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Synthesis and luminescent properties of red-emitting phosphors: ZnSiF6·6H2O and ZnGeF6·6H2O doped with Mn4+
Journal of Luminescence 137 (2013) 88–92
Contents lists available at SciVerse ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Synthesis and luminescent properties of red-emitting phosphors: ZnSiF6 6H2O and ZnGeF6 6H2O doped with Mn4 þ b,n ¨ ¨ Mariusz Kubus a, David Enseling b, Thomas Justel , H.-Jurgen Meyer a,nn a
Abteilung f¨ ur Festk¨ orperchemie und theoretische Anorganische Chemie, Institut f¨ ur Anorganische Chemie, Eberhard-Karls-Universit¨ at T¨ ubingen, Auf der Morgenstelle 18, D-72076 T¨ ubingen, Germany b Fachhochschule M¨ unster, Labor f¨ ur Angewandte Materialwissenschaft, Stegerwaldstrasse 39, D-48565 Steinfurt, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 September 2012 Received in revised form 6 December 2012 Accepted 27 December 2012 Available online 4 January 2013
A simple method for the synthesis of red emitting Mn4 þ activated phosphors is presented. The crystal structure of ZnGeF6 6H2O was determined by single crystal X-ray diffraction and was isotopic with NiSnCl6 6H2O (space group R3). Luminescent properties of ZnSiF6 6H2O and ZnGeF6 6H2O doped with manganese(IV) at different concentration levels were studied. The optimal Mn4 þ concentration with respect to the emission intensity was found to be 5 mol% for ZnSiF6 6H2O and 3 mol% for ZnGeF6 6H2O. & 2013 Elsevier B.V. All rights reserved.
Keywords: ZnSiF6 6H2O ZnGeF6 6H2O Mn4 þ Crystal structure Photoluminescence Synthesis
1. Introduction Manganese at different oxidation states is an efficient activator ion for photo- and cathodoluminescence in many solid state compounds. However, in most of applied phosphors manganese is present in the oxidation states (II) or (IV), due to the high efficiency [1]. Manganese(IV) is used as an activator in a large number of luminescent materials, for example in (i) oxides: Y3Al5O12 [2], YAlO3 [3], Gd3Ga5O12 [4], MAl12O19 (where M is Ca, Sr, or Ba) [5], LiGa5O8 [6], Mg2TiO4 [7], (ii) nitrides: GaN [8], or (iii) fluorides: Na2SiF6 [9], K2GeF6 [10]. If a trivalent ion such as aluminum(III) is substituted by manganese(IV), the additional charge is often compensated by co-doping with a divalent ion, e.g. Ca2 þ or Mg2 þ [11]. Most of the Mn4 þ phosphors are synthesized at a high temperature, which requires special equipment and thus causes rather high costs [9]. Novel synthesis methods as well as the study of the photoluminescence properties of hexafluoro complexes doped by manganese(IV) are in the focus of the interest of many research groups at academia and industry due to their potential application in solid state light sources [12]. Red emitting phosphors derived from alkaline hexafluorides, such as K2SiF6:Mn4 þ , exhibit high n
Corresponding author. Fax: þ49 2551 9 62502. Corresponding author. Tel.: þ 49 7071 29 76226; fax: þ 49 7071 29 5702. ¨ E-mail addresses: [email protected] (T. Justel). [email protected] (H.-J. Meyer). nn
0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2012.12.038
PL quantum efficiency and a suitable color point for fluorescent light sources with a high color rendering for red colors [13]. The most common method to synthesize fluorides doped by Mn4 þ is the direct method, i.e. the chemical etching of Si wafers or Ge shots in aqueous HF solution upon an addition of potassium permanganate [1,9,10,14]. Another method of synthesizing doped fluorides is based on the dissolution of an appropriate hexafluoride compound KMF6 (M ¼Si, Ti, and Ge) with K2MnF6 and its subsequent re-crystallization [15,16]. Hexafluorosilicates doped with Mn4 þ may be as well precipitated from a solution of e.g. hexafluorosilicic acid in HF and K2MnF6 by the addition of the appropriate metal sources. K2MnF6 used as a source of Mn4 þ is mostly synthesized through reduction of permanganate by the Bode’s method [17] or with diethylether [18]. Beyond the already described hexafluorides, also hydrates of the hexafluorides can be doped by manganese(IV). Hexafluorides ZnSiF6 6H2O and ZnGeF6 6H2O belong to the group of compounds with the general formula MIISiF6 6H2O (wherein e.g. MII is Co, Ni, or Mn) and are isotopic to NiSnCl6 6H2O (space group R3) [19]. To our best knowledge, this work on the luminescent properties of hexafluoride-hexahydrates doped by different concentrations of manganese(IV) is reported for the first time. The crystal structure of ZnGeF6 6H2O was investigated by single-crystal X-ray diffraction. The purity of all obtained materials was confirmed by powder X-ray diffraction and thermoluminescent properties were determined by photoluminescence spectroscopy.
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2. Experimental K2MnF6 was synthesized according to the method described in the literature [18]. 2.5 g KHF2 (99.99%, Merck) was dissolved in HF (38%, Merck), after 20 min of mixing 2 g KMnO4 were added and the solution was cooled in an ice bath. After 10 min of stirring, 15 drops diethylether were added and the precipitate (K2MnF6) was filtered off and washed with glacial acetic acid and acetone. The obtained yellow powder was dried on air. The synthesis of ZnSiF6 6H2O:Mn4 þ was carried out in a polyethylene beaker. H2SiF6 (34%, Sigma-Aldrich) was dissolved in an aqueous HF solution together with varying concentrations of K2MnF6 (5, 3 and 0.5 mol% of Mn4 þ ). Then, to the above solutions ZnF2 (99%, Sigma-Aldrich), dissolved in HF, was added. The mixture was cooled until ZnSiF6 6H2O:Mn4 þ precipitated. The obtained powder was filtered off and dried on air. ZnGeF6 6H2O:Mn4 þ was synthesized by the same procedure. The appropriate amount of GeO2 (99.99%, Fluka) was dissolved in aq. HF and K2MnF6 (0.5, 3, or 5 mol% of Mn4 þ ) was added. Next, ZnF2 dissolved in HF was added and the mixture was cooled down. The obtained precipitate was filtered off and dried on air. The single crystal of ZnGeF6 6H2O was obtained by slow evaporation of the described mixture. The powder diffraction patterns of all samples were recorded with a powder diffractometer (Stoe STADI-P, Ge monochromator) using CuKa1 radiation. Excitation and emission spectra were recorded on an Edinburgh Instruments FSL920 spectrometer equipped with a 450 W Xe arc lamp, mirror optics for powder samples and a cooled ( 20 1C) single-photon counting photomultiplier from Hamamatsu (R2658P). The correction file for the emission spectra was obtained from a tungsten incandescent lamp certified by the NPL (National Physics Laboratory, UK). For thermal quenching measurements a cryostat ‘‘MicrostatN’’ from Oxford Instruments in the present spectrometer was employed. Liquid nitrogen was used for cooling.
3. Results and discussion The XRD pattern of a typical sample and the corresponding calculated pattern of ZnSiF6 6H2O are shown in Fig. 1. Fig. 2 displays the XRD pattern of ZnGeF6 6H2O:Mn4 þ (5%). All synthesized
Fig. 1. Experimental XRD pattern of ZnSiF6 6H2O:Mn4 þ (5%) (a) and the corresponding pattern calculated from single crystal data [ICSD file no. 34757] (b).
Fig. 2. Experimental XRD pattern of obtained ZnGeF6 6H2O:Mn4 þ (5%) (a) and the corresponding pattern calculated from single crystal data obtained by our group (b).
Table 1 Crystal data and structure refinement for GeF6 Zn(OH2)6. Empirical formula
H12F6GeO6Zn
Formula weight Crystal system Space group
360.06 g mol 1 Trigonal
Unit cell dimensions
R3 (no. 148) a¼ b¼ 948.7(3) pm c ¼982.1(3) pm
Volume Z Temperature Density (calculated) Absorption coefficient F(0 0 0) Crystal shape Crystal size Theta range for data collection Index ranges Reflections collected Independent reflections Data/parameters Goodness-of-fit on F2 Final R indices [I42s(I)] R indices (all data) Largest diff. peak and hole
0.7655(4) nm3 3 220(2) K 2.343 g/cm3 5.389 mm 1 528 Colorless needle 0.44 0.14 0.14 mm3 3.23–25.931 11r h r 11, 11 rkr 11, 12 r lr 12 2669 331 [R(int)¼ 0.0558] 331/31 1.100 R1 ¼0.0288, wR2¼ 0.0662 R1 ¼0.0318, wR2¼ 0.0674 0.459 and 0.500 e/A˚ 3
a ¼ b ¼ 901 g ¼ 1201
substances were crystalline and of single-phase, which means additional phases could not be detected. ZnSiF6 6H2O and ZnGeF6 6H2O have the same structure as NiSiF6 6H2O [20]. ZnGeF6 6H2O consists of octahedral [Zn(H2O)6]2þ and [GeF6]2 units, which are interconnected by hydrogen bonds. The arrangement of the polyhedra is related to CsCl-type structure [21,22]. Crystallographic data for ZnGeF6 6H2O are given in Table 1, the octahedral [Zn(H2O)6]2 þ and [GeF6]2 units are shown in Fig. 3. Atomic coordinates and equivalent isotropic displacement parameters for ZnGeF6 6H2O are reported in Table 2. The positions of the H-atoms were found from a difference Fourier map and refined freely. The crystal structure of ZnGeF6 6H2O was refined with the space group R3 (no. 148) and unit cell dimensions a¼948.7(3), c¼982.1(3) pm for Z¼3. The Si or Ge ions have point symmetry of 3. Due to presence of water molecules in the crystal structure, the compounds are not stable at higher temperatures. That means
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Fig. 3. Coordination environment of zinc (Zn1) and germanium (Ge1) in the crystal structure of ZnGeF6 6H2O.
Fig. 5. Excitation and emission spectra of ZnGeF6 6H2O:Mn4 þ (5%).
Table 2 Atomic coordinates ( 104) and equivalent isotropic displacement parameters (pm2 10 1) for GeF6 Zn(OH2)6. U(eq) is defined as one third of the trace of the orthogonalized Uij tensor.
Ge(1) F(1) Zn(1) O(1)
x
y
z
U(eq)
0 980(4) 0 34(5)
0 758(5) 0 1774(4)
0 1054(2) ½ 3786(4)
18(1) 59(1) 19(1) 29(1)
Fig. 6. Peak intensity of the Mn4 þ emission band at 630 nm (v6) for different Mn4 þ concentrations in ZnSiF6 6H2O and ZnGeF6 6H2O (lexc ¼ 470 nm).
Fig. 4. Excitation and emission spectra of ZnSiF6 6H2O:Mn4 þ (5%).
that from 358 K upwards, dehydration and decomposition of ZnSiF6 6H2O takes place, which leads to the release of gaseous H2O and SiF4 upon the formation of SiO2. At about 442 K ZnSiF6 is converted into ZnF2 [23]. The excitation and emission spectra of hexafluorides doped by manganese(IV) recorded at room temperature are shown in Figs. 4 and 5. The red emission is caused by the Mn4 þ ions located in the [MnF6]2 octahedron. The emission spectra of fluorides doped with Mn4 þ has been already studied by several authors [11,14,24]. The emission spectra were measured by excitation at lexc ¼470 nm and the excitation spectra were obtained for monitoring the emission intensity at lem ¼630 nm. For the excitation spectra two bands at about 360 and 460 nm are observed, caused
by 4A2-4T1 and 4A2-4T2 transitions, respectively [10]. The emission bands present in the region 600–670 nm can be ascribed to the electric-dipol-forbidden transition 2E-4A2 [24]. According to literature the sharp peaks at 609 (n4), 614 (n6), 630 (n6), 634 (n4), and 645 nm (n3) are caused by vibrational modes of the [MnF6]2 octahedra [24], which are well resolved due to the weak electron–phonon coupling for the 2E-4A2 transition. The intensity of the peak at 630 nm was used to compare the emission of the silicon and germanium compound doped with different Mn4 þ concentrations. By the increase of the manganese concentration the emission intensity was also rising. The highest luminescence intensity for ZnSiF6 6H2O and ZnGeF6 6H2O was observed for a Mn4 þ concentration of 5 and 3 mol%, respectively (Fig. 6). The excitation spectra in the temperature range between 100 and 500 K of ZnSiF6 6H2O and ZnGeF6 H2O with 5% of Mn4 þ were monitored for the 630 nm emission (Fig. 7). The intensity drops once the temperature is higher than 300 K and vanished almost completely at 350 K, i.e. at a higher temperature the characteristic photoluminescence of Mn4 þ was not observed. A shift of the bands as function of the temperature was also noticeable and might be caused by the thermal expansion of the host. An explanation for the higher intensity of excitation bands in the case of ZnGeF6 6H2O:Mn4 þ between 150 and 200 K
M. Kubus et al. / Journal of Luminescence 137 (2013) 88–92
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Fig. 7. The excitation spectra of ZnSiF6 6H2O and ZnGeF6 H2O with 5% of Mn4 þ for temperatures between 100 and 500 K (lem ¼630 nm).
Fig. 8. Emission spectra of ZnSiF6 6H2O:Mn4 þ (5%) and ZnGeF6 6H2O:Mn4 þ (5%) in the temperature range between 100 and 500 K.
compared to 100 K cannot be given at this point of our study and needs a more detailed analysis. The emission spectra as function of the applied temperature are presented in Fig. 8. An intense emission of Mn4 þ can only be observed up to 250 K, as well the zero-phonon line at about 620 nm (2E-4A2). At higher temperatures strong thermal quenching of the Mn4 þ luminescence sets in. However, the emission intensity between 350 and 500 K remains almost constant at intensity level of 3% relative to the low-temperature intensity. The overall dependence of the photoluminescence intensity of both compounds is rather similar to each other, which is clearly visible in Fig. 9. The CIE 1931 color coordinates of both Mn4 þ doped compounds for different temperatures are presented in Fig. 10. The chromaticity coordinates of ZnGeF6 6H2O:Mn4 þ (5%) and of ZnSiF6 6H2O:Mn4 þ (5%) are rather stable in the range between 100 and 300 K. The color coordinates of ZnGeF6 6H2O:Mn4 þ (x¼0.69, y¼0.31) and of ZnSiF6 6H2O:Mn4 þ (x ¼0.67, y¼0.32) at 300 K are similar to those of the well-known red emitting phosphor Y2O2S:Eu3 þ (x¼0.64, y¼0.35) but are considerably different from Mg14Ge5O24:Mn4 þ (1%) (x¼0.72, y¼0.27), Mg2 TiO4:Mn4 þ (1%) (x¼ 0.73, y¼0.26) or SrMgAl10O17:Mn4 þ (1.5%) (x ¼0.73, y¼0.27) [25]. This is caused by the fact that the center of the emission line multiplett of ZnGeF6 6H2O:Mn4 þ and of ZnSiF6 6H2O:Mn4 þ are blue-shifted by about 30 nm relative to that of the above mentioned compounds known from literature.
Fig. 9. Integral emission intensity of ZnSiF6 6H2O:Mn4 þ (5%) and ZnGeF6 6H2O:Mn4 þ (5%) for temperatures from 100 to 500 K.
The strong green shift of the color points of both compounds between 300 and 500 K is caused by the strong reduction of the emission intensity on the one hand, and by the presence of a very
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Fig. 10. Section of the CIE 1931 chromaticity diagram, color points as a function of temperature from 100 to 500 K for ZnSiF6 6H2O and ZnGeF6 H2O doped with 5% of Mn4 þ .
broad band emitting but weak component, which is present in all samples, on the other hand. Since the broad emitting band peaks in the green spectral range, a green-shift is a consequence of the quenching of the Mn4 þ luminescence, if the broad band emitting component does not quench, which is indeed the case. We assume that the broad emission band is caused by defect or exciton luminescence.
4. Conclusions The new red emitting phosphors ZnSiF6 6H2O:Mn4 þ and ZnGeF6 6H2O:Mn4 þ were synthesized. Both compounds show intense red photoluminescence, which is characteristic for manganese(IV). The shape and position of emission and excitation bands is almost the same for both hexafluorides. The optimal concentration of Mn4 þ with respect to PL intensity was found to be 5 mol% for ZnSiF6 6H2O and 3 mol% for ZnGeF6 6H2O. The ZnGeF6 6H2O:Mn4 þ compound showed a rather complex dependence of its photoluminescence on temperature. The chromaticity coordinates indicate that the presented compounds are excellent red emitting phosphor for high color rendering light sources, but only at a temperature below 300 K. Due to the observed thermal quenching already at about 300 K, both materials are not of interest as a luminescent converter in fluorescent light sources, such as LEDs.
Acknowledgment ¨ We would like to thank Dr. M. Strobele from Eberhard-Karls¨ Universit¨at Tubingen for the single crystal structure determination.
References [1] T. Arai, S. Adachi, J. Appl. Phys. 110 (2011) 063514. [2] L.A. Risebergt, M.J. Weber, Solid State Commun. 9 (1971) 791. [3] Y. Zhydachevskii, D. Galanciak, S. Kobyakov, M. Berkowski, A. Kamin´ska, A. Suchocki, Y. Zakharko, A. Durygin, J. Phys. Condens. Matter 18 (2006) 11385. [4] D. Galanciak, M. Grinberg, W. Gryk, S. Kobyakov, A. Suchocki, G. Boulon, A. Brenier, J. Phys. Condens. Matter 17 (2005) 7185. [5] H.G. Kang, J.K. Park, C.H. Kim, S.C. Choi, J. Ceram. Soc. Jpn. 117 (2009) 647. [6] R.J.M. Da Fonseca, T. Abritta, Physica B 190 (1993) 327. [7] J. Stade, D. Hahn, R. Dittmann, J. Lumin. 8 (1974) 318. [8] B. Han, R.Y. Korotkov, B.W. Wessels, M.P. Ulmer, Appl. Phys. Lett. 84 (2004) 5320. [9] Y.K. Xu, S. Adachi, J. Appl. Phys. 105 (2009) 013525. [10] S. Adachi, T. Takahashi, J. Appl. Phys. 106 (2009) 013516-013516. [11] J.F. Donegan, T.J. Glynn, G.F. Imbusch, J. Lumin. 36 (1986) 93. [12] E.V. Radkov, et al., U.S. Patent 2010, No.: US 7847309. [13] T. Arai, S. Adachi, Jpn. J. Appl. Phys. 50 (2011) 092401. [14] T. Arai, Y. Arai, T. Takahashi, S. Adachi, J. Appl. Phys. 108 (2010) 063506. [15] A.A. Setlur, E.V. Radkov, C.S. Henderson, J.-H. Her, A.M. Srivastava, N. Karkada, M.S. Kishore, N.P. Kumar, D. Aesram, A. Deshpande, B. Kolodin, L.S. Grigorov, U. Happek, Chem. Mater. 22 (2010) 4076. [16] C.A. Thuesen, A.-L. Barra, J. Glerup, Inorg. Chem. 48 (2009) 3198. [17] H. Bode, H. Jenssen, F. Bandte, Angew. Chem. (1953) 304. [18] W.G. Palmer, Experimental Inorganic Chemistry, University Press, Cambridge, 1954, pp. 484. [19] L. Pauling, Z. Kristallogr. 72 (1930) 482. [20] S. Ray, A. Zalkin, D.H. Templeton, Acta Crystallogr. B29 (1973) 2741. [21] S. Idziak, S.K. Hoffmann, J. Goslar, Acta Phys. Pol. A 108 (2005) 177. [22] A.M. Zoatdinov, R.L. Davidovich, Chem. Phys. Lett. 48 (1977) 443. [23] S. Folek, K. Kowol, J. Therm. Anal. 7 (1975) 199. [24] R. Kasa, Y. Arai, T. Takahashi, S. Adachi, J. Appl. Phys. 108 (2010) 113503. [25] T. Chen, et al., U.S. Patent 2010, No.: 7,846,350 B2.
Contents lists available at SciVerse ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Synthesis and luminescent properties of red-emitting phosphors: ZnSiF6 6H2O and ZnGeF6 6H2O doped with Mn4 þ b,n ¨ ¨ Mariusz Kubus a, David Enseling b, Thomas Justel , H.-Jurgen Meyer a,nn a
Abteilung f¨ ur Festk¨ orperchemie und theoretische Anorganische Chemie, Institut f¨ ur Anorganische Chemie, Eberhard-Karls-Universit¨ at T¨ ubingen, Auf der Morgenstelle 18, D-72076 T¨ ubingen, Germany b Fachhochschule M¨ unster, Labor f¨ ur Angewandte Materialwissenschaft, Stegerwaldstrasse 39, D-48565 Steinfurt, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 September 2012 Received in revised form 6 December 2012 Accepted 27 December 2012 Available online 4 January 2013
A simple method for the synthesis of red emitting Mn4 þ activated phosphors is presented. The crystal structure of ZnGeF6 6H2O was determined by single crystal X-ray diffraction and was isotopic with NiSnCl6 6H2O (space group R3). Luminescent properties of ZnSiF6 6H2O and ZnGeF6 6H2O doped with manganese(IV) at different concentration levels were studied. The optimal Mn4 þ concentration with respect to the emission intensity was found to be 5 mol% for ZnSiF6 6H2O and 3 mol% for ZnGeF6 6H2O. & 2013 Elsevier B.V. All rights reserved.
Keywords: ZnSiF6 6H2O ZnGeF6 6H2O Mn4 þ Crystal structure Photoluminescence Synthesis
1. Introduction Manganese at different oxidation states is an efficient activator ion for photo- and cathodoluminescence in many solid state compounds. However, in most of applied phosphors manganese is present in the oxidation states (II) or (IV), due to the high efficiency [1]. Manganese(IV) is used as an activator in a large number of luminescent materials, for example in (i) oxides: Y3Al5O12 [2], YAlO3 [3], Gd3Ga5O12 [4], MAl12O19 (where M is Ca, Sr, or Ba) [5], LiGa5O8 [6], Mg2TiO4 [7], (ii) nitrides: GaN [8], or (iii) fluorides: Na2SiF6 [9], K2GeF6 [10]. If a trivalent ion such as aluminum(III) is substituted by manganese(IV), the additional charge is often compensated by co-doping with a divalent ion, e.g. Ca2 þ or Mg2 þ [11]. Most of the Mn4 þ phosphors are synthesized at a high temperature, which requires special equipment and thus causes rather high costs [9]. Novel synthesis methods as well as the study of the photoluminescence properties of hexafluoro complexes doped by manganese(IV) are in the focus of the interest of many research groups at academia and industry due to their potential application in solid state light sources [12]. Red emitting phosphors derived from alkaline hexafluorides, such as K2SiF6:Mn4 þ , exhibit high n
Corresponding author. Fax: þ49 2551 9 62502. Corresponding author. Tel.: þ 49 7071 29 76226; fax: þ 49 7071 29 5702. ¨ E-mail addresses: [email protected] (T. Justel). [email protected] (H.-J. Meyer). nn
0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2012.12.038
PL quantum efficiency and a suitable color point for fluorescent light sources with a high color rendering for red colors [13]. The most common method to synthesize fluorides doped by Mn4 þ is the direct method, i.e. the chemical etching of Si wafers or Ge shots in aqueous HF solution upon an addition of potassium permanganate [1,9,10,14]. Another method of synthesizing doped fluorides is based on the dissolution of an appropriate hexafluoride compound KMF6 (M ¼Si, Ti, and Ge) with K2MnF6 and its subsequent re-crystallization [15,16]. Hexafluorosilicates doped with Mn4 þ may be as well precipitated from a solution of e.g. hexafluorosilicic acid in HF and K2MnF6 by the addition of the appropriate metal sources. K2MnF6 used as a source of Mn4 þ is mostly synthesized through reduction of permanganate by the Bode’s method [17] or with diethylether [18]. Beyond the already described hexafluorides, also hydrates of the hexafluorides can be doped by manganese(IV). Hexafluorides ZnSiF6 6H2O and ZnGeF6 6H2O belong to the group of compounds with the general formula MIISiF6 6H2O (wherein e.g. MII is Co, Ni, or Mn) and are isotopic to NiSnCl6 6H2O (space group R3) [19]. To our best knowledge, this work on the luminescent properties of hexafluoride-hexahydrates doped by different concentrations of manganese(IV) is reported for the first time. The crystal structure of ZnGeF6 6H2O was investigated by single-crystal X-ray diffraction. The purity of all obtained materials was confirmed by powder X-ray diffraction and thermoluminescent properties were determined by photoluminescence spectroscopy.
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89
2. Experimental K2MnF6 was synthesized according to the method described in the literature [18]. 2.5 g KHF2 (99.99%, Merck) was dissolved in HF (38%, Merck), after 20 min of mixing 2 g KMnO4 were added and the solution was cooled in an ice bath. After 10 min of stirring, 15 drops diethylether were added and the precipitate (K2MnF6) was filtered off and washed with glacial acetic acid and acetone. The obtained yellow powder was dried on air. The synthesis of ZnSiF6 6H2O:Mn4 þ was carried out in a polyethylene beaker. H2SiF6 (34%, Sigma-Aldrich) was dissolved in an aqueous HF solution together with varying concentrations of K2MnF6 (5, 3 and 0.5 mol% of Mn4 þ ). Then, to the above solutions ZnF2 (99%, Sigma-Aldrich), dissolved in HF, was added. The mixture was cooled until ZnSiF6 6H2O:Mn4 þ precipitated. The obtained powder was filtered off and dried on air. ZnGeF6 6H2O:Mn4 þ was synthesized by the same procedure. The appropriate amount of GeO2 (99.99%, Fluka) was dissolved in aq. HF and K2MnF6 (0.5, 3, or 5 mol% of Mn4 þ ) was added. Next, ZnF2 dissolved in HF was added and the mixture was cooled down. The obtained precipitate was filtered off and dried on air. The single crystal of ZnGeF6 6H2O was obtained by slow evaporation of the described mixture. The powder diffraction patterns of all samples were recorded with a powder diffractometer (Stoe STADI-P, Ge monochromator) using CuKa1 radiation. Excitation and emission spectra were recorded on an Edinburgh Instruments FSL920 spectrometer equipped with a 450 W Xe arc lamp, mirror optics for powder samples and a cooled ( 20 1C) single-photon counting photomultiplier from Hamamatsu (R2658P). The correction file for the emission spectra was obtained from a tungsten incandescent lamp certified by the NPL (National Physics Laboratory, UK). For thermal quenching measurements a cryostat ‘‘MicrostatN’’ from Oxford Instruments in the present spectrometer was employed. Liquid nitrogen was used for cooling.
3. Results and discussion The XRD pattern of a typical sample and the corresponding calculated pattern of ZnSiF6 6H2O are shown in Fig. 1. Fig. 2 displays the XRD pattern of ZnGeF6 6H2O:Mn4 þ (5%). All synthesized
Fig. 1. Experimental XRD pattern of ZnSiF6 6H2O:Mn4 þ (5%) (a) and the corresponding pattern calculated from single crystal data [ICSD file no. 34757] (b).
Fig. 2. Experimental XRD pattern of obtained ZnGeF6 6H2O:Mn4 þ (5%) (a) and the corresponding pattern calculated from single crystal data obtained by our group (b).
Table 1 Crystal data and structure refinement for GeF6 Zn(OH2)6. Empirical formula
H12F6GeO6Zn
Formula weight Crystal system Space group
360.06 g mol 1 Trigonal
Unit cell dimensions
R3 (no. 148) a¼ b¼ 948.7(3) pm c ¼982.1(3) pm
Volume Z Temperature Density (calculated) Absorption coefficient F(0 0 0) Crystal shape Crystal size Theta range for data collection Index ranges Reflections collected Independent reflections Data/parameters Goodness-of-fit on F2 Final R indices [I42s(I)] R indices (all data) Largest diff. peak and hole
0.7655(4) nm3 3 220(2) K 2.343 g/cm3 5.389 mm 1 528 Colorless needle 0.44 0.14 0.14 mm3 3.23–25.931 11r h r 11, 11 rkr 11, 12 r lr 12 2669 331 [R(int)¼ 0.0558] 331/31 1.100 R1 ¼0.0288, wR2¼ 0.0662 R1 ¼0.0318, wR2¼ 0.0674 0.459 and 0.500 e/A˚ 3
a ¼ b ¼ 901 g ¼ 1201
substances were crystalline and of single-phase, which means additional phases could not be detected. ZnSiF6 6H2O and ZnGeF6 6H2O have the same structure as NiSiF6 6H2O [20]. ZnGeF6 6H2O consists of octahedral [Zn(H2O)6]2þ and [GeF6]2 units, which are interconnected by hydrogen bonds. The arrangement of the polyhedra is related to CsCl-type structure [21,22]. Crystallographic data for ZnGeF6 6H2O are given in Table 1, the octahedral [Zn(H2O)6]2 þ and [GeF6]2 units are shown in Fig. 3. Atomic coordinates and equivalent isotropic displacement parameters for ZnGeF6 6H2O are reported in Table 2. The positions of the H-atoms were found from a difference Fourier map and refined freely. The crystal structure of ZnGeF6 6H2O was refined with the space group R3 (no. 148) and unit cell dimensions a¼948.7(3), c¼982.1(3) pm for Z¼3. The Si or Ge ions have point symmetry of 3. Due to presence of water molecules in the crystal structure, the compounds are not stable at higher temperatures. That means
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Fig. 3. Coordination environment of zinc (Zn1) and germanium (Ge1) in the crystal structure of ZnGeF6 6H2O.
Fig. 5. Excitation and emission spectra of ZnGeF6 6H2O:Mn4 þ (5%).
Table 2 Atomic coordinates ( 104) and equivalent isotropic displacement parameters (pm2 10 1) for GeF6 Zn(OH2)6. U(eq) is defined as one third of the trace of the orthogonalized Uij tensor.
Ge(1) F(1) Zn(1) O(1)
x
y
z
U(eq)
0 980(4) 0 34(5)
0 758(5) 0 1774(4)
0 1054(2) ½ 3786(4)
18(1) 59(1) 19(1) 29(1)
Fig. 6. Peak intensity of the Mn4 þ emission band at 630 nm (v6) for different Mn4 þ concentrations in ZnSiF6 6H2O and ZnGeF6 6H2O (lexc ¼ 470 nm).
Fig. 4. Excitation and emission spectra of ZnSiF6 6H2O:Mn4 þ (5%).
that from 358 K upwards, dehydration and decomposition of ZnSiF6 6H2O takes place, which leads to the release of gaseous H2O and SiF4 upon the formation of SiO2. At about 442 K ZnSiF6 is converted into ZnF2 [23]. The excitation and emission spectra of hexafluorides doped by manganese(IV) recorded at room temperature are shown in Figs. 4 and 5. The red emission is caused by the Mn4 þ ions located in the [MnF6]2 octahedron. The emission spectra of fluorides doped with Mn4 þ has been already studied by several authors [11,14,24]. The emission spectra were measured by excitation at lexc ¼470 nm and the excitation spectra were obtained for monitoring the emission intensity at lem ¼630 nm. For the excitation spectra two bands at about 360 and 460 nm are observed, caused
by 4A2-4T1 and 4A2-4T2 transitions, respectively [10]. The emission bands present in the region 600–670 nm can be ascribed to the electric-dipol-forbidden transition 2E-4A2 [24]. According to literature the sharp peaks at 609 (n4), 614 (n6), 630 (n6), 634 (n4), and 645 nm (n3) are caused by vibrational modes of the [MnF6]2 octahedra [24], which are well resolved due to the weak electron–phonon coupling for the 2E-4A2 transition. The intensity of the peak at 630 nm was used to compare the emission of the silicon and germanium compound doped with different Mn4 þ concentrations. By the increase of the manganese concentration the emission intensity was also rising. The highest luminescence intensity for ZnSiF6 6H2O and ZnGeF6 6H2O was observed for a Mn4 þ concentration of 5 and 3 mol%, respectively (Fig. 6). The excitation spectra in the temperature range between 100 and 500 K of ZnSiF6 6H2O and ZnGeF6 H2O with 5% of Mn4 þ were monitored for the 630 nm emission (Fig. 7). The intensity drops once the temperature is higher than 300 K and vanished almost completely at 350 K, i.e. at a higher temperature the characteristic photoluminescence of Mn4 þ was not observed. A shift of the bands as function of the temperature was also noticeable and might be caused by the thermal expansion of the host. An explanation for the higher intensity of excitation bands in the case of ZnGeF6 6H2O:Mn4 þ between 150 and 200 K
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Fig. 7. The excitation spectra of ZnSiF6 6H2O and ZnGeF6 H2O with 5% of Mn4 þ for temperatures between 100 and 500 K (lem ¼630 nm).
Fig. 8. Emission spectra of ZnSiF6 6H2O:Mn4 þ (5%) and ZnGeF6 6H2O:Mn4 þ (5%) in the temperature range between 100 and 500 K.
compared to 100 K cannot be given at this point of our study and needs a more detailed analysis. The emission spectra as function of the applied temperature are presented in Fig. 8. An intense emission of Mn4 þ can only be observed up to 250 K, as well the zero-phonon line at about 620 nm (2E-4A2). At higher temperatures strong thermal quenching of the Mn4 þ luminescence sets in. However, the emission intensity between 350 and 500 K remains almost constant at intensity level of 3% relative to the low-temperature intensity. The overall dependence of the photoluminescence intensity of both compounds is rather similar to each other, which is clearly visible in Fig. 9. The CIE 1931 color coordinates of both Mn4 þ doped compounds for different temperatures are presented in Fig. 10. The chromaticity coordinates of ZnGeF6 6H2O:Mn4 þ (5%) and of ZnSiF6 6H2O:Mn4 þ (5%) are rather stable in the range between 100 and 300 K. The color coordinates of ZnGeF6 6H2O:Mn4 þ (x¼0.69, y¼0.31) and of ZnSiF6 6H2O:Mn4 þ (x ¼0.67, y¼0.32) at 300 K are similar to those of the well-known red emitting phosphor Y2O2S:Eu3 þ (x¼0.64, y¼0.35) but are considerably different from Mg14Ge5O24:Mn4 þ (1%) (x¼0.72, y¼0.27), Mg2 TiO4:Mn4 þ (1%) (x¼ 0.73, y¼0.26) or SrMgAl10O17:Mn4 þ (1.5%) (x ¼0.73, y¼0.27) [25]. This is caused by the fact that the center of the emission line multiplett of ZnGeF6 6H2O:Mn4 þ and of ZnSiF6 6H2O:Mn4 þ are blue-shifted by about 30 nm relative to that of the above mentioned compounds known from literature.
Fig. 9. Integral emission intensity of ZnSiF6 6H2O:Mn4 þ (5%) and ZnGeF6 6H2O:Mn4 þ (5%) for temperatures from 100 to 500 K.
The strong green shift of the color points of both compounds between 300 and 500 K is caused by the strong reduction of the emission intensity on the one hand, and by the presence of a very
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Fig. 10. Section of the CIE 1931 chromaticity diagram, color points as a function of temperature from 100 to 500 K for ZnSiF6 6H2O and ZnGeF6 H2O doped with 5% of Mn4 þ .
broad band emitting but weak component, which is present in all samples, on the other hand. Since the broad emitting band peaks in the green spectral range, a green-shift is a consequence of the quenching of the Mn4 þ luminescence, if the broad band emitting component does not quench, which is indeed the case. We assume that the broad emission band is caused by defect or exciton luminescence.
4. Conclusions The new red emitting phosphors ZnSiF6 6H2O:Mn4 þ and ZnGeF6 6H2O:Mn4 þ were synthesized. Both compounds show intense red photoluminescence, which is characteristic for manganese(IV). The shape and position of emission and excitation bands is almost the same for both hexafluorides. The optimal concentration of Mn4 þ with respect to PL intensity was found to be 5 mol% for ZnSiF6 6H2O and 3 mol% for ZnGeF6 6H2O. The ZnGeF6 6H2O:Mn4 þ compound showed a rather complex dependence of its photoluminescence on temperature. The chromaticity coordinates indicate that the presented compounds are excellent red emitting phosphor for high color rendering light sources, but only at a temperature below 300 K. Due to the observed thermal quenching already at about 300 K, both materials are not of interest as a luminescent converter in fluorescent light sources, such as LEDs.
Acknowledgment ¨ We would like to thank Dr. M. Strobele from Eberhard-Karls¨ Universit¨at Tubingen for the single crystal structure determination.
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