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Mild solvothermal synthesis of complex fluorides Li2BeF4 and LiSrAlF6
Materials Letters 60 (2006) 3822 – 3825 www.elsevier.com/locate/matlet
Mild solvothermal synthesis of complex fluorides Li2BeF4 and LiSrAlF6 Ruinian Hua a,⁎, Huiming Jiang a , Liyan Na a , Chunshan Shi b,⁎ a
b
College of Life Science, Dalian Nationalities University, Dalian 116600, PR China Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China Received 22 December 2005; accepted 31 March 2006 Available online 27 April 2006
Abstract The complex fluorides Li2BeF4 and LiSrAlF6 were synthesized solvothermally at 180–240 °C and characterized by means of X-ray powder diffraction (XRD), Scanning electron microscopy (SEM), thermogravimetric analysis (TGA), infrared spectroscopy (IR) and X-ray photoelectron spectroscopy (XPS). The different influence factors such as solvents, molar ratios of initial mixtures, reaction temperature and reaction time were investigated. The experimental results indicated that Li2BeF4 and LiSrAlF6 powders could be controllably synthesized in the solvothermal process. © 2006 Elsevier B.V. All rights reserved. Keywords: Li2BeF4; LiSrAlF6; Solvothermal synthesis; Characterization
1. Introduction Complex fluorides which were synthesized by mild hydrothermal and solvothermal processes (so called softchemical processes) have attracted growing interest during the last decade [1–4]. The complex fluorides of different structures show various properties such as piezoelectric characteristics [5], ferromagnetic [6], nonmagnetic insulator behaviour [7] and photoluminescence host materials [8], which have been studied extensively. As the complex fluoride Li2BeF4 is a sort of potential liquid tritium breeding material [9] and LiSrAlF6 is a tunable, room-temperature laser material when doped with the ion Cr3+ [10]. Meanwhile, LiSrAlF6 doped with Ce3+ shows UV solid-state laser material that is continuously tunable over 4000 cm− 1, had numerous applications in scientific, engineering, and medical fields [11]. It is well known that complex fluorides can be prepared by conventional solid-state reaction [12], Bridgman–Stockbarger method [13] and high temperature (N400 °C), high-pressure (N100 MPa) hydrothermal technique [14]. High temperature is required to enhance the diffusion between raw solid materials during the solid-state reaction. Meanwhile, this synthetic process requires a complicated set-up
⁎ Corresponding authors. Tel.: +86 411 87633470. E-mail address: [email protected] (R. Hua). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.03.121
because of the corrosive nature of fluoride. The high temperature, high-pressure hydrothermal techniques require special devices. For improving the drawbacks of the solid-state reaction and the high temperature, high-pressure hydrothermal techniques, soft-chemical processes have been investigated. Mild hydrothermal and solvothermal synthesis of complex fluorides at 120–240 °C were accepted to be an effective method for synthesizing complex fluorides [1,3]. The contents of oxygen in complex fluorides synthesized by hydrothermal and solvothermal method is lower than that of the corresponding complex fluorides synthesized by high temperature solid-state reaction [15,16]. In order to develop new routes to synthesize complex fluorides without complicated synthesis apparatus, herein we report a convenient solvothermal method for the synthesis of the complex fluorides Li2BeF4 and LiSrAlF6. The different influence factors such as solvents, molar ratios of initial mixtures, reaction temperature and reaction time were investigated. 2. Experimental Synthesis of Li2BeF4 and LiSrAlF6 powders were carried out in a 20 mL Teflon-lined stainless steel autoclave under autogenous pressure. The starting reactants were LiF (A.R.), BeF2 (A.R.), SrF2 (A.R.), and AlF3·7/2H2O (A.R.). For the synthesis of Li2BeF4, the molar ratios of initial mixtures were
R. Hua et al. / Materials Letters 60 (2006) 3822–3825
3823
Fig. 1. XRD patterns of Li2BeF4 (a) and LiSrAlF6 (b).
2.0 (or 3.0) LiF:1.0 BeF2. The typical synthetic procedure was as follows: 0.02 mol (0.5188 g) LiF and 0.01 mol (0.4701 g) BeF2 were mixed and added into a Teflon-lined autoclave. Then the autoclave was filled with ethylene glycol up to 80% of the total volume. For the synthesis of LiSrAlF6, the molar ratios of the initial mixtures were 1.5 (or 2) LiF:1.0SrF2: 1.0 AlF3. The typical synthetic procedure was as follows: 0.006 mol (0.1556 g) LiF, 0.004 mol (0.5025 g) SrF2 and 0.004 mol (0.5881 g) AlF3·7/2H2O were mixed and added into a Teflon-lined autoclave. Then the autoclave was also filled with ethylene glycol up to 80% of the total volume. The autoclave was sealed into a stainless steel tank and heated in an oven at 180 °C (for the synthesis of Li2BeF4) and 240 °C (for the synthesis of LiSrAlF6) for 24 h. After being cooled to room temperature naturally, the final powder products were filtered off, washed with absolute ethanol and distilled water, and then dried in air at ambient temperature. All products were characterized by X-ray powder diffraction (XRD), using a Japan Rigaku D/max-IIB diffractometer with Cu Kα radiation (λ = 0.1541 nm). The XRD data for index and cell-parameter calculations were collected by a scanning mode with a step of 0.02° in the 2θ range from 10° to 100° and a scanning rate of 4.0° min− 1 with silicon used as an internal standard. Observation of crystallites by SEM was performed on a Hitachi S-570 scanning electron microscopy, aurum was used to coat the particles as a means to reduce charging effects. Thermogravimetric analysis (TGA) was conducted using a DT30 thermogravimetric system in air. IR spectra were obtained with a Magna 560 spectrometer in the range of 400–4000 cm− 1. The samples were pressed KBr pellets for the spectral measurements. X-ray photoelectron spectroscopy (XPS) experiment was conducted with a VGESCALAB MK-II spectrometer using Mg Kα (1253.6 eV) source, C1s (Eb = 285.00 eV) as an internal standard, and at a pressure of 1.33 × 10 − 7 Pa. Elementary analysis was conducted on a POEMS-II inductively coupled plasma–atomic emission spectrometer. 3. Results and discussion 3.1. Synthesis conditions During the synthesis of Li2BeF4 and LiSrAlF6, the Li/Be and Li/Sr/ Al ratios were found to be crucial to the formation, crystallization and
purity of the products. When the molar ratios Li/Be and Li/Sr/Al of mixture was 2 (or 3):1 and 1.5 (or 2):1:1, respectively, ethylene glycol was used as solvent, the pure and well-crystallized Li2BeF4 and LiSrAlF6 products were prepared. When the Li/Be molar ratio is less than 2 or larger than 3, an impurity phase of BeF2 and LiF appeared in the Li2BeF4 products. Meanwhile, when the Li/Sr (immobility of Sr/Al
Table 1 XRD data of Li2BeF4 and LiSrAlF6 Li2BeF4
LiSrAlF6
hkl
dA
I/I1
hkl
dA
I/I1
110 012 211 300 202 220 122 311 113 410 033 322 330 104 502 422 143 250 314 333 432 611 205 125 603 621 244 170 154 262 006 451 713 363 900 336 553
6.6515 4.1602 3.9139 3.8405 3.5228 3.3262 3.1164 3.0074 2.7121 2.5129 2.3504 2.3270 2.2160 2.1878 2.0145 1.9561 1.9194 1.8475 1.8270 1.7775 1.7428 1.7251 1.7048 1.6499 1.6117 1.5720 1.5561 1.5253 1.5150 1.5016 1.4852 1.4531 1.3565 1.3042 1.2790 1.2338 1.2148
24 10 27 22 2 59 5 2 33 26 93 16 100 2 21 3 14 8 2 4 3 2 2 2 14 3 5 6 1 2 7 3 15 7 5 5 6
100 101 102 110 112 104 200 202 114 120 106 122 300 116 124 206 222 126 132 118 224
4.4053 4.0447 3.3384 2.5418 2.2750 2.2118 2.2015 2.0213 1.8030 1.6681 1.5898 1.5818 1.4671 1.6060 1.3944 1.3479 1.2335 1.1903 1.1877 1.1427 1.1377
25 5 100 8 38 22 2 17 31 4 10 14 8 15 6 6 2 8 6 5 4
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R. Hua et al. / Materials Letters 60 (2006) 3822–3825
Fig. 2. SEM photographs of Li2BeF4 (a) and LiSrAlF6 (b).
molar ratio is 1) molar ratio is less than 1 or larger than 2, an impurity phase is obtained in the LiSrAlF6 products. The formation of pure Li2BeF4 and LiSrAlF6 products also depended on the solvents. Ethylene glycol was found to be the most effective solvent for the synthesis of Li2BeF4 and LiSrAlF6. Other solvents were used to synthesize Li2BeF4 and LiSrAlF6, such as n-butanol, ethylenediamine, pyridine and phenol etc., the result reveals that all the other solvents can not form pure and well-crystallized products. Temperature was also an important factor for an effective synthesis. Although Li2BeF4 products could be synthesized at temperature below 180 °C, however, lower temperature requires longer reaction times. For instance, in the LiF– BeF2–ethylene glycol system, Li2BeF4 was obtained after 1 day at 180 °C, but could form the pure crystals of Li2BeF4 at 150 °C for 2 days or at 120 °C for 7 days. In the LiF–SrF2–AlF3–ethylene glycol system, LiSrAlF6 was obtained after 1 day at 240 °C, but could not form the pure crystals of LiSrAlF6 below 240 °C (such as 220 °C) for 7 days. The elemental analysis showed that the actual molar ratios of metal elements in Li2BeF4 and LiSrAlF6 are about 2:1 and 1:1:1, respectively. 3.2. Description of the structure The XRD patterns of Li2BeF4 and LiSrAlF6 are shown in Fig. 1 and can be indexed in the primitive hexagonal system. The unit-cellparameters for Li2BeF4 are a0 = 13.2801 Å, c0 = 8.9134 Å and for LiSrAlF6 are a0 = 5.0838 Å, c0 = 10.2167 Å. The XRD data of Li2BeF4 and LiSrAlF6 are listed in Table 1. The values for Li2BeF4 and LiSrAlF6 are similar to those of the corresponding Li2BeF4 [JCPDS Card 20-116, a0 = 13.29, c0 = 8.91] and LiSrAlF6 [JCPDS Card 481640, a0 = 5.084, c0 = 10.218], which were synthesized by a solid-state reaction. The powder XRD patterns also showed that there were not any detectable impurities present in the samples, which indicated that the products were single phase. 3.3. SEM observations The SEM observations of Li2BeF4 and LiSrAlF6 are shown in Fig. 2. As Fig. 2 clearly indicates, the crystallites have a regular morphology which implies that the products are pure and single phase. The complex fluoride Li2BeF4 and LiSrAlF6 crystallites have
the same cubic morphology, and the average grain sizes are ca. 0.5 and 1.0 μm, respectively. 3.4. Thermal analysis The thermal stability of Li2BeF4 and LiSrAlF6 powders were studied by TG-DTA analysis in air. Neither Li2BeF4 is decomposed up to 600 °C nor LiSrAlF6 up to 750 °C. A small amount of ca. 0.555% surface water was evidently lost for LiSrAlF6 between 25 and 120 °C. The presence of water is confirmed by IR at 3447 and 1635 cm− 1. A small amount of ca. 10% is lost for Li2BeF4 between 600 and 900 °C, and ca. 1.25% are lost for LiSrAlF6 between 400 and 750 °C. 3.5. XPS analysis XPS analysis was performed in order to further examine the composition and bond types of the as-prepared samples. The XPS analytical data are displayed in Table 2. It is well known that, if oxygen enters fluorides crystal lattice, or oxygen content is high in fluoride products, the binding energies and the symmetry of the XPS peaks of each element will be changed remarkably because of M–O (M = metal element) and F–O bonds[17]. As Table 2 clearly indicates, the binding energies of each element in the samples are similar to those of the corresponding standard materials. In the meantime, the XPS peaks of each element have the well symmetry, which implies that all bond types are M–F (M = Li, Be, Sr, and Al) bonds, while the M–O and F–O bonds were not detected. This result is in agreement with the previous report that the oxygen content is lower in complex fluorides synthesized by
Table 2 XPS analysis data of Li2BeF4 and LiSrAlF6 Compounds
F1s
Li1s
Be1s
Li2BeF4 LiSrAlF6 LiF BeF2 AlF3 SrF2
685.9 685.7 685.4 685.8 685.6 685.4
55.7 55.5 55.7
116.8
Al2p
Sr3d
75.8
134.3
116.9 75.9 134.3
R. Hua et al. / Materials Letters 60 (2006) 3822–3825
mild solvothermal process than those of fluorides prepared by a conventional solid-state reaction at high temperature [16].
4. Conclusion A new method for the synthesis of Li2BeF4 and LiSrAlF6 by solvothermal crystallization at 180 and 240 °C is presented. Both Li2BeF4 and LiSrAlF6 crystallites are hexagonal systems, and have uniform grain shapes and sizes. The molar ratios of the initial mixtures, reaction temperature and reaction time are important factors for the effective synthesis. The solvothermally synthetic method to prepare complex fluorides appears to be advantageous in terms of simple devices and operation, lower synthesis temperature, single phase, well-crystallization of products and lower oxygen contents. We believe that our simple procedure can be extended to prepare other metal complex fluorides with lower oxygen content. The complex fluorides Li2BeF4 and LiSrAlF6 doped with rare earth ions synthesized via solvothermal process are underway. Acknowledgments This work was supported by the Postdoctoral Fund of Dalian Nationalities University (20056110) and the National Natural Science Foundation of China (90201032).
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References [1] C. Zhao, S. Feng, Z. Chao, C. Shi, R. Xu, J. Ni, Chem. Commun. (1996) 1641. [2] C. Zhao, S. Feng, R. Xu, C. Shi, J. Ni, Chem. Commun. (1997) 945. [3] R. Hua, Z. Jia, D. Xie, C. Shi, Chem. Lett. 31 (2002) 538. [4] R. Hua, Z. Jia, D. Xie, C. Shi, Mater. Res. Bull. 37 (2002) 1189. [5] M. Eibschutz, H.J. Guggenheim, S.H. Wemple, I. Camlibel, M. Didomenico, Phys. Lett. 7 (1969) 409. [6] A.H. Cooke, D.A. Jones, J.F.A. Silva, M.R. Weils, J. Phys. C. Solid State Phys. 8 (1975) 4083. [7] R.A. Heaton, C. Lin, Phys. Rev., B 25 (1982) 3538. [8] D.K. Sardar, W.A. Sibley, R. Alcala, J. Lumin. 27 (1982) 401. [9] A. Suzuki, T. Terai, S. Tanaka, J. Nucl. Mater. 258–263 (1998) 519. [10] S.A. Payne, L.L. Chase, G.D. Wilke, J. Lumin. 44 (1989) 167. [11] C.D. Marshall, J.A. Speth, S.A. Payne, W.F. Krupke, G.J. Quarles, V. Castillo, B.H.T. Chai, J. Opt. Soc. Am., B, Opt. Phys. 11 (1994) 2054. [12] G.D. Dzik, I. Sokolska, S. Golab, M. Baluka, J. Alloys Compd. 300–301 (2000) 254. [13] H. Su, Z. Jia, C. Shi, Chem. Mater. 14 (2002) 310. [14] S. Somiya, S.I. Hirano, M. Yoshimura, K. Yanagisawa, J. Mater. Sci. 16 (1981) 813. [15] H. Li, Z. Jia, S. Feng, C. Shi, Chem. J. Chin. Univ. 12 (2000) 1805. [16] R. Hua, B. Lei, D. Xie, C. Shi, J. Solid State Chem. 175 (2003) 284. [17] X. Liu, Y. Zhang, C. Shi, D. Xie, J. Solid State Chem. 178 (2005) 2167.
Mild solvothermal synthesis of complex fluorides Li2BeF4 and LiSrAlF6 Ruinian Hua a,⁎, Huiming Jiang a , Liyan Na a , Chunshan Shi b,⁎ a
b
College of Life Science, Dalian Nationalities University, Dalian 116600, PR China Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China Received 22 December 2005; accepted 31 March 2006 Available online 27 April 2006
Abstract The complex fluorides Li2BeF4 and LiSrAlF6 were synthesized solvothermally at 180–240 °C and characterized by means of X-ray powder diffraction (XRD), Scanning electron microscopy (SEM), thermogravimetric analysis (TGA), infrared spectroscopy (IR) and X-ray photoelectron spectroscopy (XPS). The different influence factors such as solvents, molar ratios of initial mixtures, reaction temperature and reaction time were investigated. The experimental results indicated that Li2BeF4 and LiSrAlF6 powders could be controllably synthesized in the solvothermal process. © 2006 Elsevier B.V. All rights reserved. Keywords: Li2BeF4; LiSrAlF6; Solvothermal synthesis; Characterization
1. Introduction Complex fluorides which were synthesized by mild hydrothermal and solvothermal processes (so called softchemical processes) have attracted growing interest during the last decade [1–4]. The complex fluorides of different structures show various properties such as piezoelectric characteristics [5], ferromagnetic [6], nonmagnetic insulator behaviour [7] and photoluminescence host materials [8], which have been studied extensively. As the complex fluoride Li2BeF4 is a sort of potential liquid tritium breeding material [9] and LiSrAlF6 is a tunable, room-temperature laser material when doped with the ion Cr3+ [10]. Meanwhile, LiSrAlF6 doped with Ce3+ shows UV solid-state laser material that is continuously tunable over 4000 cm− 1, had numerous applications in scientific, engineering, and medical fields [11]. It is well known that complex fluorides can be prepared by conventional solid-state reaction [12], Bridgman–Stockbarger method [13] and high temperature (N400 °C), high-pressure (N100 MPa) hydrothermal technique [14]. High temperature is required to enhance the diffusion between raw solid materials during the solid-state reaction. Meanwhile, this synthetic process requires a complicated set-up
⁎ Corresponding authors. Tel.: +86 411 87633470. E-mail address: [email protected] (R. Hua). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.03.121
because of the corrosive nature of fluoride. The high temperature, high-pressure hydrothermal techniques require special devices. For improving the drawbacks of the solid-state reaction and the high temperature, high-pressure hydrothermal techniques, soft-chemical processes have been investigated. Mild hydrothermal and solvothermal synthesis of complex fluorides at 120–240 °C were accepted to be an effective method for synthesizing complex fluorides [1,3]. The contents of oxygen in complex fluorides synthesized by hydrothermal and solvothermal method is lower than that of the corresponding complex fluorides synthesized by high temperature solid-state reaction [15,16]. In order to develop new routes to synthesize complex fluorides without complicated synthesis apparatus, herein we report a convenient solvothermal method for the synthesis of the complex fluorides Li2BeF4 and LiSrAlF6. The different influence factors such as solvents, molar ratios of initial mixtures, reaction temperature and reaction time were investigated. 2. Experimental Synthesis of Li2BeF4 and LiSrAlF6 powders were carried out in a 20 mL Teflon-lined stainless steel autoclave under autogenous pressure. The starting reactants were LiF (A.R.), BeF2 (A.R.), SrF2 (A.R.), and AlF3·7/2H2O (A.R.). For the synthesis of Li2BeF4, the molar ratios of initial mixtures were
R. Hua et al. / Materials Letters 60 (2006) 3822–3825
3823
Fig. 1. XRD patterns of Li2BeF4 (a) and LiSrAlF6 (b).
2.0 (or 3.0) LiF:1.0 BeF2. The typical synthetic procedure was as follows: 0.02 mol (0.5188 g) LiF and 0.01 mol (0.4701 g) BeF2 were mixed and added into a Teflon-lined autoclave. Then the autoclave was filled with ethylene glycol up to 80% of the total volume. For the synthesis of LiSrAlF6, the molar ratios of the initial mixtures were 1.5 (or 2) LiF:1.0SrF2: 1.0 AlF3. The typical synthetic procedure was as follows: 0.006 mol (0.1556 g) LiF, 0.004 mol (0.5025 g) SrF2 and 0.004 mol (0.5881 g) AlF3·7/2H2O were mixed and added into a Teflon-lined autoclave. Then the autoclave was also filled with ethylene glycol up to 80% of the total volume. The autoclave was sealed into a stainless steel tank and heated in an oven at 180 °C (for the synthesis of Li2BeF4) and 240 °C (for the synthesis of LiSrAlF6) for 24 h. After being cooled to room temperature naturally, the final powder products were filtered off, washed with absolute ethanol and distilled water, and then dried in air at ambient temperature. All products were characterized by X-ray powder diffraction (XRD), using a Japan Rigaku D/max-IIB diffractometer with Cu Kα radiation (λ = 0.1541 nm). The XRD data for index and cell-parameter calculations were collected by a scanning mode with a step of 0.02° in the 2θ range from 10° to 100° and a scanning rate of 4.0° min− 1 with silicon used as an internal standard. Observation of crystallites by SEM was performed on a Hitachi S-570 scanning electron microscopy, aurum was used to coat the particles as a means to reduce charging effects. Thermogravimetric analysis (TGA) was conducted using a DT30 thermogravimetric system in air. IR spectra were obtained with a Magna 560 spectrometer in the range of 400–4000 cm− 1. The samples were pressed KBr pellets for the spectral measurements. X-ray photoelectron spectroscopy (XPS) experiment was conducted with a VGESCALAB MK-II spectrometer using Mg Kα (1253.6 eV) source, C1s (Eb = 285.00 eV) as an internal standard, and at a pressure of 1.33 × 10 − 7 Pa. Elementary analysis was conducted on a POEMS-II inductively coupled plasma–atomic emission spectrometer. 3. Results and discussion 3.1. Synthesis conditions During the synthesis of Li2BeF4 and LiSrAlF6, the Li/Be and Li/Sr/ Al ratios were found to be crucial to the formation, crystallization and
purity of the products. When the molar ratios Li/Be and Li/Sr/Al of mixture was 2 (or 3):1 and 1.5 (or 2):1:1, respectively, ethylene glycol was used as solvent, the pure and well-crystallized Li2BeF4 and LiSrAlF6 products were prepared. When the Li/Be molar ratio is less than 2 or larger than 3, an impurity phase of BeF2 and LiF appeared in the Li2BeF4 products. Meanwhile, when the Li/Sr (immobility of Sr/Al
Table 1 XRD data of Li2BeF4 and LiSrAlF6 Li2BeF4
LiSrAlF6
hkl
dA
I/I1
hkl
dA
I/I1
110 012 211 300 202 220 122 311 113 410 033 322 330 104 502 422 143 250 314 333 432 611 205 125 603 621 244 170 154 262 006 451 713 363 900 336 553
6.6515 4.1602 3.9139 3.8405 3.5228 3.3262 3.1164 3.0074 2.7121 2.5129 2.3504 2.3270 2.2160 2.1878 2.0145 1.9561 1.9194 1.8475 1.8270 1.7775 1.7428 1.7251 1.7048 1.6499 1.6117 1.5720 1.5561 1.5253 1.5150 1.5016 1.4852 1.4531 1.3565 1.3042 1.2790 1.2338 1.2148
24 10 27 22 2 59 5 2 33 26 93 16 100 2 21 3 14 8 2 4 3 2 2 2 14 3 5 6 1 2 7 3 15 7 5 5 6
100 101 102 110 112 104 200 202 114 120 106 122 300 116 124 206 222 126 132 118 224
4.4053 4.0447 3.3384 2.5418 2.2750 2.2118 2.2015 2.0213 1.8030 1.6681 1.5898 1.5818 1.4671 1.6060 1.3944 1.3479 1.2335 1.1903 1.1877 1.1427 1.1377
25 5 100 8 38 22 2 17 31 4 10 14 8 15 6 6 2 8 6 5 4
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R. Hua et al. / Materials Letters 60 (2006) 3822–3825
Fig. 2. SEM photographs of Li2BeF4 (a) and LiSrAlF6 (b).
molar ratio is 1) molar ratio is less than 1 or larger than 2, an impurity phase is obtained in the LiSrAlF6 products. The formation of pure Li2BeF4 and LiSrAlF6 products also depended on the solvents. Ethylene glycol was found to be the most effective solvent for the synthesis of Li2BeF4 and LiSrAlF6. Other solvents were used to synthesize Li2BeF4 and LiSrAlF6, such as n-butanol, ethylenediamine, pyridine and phenol etc., the result reveals that all the other solvents can not form pure and well-crystallized products. Temperature was also an important factor for an effective synthesis. Although Li2BeF4 products could be synthesized at temperature below 180 °C, however, lower temperature requires longer reaction times. For instance, in the LiF– BeF2–ethylene glycol system, Li2BeF4 was obtained after 1 day at 180 °C, but could form the pure crystals of Li2BeF4 at 150 °C for 2 days or at 120 °C for 7 days. In the LiF–SrF2–AlF3–ethylene glycol system, LiSrAlF6 was obtained after 1 day at 240 °C, but could not form the pure crystals of LiSrAlF6 below 240 °C (such as 220 °C) for 7 days. The elemental analysis showed that the actual molar ratios of metal elements in Li2BeF4 and LiSrAlF6 are about 2:1 and 1:1:1, respectively. 3.2. Description of the structure The XRD patterns of Li2BeF4 and LiSrAlF6 are shown in Fig. 1 and can be indexed in the primitive hexagonal system. The unit-cellparameters for Li2BeF4 are a0 = 13.2801 Å, c0 = 8.9134 Å and for LiSrAlF6 are a0 = 5.0838 Å, c0 = 10.2167 Å. The XRD data of Li2BeF4 and LiSrAlF6 are listed in Table 1. The values for Li2BeF4 and LiSrAlF6 are similar to those of the corresponding Li2BeF4 [JCPDS Card 20-116, a0 = 13.29, c0 = 8.91] and LiSrAlF6 [JCPDS Card 481640, a0 = 5.084, c0 = 10.218], which were synthesized by a solid-state reaction. The powder XRD patterns also showed that there were not any detectable impurities present in the samples, which indicated that the products were single phase. 3.3. SEM observations The SEM observations of Li2BeF4 and LiSrAlF6 are shown in Fig. 2. As Fig. 2 clearly indicates, the crystallites have a regular morphology which implies that the products are pure and single phase. The complex fluoride Li2BeF4 and LiSrAlF6 crystallites have
the same cubic morphology, and the average grain sizes are ca. 0.5 and 1.0 μm, respectively. 3.4. Thermal analysis The thermal stability of Li2BeF4 and LiSrAlF6 powders were studied by TG-DTA analysis in air. Neither Li2BeF4 is decomposed up to 600 °C nor LiSrAlF6 up to 750 °C. A small amount of ca. 0.555% surface water was evidently lost for LiSrAlF6 between 25 and 120 °C. The presence of water is confirmed by IR at 3447 and 1635 cm− 1. A small amount of ca. 10% is lost for Li2BeF4 between 600 and 900 °C, and ca. 1.25% are lost for LiSrAlF6 between 400 and 750 °C. 3.5. XPS analysis XPS analysis was performed in order to further examine the composition and bond types of the as-prepared samples. The XPS analytical data are displayed in Table 2. It is well known that, if oxygen enters fluorides crystal lattice, or oxygen content is high in fluoride products, the binding energies and the symmetry of the XPS peaks of each element will be changed remarkably because of M–O (M = metal element) and F–O bonds[17]. As Table 2 clearly indicates, the binding energies of each element in the samples are similar to those of the corresponding standard materials. In the meantime, the XPS peaks of each element have the well symmetry, which implies that all bond types are M–F (M = Li, Be, Sr, and Al) bonds, while the M–O and F–O bonds were not detected. This result is in agreement with the previous report that the oxygen content is lower in complex fluorides synthesized by
Table 2 XPS analysis data of Li2BeF4 and LiSrAlF6 Compounds
F1s
Li1s
Be1s
Li2BeF4 LiSrAlF6 LiF BeF2 AlF3 SrF2
685.9 685.7 685.4 685.8 685.6 685.4
55.7 55.5 55.7
116.8
Al2p
Sr3d
75.8
134.3
116.9 75.9 134.3
R. Hua et al. / Materials Letters 60 (2006) 3822–3825
mild solvothermal process than those of fluorides prepared by a conventional solid-state reaction at high temperature [16].
4. Conclusion A new method for the synthesis of Li2BeF4 and LiSrAlF6 by solvothermal crystallization at 180 and 240 °C is presented. Both Li2BeF4 and LiSrAlF6 crystallites are hexagonal systems, and have uniform grain shapes and sizes. The molar ratios of the initial mixtures, reaction temperature and reaction time are important factors for the effective synthesis. The solvothermally synthetic method to prepare complex fluorides appears to be advantageous in terms of simple devices and operation, lower synthesis temperature, single phase, well-crystallization of products and lower oxygen contents. We believe that our simple procedure can be extended to prepare other metal complex fluorides with lower oxygen content. The complex fluorides Li2BeF4 and LiSrAlF6 doped with rare earth ions synthesized via solvothermal process are underway. Acknowledgments This work was supported by the Postdoctoral Fund of Dalian Nationalities University (20056110) and the National Natural Science Foundation of China (90201032).
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