Novel red-emitting garnet Na₂CaTi₂Ge₃O₁₂:Pr³⁺,Na⁺ phosphors

Abstract

A series of red-emitting garnet Na₂CaTi₂Ge₃O₁₂:Pr³⁺,Na⁺ phosphors were synthesized by a high-temperature solid-state reaction. The crystal structure, luminescence properties, decay lifetime, and thermal stability were initially studied. The excitation spectra show one broad band centered at 337 nm and several sharp bands centered at 451, 462, 474 and 488 nm, which are ascribed to the 4f² → 4f¹5d¹ and 4f² → 4f² transitions of Pr³⁺. The emission spectra show narrow band emissions centered at 595, 609, 622 and 630 nm, which correspond to the parity-forbidden intra-configuration 4f² → 4f² transitions of the Pr³⁺ ion. The optimum concentration of Pr³⁺ in Na₂CaTi₂Ge₃O₁₂:Pr³⁺,Na⁺ was determined to be 2 mol%. Upon excitation at 451 nm, the Na₂CaTi₂Ge₃O₁₂:Pr³⁺,Na⁺ phosphors exhibit an intense red emission at 609 nm with CIE chromaticity coordinates of (0.6524, 0.3457). The wider color gamut of the as-synthesized Na₂CaTi₂Ge₃O₁₂:Pr³⁺,Na⁺ is much higher than that of commercially available materials, such as CaSi₅N₈:Eu²⁺ and CaAlSiN₃:Eu²⁺. The results indicate that the novel red-emitting phosphors could be a potential candidate for display applications.

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Introduction

Rare-earth doped inorganic materials have attracted a great deal of attention because of their strong luminescent properties, high chemical and thermal stability. In recent years, Pr³⁺-doped inorganic materials have been intensively studied and applied in many luminescent materials due to their color-tunable nature over a very wide range from the UV region to the red region via a change in the coordinating environment of the activator, Pr³⁺, in the host lattice.^1–5 Liu et al.⁶ reported a highly stable red-emitting oxynitride β-SiAlON:Pr³⁺ phosphor for application in light-emitting diodes. In addition, the excitation spectra of β-SiAlON:Pr³⁺ with a broad band in the VUV region and several sharp lines in the 440–520 region could be excited by Xe atoms and InGaN blue LED chips. Jeon et al.⁷ demonstrated that a Ce³⁺–Pr³⁺ co-doped YAG phosphor, which giving a sharp emission peak at 610 nm through a ¹D₂ → ³H₄ transition of Pr³⁺ ions. By fabrication with a blue chip, the CRI values are as high as 83, which is higher than that of the conventional combination of a blue chip and the YAG:Ce phosphor with a fair value of 71. Zhang et al.⁸ reported a white-light long-lasting Sr₂SiO₄:Pr³⁺ phosphor. The Sr₂SiO₄:Pr³⁺ exhibits a white-light afterglow, which is composed of bluish purple (peaking at 390 nm), green (peaking at 535 nm) and red (peaking at 604 nm) light from the transitions of 4f → 5d, ³P₀ → ³H₅ and ¹D₂ → ³H₄ of Pr³⁺, and the afterglow can last over 40 min after the irradiation at 254 nm. In our previous work,⁹ we studied the broad-band UVC-emitting Ca₉Y(PO₄)₇:Pr³⁺ phosphors and concluded that the PL spectra of Ca₉Y(PO₄)₇:0.2Pr³⁺ show two broad emission bands at 230–340 nm, centered at 250 and 275 nm; these bands are attributed to the transition of Pr³⁺ ions from 5d to 4f levels. The luminescence intensity of the UVC-emitting Ca₉Y(PO₄)₇:0.2Pr³⁺ phosphor is almost 1.5 times that of the LaPO₄:0.1Pr³⁺ phosphor and the phosphor exhibits an excellent thermal quenching stability greater than that of the LaPO₄:0.1Pr³⁺ phosphor.

To the best of our knowledge, there have been no reports yet on the luminescence and thermal properties of Pr³⁺-activated Na₂CaTi₂Ge₃O₁₂ phosphors. In this paper, we report the luminescence properties, critical energy transfer distance-R_c between Pr³⁺ ions, thermal stability, and activation energy E_a of red-emitting Na₂CaTi₂Ge₃O₁₂:Pr³⁺,Na⁺ phosphors.

Experimental

Materials and synthesis

A series of Pr³⁺-doped garnet Na_2+x(Ca_1−2xPr_x)Ti₂Ge₃O₁₂ (x = 0–0.05 mol) phosphors were synthesized by a conventional high-temperature solid-state reaction in which the constituent raw materials Na₂CO₃ (≧99.8%, Sigma-Aldrich), CaCO₃ (99.9%, Aldrich), TiO₂ (≧99%, Sigma-Aldrich), GeO₂ (≧99.99%, Aldrich) and Pr₂O₃ (99.9%, Alfa) were weighed in stoichiometric proportions. The powder reactants were blended and ground thoroughly in an agate mortar, and the homogeneous mixture was transferred to an alumina crucible and calcined in a furnace at 950 °C for 8 h under a reducing atmosphere. The products were then cooled to room temperature in the furnace, ground, and pulverized for further measurements.

Material characterization

All crystal structure compositions were checked for phase formation by using powder X-ray diffraction (XRD) analysis with a Bruker AXS D8 advanced automatic diffractometer with Cu Kα radiation (λ = 1.5418 Å) over the angular range 10° ≦ 2θ ≦ 80°, operating at 40 kV and 40 mA. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra of the samples were analyzed by using a Spex Fluorolog-3 Spectrofluorometer equipped with a 450-W Xe light source. Time-resolved measurements were performed with a tunable nanosecond optical-parametric-oscillator/Q-switch-pumped YAG:Nd³⁺ laser system (NT341/1/UV, Ekspla). Emission transients were collected with a nanochromater (SpectraPro-300i, ARC), detected with a photomultiplier tube (R928HA, Hamamatsu), connected to a digital oscilloscope (LT372, LeCrop) and transferred to a computer for kinetic analysis. The Commission International de I'Eclairage (CIE) chromaticity coordinates for all samples were measured by a Laiko DT-101 color analyzer equipped with a CCD detector (Laiko Co., Tokyo, Japan). Thermal quenching measurements were investigated using a heating apparatus (THMS-600) in combination with a Jobin-Yvon Spex, Model FluoroMax-3 spectrophotometer.

Results and discussion

Crystal structure

All Powder X-ray diffraction patterns of Na₂CaTi₂Ge₃O₁₂ and Na₂CaTi₂Ge₃O₁₂:0.05Pr³⁺,0.05Na⁺ phosphors are shown in Fig. 1. All phase purity of the as-prepared phosphors was investigated and matched well with ICSD file no. 15418,¹⁰ indicating that the host nor Pr³⁺/Na⁺-doped ions did not generate any impurities or induce significant changes in the Na₂CaTi₂Ge₃O₁₂ host structure.

Fig. 1 Powder XRD patterns of Na₂CaTi₂Ge₃O₁₂ and Na₂CaTi₂Ge₃O₁₂:0.05Pr³⁺,0.05Na⁺. (Na₂CaTi₂Ge₃O₁₂ file no. ICSD 59722).

The crystal structure of Na₂CaTi₂Ge₃O₁₂ is shown in Fig. 2. Na₂CaTi₂Ge₃O₁₂ was first reported by Durif,¹¹ it crystallizes in the cubic space group Ia3̄d (230) with unit cell parameters of a = b = c = 12.359 Å, α = β = γ = 90°, V = 1887.77 ų, and N = 8. In the Na₂CaTi₂Ge₃O₁₂ structure, the crystal structure of Na₂CaTi₂Ge₃O₁₂ has three types of coordination-polyhedra linking in a complex manner; a tetrahedron shares its edges with two dodecahedra, an octahedron with six dodecahedra and a dodecahedron with two tetrahedra, four octahedra and four other dodecahedra, and the linkage between tetrahedra and octahedra is made by mutually sharing all of the corners.¹² Each cation has different coordination environments: Na⁺ and Ca²⁺ occupy the dodecahedral site with occupancy parameters of 0.6667 and 0.3333; Ti⁴⁺ occupies the octahedral site; Ge⁴⁺ occupies the tetrahedral site by the O²⁻ ions. The ionic radii of the eight-coordinated Na⁺ and Ca²⁺ are 1.18 and 1.12 Å; six-coordinated Ti⁴⁺ is 0.74 Å. The ionic radii for eight-coordinated Pr³⁺ is 1.126 Å. Therefore, based on the effective ionic radii of cations with different coordination numbers, in this study, we have proposed that Pr³⁺ ions are expected to randomly occupy a dodecahedral site in the Na₂CaTi₂Ge₃O₁₂ crystal structure.

Fig. 2 The crystal structure of Na₂CaTi₂Ge₃O₁₂.

Photoluminescence properties

Fig. 3 shows the PL/PLE spectra of the Na₂CaTi₂Ge₃O₁₂:Pr³⁺ (λ_ex: 451 nm, λ_em: 609 nm) phosphor. The excitation spectra (PLE) were monitored at an emission wavelength of 609 nm for the ¹D₂ → ³H₄ transition of Pr³⁺ ions. The asymmetric broad band absorption in the 240–410 nm region centered at 337 nm is assigned to the 4f² → 4f¹5d¹ transitions of Pr³⁺. The sharp peaks from 430 to 500 nm centered at 451(³H₄ → ³P₂), 462(³H₄ → ¹I₆), 474(³H₄ → ³P₁) and 488(³H₄ → ³P₀) nm were attributed to the 4f² → 4f² transitions of Pr³⁺.¹³ Upon 451 nm excitation, the PL spectra indicate that the as-synthesized Na₂CaTi₂Ge₃O₁₂:Pr³⁺ phosphor exhibited four narrow red-emitting bands peaking at 595 (³P₁ → ³H₆), 609 (¹D₂ → ³H₄), 622 (³P₀ → ³H₆) and 630 (³P₁ → ³F₂) nm, which correspond to the parity-forbidden intra-configuration 4f² → 4f² transitions of the Pr³⁺ ion.^14,15 The strongest emission line at 609 nm contributes to the ¹D₂ → ³H₄ transition of the Pr³⁺ ions in the red region.

Fig. 3 PL/PLE spectra of the Na₂CaTi₂Ge₃O₁₂:Pr³⁺,Na⁺ (λ_ex: 451 nm, λ_em: 609 nm) phosphor.

Fig. 4 shows the concentration dependence of the emission intensity of Na₂Ca_1−2xTi₂Ge₃O₁₂:xPr³⁺,xNa⁺ (x = 0.005–0.05) with various concentrations of Pr³⁺. Upon 451 nm excitation, the photoluminescence (PL) intensity was observed to increase when the Pr³⁺-dopant concentration increased up to x < 0.02 mol, and the optimal concentration of the Pr³⁺-doped content was found to be 0.02 mol. Concentration quenching was observed for samples with concentrations of Pr³⁺ ions higher than 0.02 mol, and the PL intensity was found to decrease with an increase in Pr³⁺-doped content. The critical distance of energy transfer, R_c, was calculated by using the concentration quenching method. The critical distance between Pr³⁺ ions in the Na₂CaTi₂Ge₃O₁₂ phosphor can be calculated by the following equation:¹⁶

where V is the volume of the unit cell, x_c is the critical concentration of the Pr³⁺ ion, and N is the number of host cations in the unit cell. For the Na₂CaTi₂Ge₃O₁₂ crystal structure analytic and experimental values were V = 1887.77 ų and N = 8. Thus, the R_Pr–Pr of Na₂CaTi₂Ge₃O₁₂:xPr³⁺,xNa⁺ was determined to be 44.84, 35.59, 31.09, 28.25, 26.22, 24.67, 22.42 and 20.81 Å for x = 0.005, 0.01, 0.015, 0.02, 0.025, 0.03, 0.04 and 0.05, respectively. The critical concentration (x_c) of Pr³⁺ in the Na₂CaTi₂Ge₃O₁₂ host was found to be 0.02 mol. Therefore, the critical distance of energy transfer was calculated to be 28.25 Å.

Fig. 4 Concentration dependence of the emission intensity of Na₂CaTi₂Ge₃O₁₂:xPr³⁺,xNa⁺ (x = 0.005–0.05) phosphors under 451 nm excitation.

Decay lifetime properties

The decay curves of Na₂CaTi₂Ge₃O₁₂:xPr³⁺ phosphors excited at 451 nm and 488 nm, monitored at 609 nm were investigated and are shown in Fig. 5. The corresponding luminescent decay times can be fitted well with a single-order exponential decay curve by the following equation:¹⁷

I = A₁ exp(−t/τ₁) (2)

where I is the luminescence intensity; A₁ is a constant; t is the time; and τ₁ is lifetime for exponential components. Upon 451 nm excitation, the decay times (τ) were calculated to be 62.89, 56.37 and 52.51 μs for Pr^{3+ 3}H₄ → ³P₂ energy level with x = 0.01, 0.03, and 0.05. For ³H₄ → ³P₀ energy level, the decay times were determined to be 72.16, 60.55, and 52.65 μs for x = 0.01, 0.03, and 0.05, respectively. This result indicates that when excited at the ³H₄ → ³P₂ or ³H₄ → ³P₀ energy level, the decay life times were found to become faster with increasing Pr³⁺ content (x), whereas the decay lifetime of the ³H₄ → ³P₀ energy level at 488 nm was observed to be slower than the ³H₄ → ³P₂ energy level (451 nm). The good fit of the results by an exponential decay with a single component indicates that there is only one crystallographically distinct Ca²⁺ site in the Na₂CaTi₂Ge₃O₁₂ lattice, which is consistent with the crystal structure of Na₂CaTi₂Ge₃O₁₂.¹⁸

Fig. 5 Decay curves of Pr³⁺ emission for Na₂CaTi₂Ge₃O₁₂:xPr³⁺,xNa⁺ (x = 0.005–0.05) phosphors under excitation at (a) 451 nm, (b) 488 nm, monitored at 609 nm.

Chromaticity coordinate properties

The chromaticity coordinates (x, y) for Na₂CaTi₂Ge₃O₁₂:xPr³⁺,xNa⁺ phosphors were measured to be (0.6504, 0.3481), (0.6516, 0.3466), (0.6507, 0.3474), (0.6524, 0.3457), (0.6527, 0.3455), (0.6517, 0.3463), (0.6512, 0.3466), and (0.6502, 0.3475) with x = 0.005, 0.01, 0.015, 0.02, 0.025, 0.03, 0.04, and 0.05, respectively. These results indicate that the CIE chromaticity coordinate of the red-emitting Na₂CaTi₂Ge₃O₁₂:xPr³⁺,xNa⁺ phosphor experience a smaller shift upon increasing the Pr³⁺-dopant molar concentration. The CIE chromaticity diagram and chromaticity coordinates (x, y) of the red-emitting Na₂CaTi₂Ge₃O₁₂:Pr³⁺,Na⁺ (NCTG:Pr³⁺,Na⁺), CaAlSiN₃:Eu²⁺ and Ca₂Si₅N₈:Eu²⁺ phosphors are shown in Fig. 6. The CIE chromaticity coordinates of Na₂CaTi₂Ge₃O₁₂:0.02Pr³⁺,0.02Na⁺,CaAlSiN₃:Eu²⁺ (BR102C; Mitsubishi Chemical Corporation) and Ca₂Si₅N₈:Eu²⁺ (ZYP650G3; Nakamura Yuji S&T) are (0.652, 0.346), (0.619, 0.380) and (0.647, 0.352), respectively. Nevertheless, the dominant wavelengths of the red-emitting Na₂CaTi₂Ge₃O₁₂:0.02Pr³⁺,0.02Na⁺ phosphors are longer than those of CaAlSiN₃:Eu²⁺, and Ca₂Si₅N₈:Eu²⁺ phosphors. The result indicate that the Na₂CaTi₂Ge₃O₁₂:0.02Pr³⁺,0.02Na⁺ phosphor has a wider color gamut than those of CaAlSiN₃:Eu²⁺, and Ca₂Si₅N₈:Eu²⁺ phosphors in display applications with the same combination of blue and green phosphors.

Fig. 6 CIE chromaticity diagram and chromaticity coordinates (x, y) of red emission Na₂CaTi₂Ge₃O₁₂:Pr³⁺,Na⁺ (blue circle), CaAlSiN₃:Eu²⁺ (black circle) and Ca₂Si₅N₈:Eu²⁺ (green circle) phosphors under 451 nm excitation.

Thermal properties

Fig. 7 shows the temperature dependence of the relative emission intensity as a function of the temperature (25–200 °C) of the Na₂CaTi₂Ge₃O₁₂:0.02Pr³⁺,0.02Na⁺ phosphor under 451 nm excitation. The relative emission intensity of four narrow emission bands at 595, 609, 622 and 630 nm from the ³P₁ → ³H₆, ¹D₂ → ³H₄, ³P₀ → ³H₆ and ³P₁ → ³F₂ transitions decrease with increasing temperature in the range of 25 °C–200 °C. The temperature decays were monitored at the 609 nm peak, decays of 16%, 29%, 40%, 50%, 62%, 72% and 81% were observed at 50, 75, 100, 125, 150, 175 and 200 °C, respectively. The emission wavelength of the Na₂CaTi₂Ge₃O₁₂:0.02Pr³⁺,0.02Na⁺ phosphor at 630 nm (³P₁ → ³F₂) has a higher thermal quenching than that at 595 nm (³P₁ → ³H₆) with increasing temperature. As seen in the inset of Fig. 5, the relationship of the relative emission intensities with temperature can be used to calculate the activation energy (E_a) from thermal quenching using the following equation:¹⁹where I₀ and I are the luminescence intensity of the Na₂CaTi₂Ge₃O₁₂:0.02Pr³⁺,0.02Na⁺ phosphor at room temperature and the testing temperature (25–200 °C), respectively, A is a constant, and k_B is the Boltzmann constant (8.617 × 10⁻⁵ eV K⁻¹). The E_a was calculated to be 0.1094 eV.

Fig. 7 Temperature dependence of the relative emission intensity as a function of temperatures of Na₂CaTi₂Ge₃O₁₂:0.02Pr³⁺,0.02Na⁺. The inset shows the activation energy (E_a) of the Na₂CaTi₂Ge₃O₁₂:0.02Pr³⁺,0.02Na⁺ phosphor.

Conclusions

In summary, we synthesized novel red-emitting garnet Na₂CaTi₂Ge₃O₁₂:Pr³⁺,Na⁺ phosphors by solid-state reactions. The corresponding luminescence properties, decay lifetime, crystal structure as well as critical distance of energy transfer and thermal stability were initially investigated. Na₂CaTi₂Ge₃O₁₂:Pr³⁺,Na⁺ phosphors exhibit one broad and four sharp excitation bands between 240–410 nm and 430–500 nm, which were due to the 4f² → 4f¹5d¹ and 4f² → 4f² transitions of Pr³⁺. The optimum Pr³⁺-dopant concentration of these phosphors is 0.02 mol and the emission centered at 609 nm corresponds to the parity-forbidden intra-configuration 4f² → 4f² transitions of Pr³⁺ with CIE = (0.6524, 0.3457). The critical distance R_c between the Pr³⁺ ions was calculated to be 28.25 Å by using the concentration quenching methods and the activation energy E_a was calculated to be 0.1094 eV by thermal quenching. These results indicate that Na₂CaTi₂Ge₃O₁₂:Pr³⁺,Na⁺ is a potential red-emitting phosphor for display applications.

Acknowledgements

This research was supported by the Industrial Technology Research Institute under contract no. D354DM4400 (Y. T. Y.) and in part by the National Science Council of Taiwan under contract no. NSC 102-2622-E-007-016-CC1 (W. R. L.).

Notes and references

1. T. Jüstel, P. Huppertz, W. Mayr and D. U. Wiechert, J. Lumin., 2004, 106, 225.
2. A. P. Vink, P. Dorenbos and C. W. E. van Eijk, Phys. Rev. B: Condens. Matter, 2002, 66, 075118.
3. S. kück and I. sokólska, Appl. Phys. A: Mater. Sci. Process., 2003, 77, 469.
4. X. He, M. Guan, Z. Li, T. Shang, N. Lian and Q. Zhou, J. Am. Ceram. Soc., 2011, 94, 2483.
5. Y. Zorenko, V. Gorbenko, E. Mihokova, M. Nikl, K. Nejezchleb, A. Vedda, V. Kolobanove and D. Spassky, Radiat. Meas., 2007, 42, 521.
6. T. C. Liu, B. M. Cheng, S. F. Hu and R. S. Liu, Chem. Mater., 2011, 23, 3698.
7. H. S. Jang, W. B. Im, D. C. Lee, D. Y. Jeon and S. S. Kim, J. Lumin., 2007, 126, 371.
8. L. Zhang, X. Zhou, H. Zeng, H. Chen and X. Dong, Mater. Lett., 2008, 62, 2539.
9. C. H. Huang, T. M. Chen and B. M. Cheng, Inorg. Chem., 2011, 50, 6552.
10. ICSD file no. 15418.
11. A. Durif and G. Maupin, Acta Crystallogr, 1961, 14, 440.
12. A. Nakatsuka, Y. Ikuta, A. Yoshiasa and K. Iishi, Mater. Res. Bull., 2004, 39, 949.
13. Y. Ji, J. Cao, Z. Zhu, J. Li, Y. Wang and C. Tu, Mater. Express, 2011, 1, 231.
14. B. Xu, J. Liu, C. Song, H. Luo, Y. J. Peng and X. Yu, J. Am. Ceram. Soc., 2012, 95, 250.
15. A. Lazarowska, S. Mahlik, M. Grinberg, T. C. Liu and R. S. Liu, Opt. Mater., 2013, 35, 2001.
16. D. Geng, G. Li, M. Shang, D. Yang, Y. Zhang, Z. Cheng and J. Lin, J. Mater. Chem., 2012, 22, 14262.
17. R. Pang, C. Li, L. Shi and Q. Su, J. Phys. Chem. Solids, 2009, 70, 303.
18. W. R. Liu, C. H. Huang, C. P. Wu, Y. C. Chiu, Y. T. Yeh and T. M. Chen, J. Mater. Chem., 2011, 21, 6869.
19. C. H. Huang and T. M. Chen, Inorg. Chem., 2011, 50, 5725.

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