogeneous distribution of Er3 + ions in lead fluoride nanocrystals,” J. Non-Cryst. Solids 365, 1–5 (2013). [CrossRef]

38.

K. Leśniak, “Model simulation of the tetragonal symmetry centre of a rare-earth ion in a fluorite lattice,” J. Phys. C Solid State Phys. 19(15), 2721–2727 (1986). [CrossRef]

39.

K. Leśniak, “Crystal fields and dopant-ligand separations in cubic centres of rare-earth ions in fluorites,” J. Phys. Condens. Matter 2(25), 5563–5574 (1990). [CrossRef]

40.

J. M. Baker, W. Hayes, and D. A. Jones, “Paramagnetic resonance of impurities in CaF2,” Proc. Phys. Soc. 73(6), 942–945 (1959). [CrossRef]

41.

C. W. Rector, B. C. Pandey, and H. W. Moos, “Electron paramagnetic resonance and optical Zeeman spectra of type II CaF2:Er3+,” J. Chem. Phys. 45(1), 171–179 (1966). [CrossRef]

42.

M. J. Weber and R. W. Bierig, “Paramagnetic resonance and relaxation of trivalent rare-earth ions in calcium fluoride. I. resonance spectra and crystal fields,” Phys. Rev. 134(6A), A1492–A1503 (1964). [CrossRef]

43.

Q. Ju, Y. S. Liu, R. F. Li, L. Q. Liu, W. Q. Luo, and X. Y. Chen, “Optical spectroscopy of Eu3+-doped BaFCl nanocrystals,” J. Phys. Chem. C 113(6), 2309–2315 (2009). [CrossRef]

44.

G. Dantelle, M. Mortier, D. Vivien, and G. Patriarche, “Effect of CeF3 addition on the nucleation and up-conversion luminescence in transparent oxyfluoride glass-ceramics,” Chem. Mater. 17(8), 2216–2222 (2005). [CrossRef]

45.

B. Delley, “An all-electron numerical method for solving the local density functional for polyatomic molecules,” J. Chem. Phys. 92(1), 508–517 (1990). [CrossRef]

46.

B. Delley, “From molecules to solids with the DMol3 approach,” J. Chem. Phys. 113(18), 7756–7764 (2000). [CrossRef]

47.

J. P. Perdew and Y. Wang, “Accurate and simple analytic representation of the electron-gas correlation energy,” Phys. Rev. B Condens. Matter 45(23), 13244–13249 (1992). [CrossRef] [PubMed]

48.

V. K. Tikhomirov, D. Furniss, A. B. Seddon, I. M. Reaney, M. Beggiora, M. Ferrari, M. Montagna, and R. Rolli, “Fabrication and characterization of nanoscale, Er3+-doped, ultratransparent oxy-fluoride glass ceramics,” Appl. Phys. Lett. 81(11), 1937–1939 (2002). [CrossRef]

49.

Q. Luo, X. S. Qiao, X. P. Fan, and X. H. Zhang, “Preparation and luminescence properties of Ce3+ and Tb3+ co-doped glasses and glass ceramics containing SrF2 nanocrystals,” J. Non-Cryst. Solids 356(50–51), 2875–2879 (2010). [CrossRef]

50.

J. J. Pan, R. R. Xu, M. Wang, G. J. Gao, J. M. Chen, L. L. Hu, and J. J. Zhang, “Enhanced 2.0 μm emission in Tm3+/Ho3+ codoped transparent oxyfluoride glass ceramics containing β-PbF2 nano-crystals,” Solid State Commun. 150(1–2), 78–80 (2010). [CrossRef]

51.

D. T. Tu, Y. S. Liu, H. M. Zhu, R. F. Li, L. Q. Liu, and X. Y. Chen, “Breakdown of crystallographic site symmetry in lanthanide-doped NaYF4 crystals,” Angew. Chem. Int. Ed. Engl. 52(4), 1128–1133 (2013). [CrossRef] [PubMed]

52.

C. Bensalem, M. Mortier, D. Vivien, and M. Diaf, “Optical investigation of Eu3+:PbF2 ceramics and transparent glass-ceramics,” Opt. Mater. 33(6), 791–798 (2011). [CrossRef]

53.

M. Gu, Q. C. Gao, S. M. Huang, X. L. Liu, B. Liu, and C. Ni, “Luminescence properties of Pr3+-doped transparent oxyfluoride glass-ceramics containing BaYF5 nanocrystals,” J. Lumin. 132(10), 2531–2536 (2012). [CrossRef]

54.

B. C. Jamalaiah, M. V. Vijaya Kumar, and K. Rama Gopal, “Fluorescence properties and energy transfer mechanism of Sm3+ ion in lead telluroborate glasses,” Opt. Mater. 33(11), 1643–1647 (2011). [CrossRef]

55.

P. Babu, K. H. Jang, E. S. Kim, L. Shi, R. Vijaya, V. Lavín, C. K. Jayasankar, and H. J. Seo, “Optical properties and energy transfer of Dy3+-doped transparent oxyfluoride glasses and glass-ceramics,” J. Non-Cryst. Solids 356(4–5), 236–243 (2010). [CrossRef]

56.

Z. J. Hu, E. Ma, Y. S. Wang, and D. Q. Chen, “Fluorescence property investigations on Er3+-doped oxyfluoride glass ceramics containing LaF3 nanocrystals,” Mater. Chem. Phys. 100(2–3), 308–312 (2006). [CrossRef]

57.

W. J. Zhang, Q. Y. Zhang, Q. J. Chen, Q. Qian, Z. M. Yang, J. R. Qiu, P. Huang, and Y. S. Wang, “Enhanced 2.0 μm emission and gain coefficient of transparent glass ceramic containing BaF2: Ho3+,Tm3+ nanocrystals,” Opt. Express 17(23), 20952–20958 (2009). [CrossRef] [PubMed]

OCIS Codes
(160.2540) Materials : Fluorescent and luminescent materials
(160.5690) Materials : Rare-earth-doped materials
(300.6280) Spectroscopy : Spectroscopy, fluorescence and luminescence
(220.4241) Optical design and fabrication : Nanostructure fabrication

ToC Category:
Materials

History
Original Manuscript: June 27, 2013
Revised Manuscript: September 24, 2013
Manuscript Accepted: September 30, 2013
Published: October 9, 2013

Citation
Hui Guo, Hua Yu, Xinxing Zhang, Lifen Chang, Zijian Lan, Yiming Li, and Lijuan Zhao, "Doping concentration induced phase transition in Eu3+-doped β-PbF2 nano-particles," Opt. Express 21, 24742-24752 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-21-24742


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References

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  11. S. W. Hao, L. Sun, G. Y. Chen, H. L. Qiu, C. Xu, T. N. Soitah, Y. Sun, and C. H. Yang, “Synthesis of monoclinic Na3ScF6:1 mol% Er3+/2 mol% Yb3+ microcrystals by a facile hydrothermal approach,” J. Alloy. Comp.522, 74–77 (2012). [CrossRef]
  12. M. H. V. Werts, R. T. F. Jukes, and J. W. Verhoeven, “The emission spectrum and the radiative lifetime of Eu3+ in luminescent Lanthanide complexes,” Phys. Chem. Chem. Phys.4(9), 1542–1548 (2002). [CrossRef]
  13. J. C. Boyer, F. Vetrone, J. A. Capobianco, A. Speghini, and M. Bettinelli, “Variation of fluorescence lifetimes and Judd-Ofelt parameters between Eu3+ doped bulk and nanocrystalline cubic Lu2O3,” J. Phys. Chem. B108(52), 20137–20143 (2004). [CrossRef]
  14. A. M. Cross, P. S. May, F. C. J. M. van Veggel, and M. T. Berry, “Dipicolinate sensitization of europium luminescence in dispersible 5%Eu:LaF3 nanoparticles,” J. Phys. Chem. C114(35), 14740–14747 (2010). [CrossRef]
  15. Y. H. Wang, Y. S. Liu, Q. B. Xiao, H. M. Zhu, R. F. Li, and X. Y. Chen, “Eu3+ doped KYF4 nanocrystals: synthesis, electronic structure, and optical properties,” Nanoscale3(8), 3164–3169 (2011). [CrossRef] [PubMed]
  16. A. Kar and A. Patra, “Impacts of core-shell structures on properties of lanthanide-based nanocrystals: crystal phase, lattice strain, downconversion, upconversion and energy transfer,” Nanoscale4(12), 3608–3619 (2012). [CrossRef] [PubMed]
  17. Y. Wang and J. Ohwaki, “New transparent vitroceramics codoped with Er3+ and Yb3+ for efficient frequency upconversion,” Appl. Phys. Lett.63(24), 3268–3270 (1993). [CrossRef]
  18. P. A. Tick, N. F. Borrellia, L. K. Cornelius, and M. A. Newhouse, “Transparent glass ceramics for 1300 nm amplifier applications,” J. Appl. Phys.78(11), 6367–6374 (1995). [CrossRef]
  19. K. Driesen, V. K. Tikhomirov, C. Görller-Walrand, V. D. Rodríguez, and A. B. Seddon, “Transparent Ho3+-doped nano-glass-ceramics for efficient infrared emission,” Appl. Phys. Lett.88(7), 073111 (2006). [CrossRef]
  20. V. K. Tikhomirov, K. Driesen, C. Görller-Walrand, and M. Mortier, “Broadband telecommunication wavelength emission in Yb(3+)-Er(3+)-Tm(3+) co-doped nano-glassceramics,” Opt. Express15(15), 9535–9540 (2007). [CrossRef] [PubMed]
  21. M. Mortier, P. Goldner, C. Chateau, and M. Genotelle, “Erbium doped glass-ceramics: concentration effect on crystal structure and energy transfer between active ions,” J. Alloys Compd. 323&324, 245–249 (2001).
  22. M. Beggiora, I. M. Reaney, and M. S. Islam, “Structure of the nanocrystals in oxyfluoride glass ceramics,” Appl. Phys. Lett.83(3), 467–469 (2003). [CrossRef]
  23. R. J. Hamers, J. R. Wietfeld, and J. C. Wright, “Defect chemistry in CaF2:Eu3+,” J. Chem. Phys.77(2), 683–692 (1982). [CrossRef]
  24. S. Mho and J. C. Wright, “Site selective spectroscopy of defect chemistry in CdF2:Eu,” J. Chem. Phys.77(3), 1183–1192 (1982). [CrossRef]
  25. F. J. Weesner, J. C. Wright, and J. J. Fontanella, “Laser spectroscopy of ion-size effects on point-defect equilibria in PbF2:Eu3+,” Phys. Rev. B Condens. Matter33(2), 1372–1380 (1986). [CrossRef] [PubMed]
  26. M. Bouffard, J. P. Jouart, and M. F. Joubert, “Red-to-blue up-conversion spectroscopy of Tm3+ in SrF2, CaF2, BaF2 and CdF2,” Opt. Mater.14(1), 73–79 (2000). [CrossRef]
  27. J. Méndez-Ramos, V. Lavín, I. R. Martín, U. R. Rodríguez-Mendoza, V. D. Rodríguez, A. D. Lozano-Gorrín, and P. Núñez, “Site selective study of Eu3+-doped transparent oxyfluoride glass ceramics,” J. Appl. Phys.94(4), 2295–2301 (2003). [CrossRef]
  28. K. Driesen, V. K. Tikhomirov, and C. Görller-Walrand, “Eu3+ as a probe for rare-earth dopant site structure in nano-glass-ceramics,” J. Appl. Phys.102(2), 024312 (2007). [CrossRef]
  29. N. Hu, H. Yu, M. Zhang, P. Zhang, Y. Z. Wang, and L. J. Zhao, “The tetragonal structure of nanocrystals in rare-earth doped oxyfluoride glass ceramics,” Phys. Chem. Chem. Phys.13(4), 1499–1505 (2011). [CrossRef] [PubMed]
  30. H. Yu, H. Guo, M. Zhang, Y. Liu, M. Liu, and L. J. Zhao, “Distribution of Nd3+ ions in oxyfluoride glass ceramics,” Nanoscale Res. Lett.7(1), 275 (2012). [CrossRef] [PubMed]
  31. H. Yu, N. Hu, Y. N. Wang, Z. L. Wang, Z. S. Gan, and L. J. Zhao, “The fabrication of nano-particles in aqueous solution from oxyfluoride glass ceramics by thermal induction and corrosion treatment,” Nanoscale Res. Lett.3(12), 516–520 (2008). [CrossRef] [PubMed]
  32. J. Rodriguez-Carvajal, “FULLPROF program for Rietveld, Profile Matching and Integrated Intensities Refinement of X-ray and/or Neutron Data, Satellite Meeting on Powder Diffraction of the XVth Congress of IUCr,” Toulouse, France, p. 127 (1990).
  33. T. Roisnel, and J. Rodriguez-Carvajal, “WinPLOTR: A Windows tool for powder diffraction patterns analysis,” in Epdic 7: European Powder Diffraction Pts 1 and 2, 378, 118 (2001).
  34. C. Bensalem, M. Mortier, D. Vivien, and M. Diaf, “Thermal and optical investigation of EuF3-doped lead fluorogermanate glasses,” J. Non-Cryst. Solids356(1), 56–64 (2010). [CrossRef]
  35. M. Mortier and F. Auzel, “Rare-earth doped transparent glass-ceramics with high cross-sections,” J. Non-Cryst. Solids 256&257, 361–365 (1999).
  36. C. Liu and J. Heo, “Electron energy loss spectroscopy analysis on the preferential incorporation of Er3+ ions into fluoride nanocrystals in oxyfluoride glass-ceramics,” J. Am. Ceram. Soc.95(7), 2100–2102 (2012). [CrossRef]
  37. C. Liu, X. J. Zhao, and J. Heo, “Direct imaging of inhomogeneous distribution of Er3 + ions in lead fluoride nanocrystals,” J. Non-Cryst. Solids365, 1–5 (2013). [CrossRef]
  38. K. Leśniak, “Model simulation of the tetragonal symmetry centre of a rare-earth ion in a fluorite lattice,” J. Phys. C Solid State Phys.19(15), 2721–2727 (1986). [CrossRef]
  39. K. Leśniak, “Crystal fields and dopant-ligand separations in cubic centres of rare-earth ions in fluorites,” J. Phys. Condens. Matter2(25), 5563–5574 (1990). [CrossRef]
  40. J. M. Baker, W. Hayes, and D. A. Jones, “Paramagnetic resonance of impurities in CaF2,” Proc. Phys. Soc.73(6), 942–945 (1959). [CrossRef]
  41. C. W. Rector, B. C. Pandey, and H. W. Moos, “Electron paramagnetic resonance and optical Zeeman spectra of type II CaF2:Er3+,” J. Chem. Phys.45(1), 171–179 (1966). [CrossRef]
  42. M. J. Weber and R. W. Bierig, “Paramagnetic resonance and relaxation of trivalent rare-earth ions in calcium fluoride. I. resonance spectra and crystal fields,” Phys. Rev.134(6A), A1492–A1503 (1964). [CrossRef]
  43. Q. Ju, Y. S. Liu, R. F. Li, L. Q. Liu, W. Q. Luo, and X. Y. Chen, “Optical spectroscopy of Eu3+-doped BaFCl nanocrystals,” J. Phys. Chem. C113(6), 2309–2315 (2009). [CrossRef]
  44. G. Dantelle, M. Mortier, D. Vivien, and G. Patriarche, “Effect of CeF3 addition on the nucleation and up-conversion luminescence in transparent oxyfluoride glass-ceramics,” Chem. Mater.17(8), 2216–2222 (2005). [CrossRef]
  45. B. Delley, “An all-electron numerical method for solving the local density functional for polyatomic molecules,” J. Chem. Phys.92(1), 508–517 (1990). [CrossRef]
  46. B. Delley, “From molecules to solids with the DMol3 approach,” J. Chem. Phys.113(18), 7756–7764 (2000). [CrossRef]
  47. J. P. Perdew and Y. Wang, “Accurate and simple analytic representation of the electron-gas correlation energy,” Phys. Rev. B Condens. Matter45(23), 13244–13249 (1992). [CrossRef] [PubMed]
  48. V. K. Tikhomirov, D. Furniss, A. B. Seddon, I. M. Reaney, M. Beggiora, M. Ferrari, M. Montagna, and R. Rolli, “Fabrication and characterization of nanoscale, Er3+-doped, ultratransparent oxy-fluoride glass ceramics,” Appl. Phys. Lett.81(11), 1937–1939 (2002). [CrossRef]
  49. Q. Luo, X. S. Qiao, X. P. Fan, and X. H. Zhang, “Preparation and luminescence properties of Ce3+ and Tb3+ co-doped glasses and glass ceramics containing SrF2 nanocrystals,” J. Non-Cryst. Solids356(50–51), 2875–2879 (2010). [CrossRef]
  50. J. J. Pan, R. R. Xu, M. Wang, G. J. Gao, J. M. Chen, L. L. Hu, and J. J. Zhang, “Enhanced 2.0 μm emission in Tm3+/Ho3+ codoped transparent oxyfluoride glass ceramics containing β-PbF2 nano-crystals,” Solid State Commun.150(1–2), 78–80 (2010). [CrossRef]
  51. D. T. Tu, Y. S. Liu, H. M. Zhu, R. F. Li, L. Q. Liu, and X. Y. Chen, “Breakdown of crystallographic site symmetry in lanthanide-doped NaYF4 crystals,” Angew. Chem. Int. Ed. Engl.52(4), 1128–1133 (2013). [CrossRef] [PubMed]
  52. C. Bensalem, M. Mortier, D. Vivien, and M. Diaf, “Optical investigation of Eu3+:PbF2 ceramics and transparent glass-ceramics,” Opt. Mater.33(6), 791–798 (2011). [CrossRef]
  53. M. Gu, Q. C. Gao, S. M. Huang, X. L. Liu, B. Liu, and C. Ni, “Luminescence properties of Pr3+-doped transparent oxyfluoride glass-ceramics containing BaYF5 nanocrystals,” J. Lumin.132(10), 2531–2536 (2012). [CrossRef]
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