Synthesis of the Complex Fluoride LiBaF3 through a Solvothermal Process

Chinese Chemical Letters Vol. 13, No.10, pp 1021- 1024, 20021021http://www.imm.ac.cn/jourmalccl.htmlSynthesis of the Complex Fluoride LiBaF3 through a SolvothermalProcessRui Nian HUA' , Zhi Hong JIA3 De Min XIE', Chun Shan SHP*'Faculty of Chemistry, Northeast Normal University, Changchun 130024'Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry,Chinese Academy of Sciences, Changchun 130022Abstract The complex fluoride LiBaF; is solvothermally synthesized at 180C and characterizedby means of X-ray powder diffraction, scanning electron microscopy, thermogravimetric analysisand infrared spectroscopy. In the solvothermal process, the solvents, mole ratios of initialmixtures and reaction temperature play important roles in the growth of the single crystal.Key words: Solvothermal synthesis, complex fluoride.Since the 1970s a number of papers!2 have reported the high temperature solid statesynthesis of complex fluoride LiBaF3 in LiF-BaF2 system. Some complex fluorideswith general formula ABF3 (A = Li, Na, K; B = divalent element) possess the perovskitestructure. The complex fluoride, ABF3 has become important as it shows lasing action3when doped with a proper dopant.Various isomorphous replacements in the frameworkof complex fluorides lead to many controllable properties*. However, the solid statesynthesis requires a complicated set-up because of the erosion of the fluorides. Recently,a mild hydrothermal synthesis of the complex fluorides at 120-2409C has been reported-8. The oxygen content in ABF; synthesized by hydrothermal method is lower than thatof the corresponding compound prepared by high temperature solid state reaction' andthus has limited the study of the fluorides. Taking into account some of the interestingproperties of the complex fluorides such as piezoelectric characteristics"0, ferromagnetic",nonmagnetic insulators behaviour', and with an aim to develop new routes withoutcomplicated apparatus.Herein we report a convenient solvothermal synthesis ofLiBaF; with perovskite structure.ExperimentalLiBaF3 crystal was grown in a 20 mL Teflon-lined stainless steel autoclave underautogenous pressure. The starting reactants were LiF (A.R.) and BaF2 (A.R.). Themole ratios ofLiF : BaF2 were 1.0: 1.0. The typical synthetic procedure was as follows:0.130 g (0.005 moL) LiF and 0.877 g (0.005 moL) BaF2 were mixed and added into a"E-mail: cshi@ ciac.jl.cn中国煤化工MHCNMH G.1022Rui Nian HUA et al.Teflon-lined autoclave. Then the autoclave was flldl with ethylene glycol up to 80%of the total volume.The autoclave was sealed into a stainless steel tank and heated inan oven at 180°C for 2 days. After being cooled to room temperature, the final powderproduct was filtered off, washed with absolute ethanol and distilled water, and then driedin air at ambient temperature.All products were characterized by X-ray powder diffraction(XRD), using a JapanRigaku D/max- II B diffractometer with Cu K 1 radiation (A=1.5405 A. The XRDdata for index and cell-parameter calculations were collected by a scanning mode with astep of 0.02° in the 20 range from 10° to 100° and a scanning rate of 4.0°●min' withsilicon used as an internal standard. Observation of crysallites by SEM was performedon a JXA-840 scanning electron microscope. Thermogravimetric analysis (TGA) wasconducted using a DT-30 thermogravimetric system. IR spectra were obtained with aMagna 560 spectrometer in the range 400-4000cm'. The samples for the spectralmeasurements were in KBr pellets.Results and DiscussionTable 1 shows the solvothermal synthesis conditions for LiBaFs In the synthesis ofLiBaF, the Li/Ba ratio and the solvent were found to be crucial to the formation,crystallization and purity of the product. When the mole ratio Li/Ba of mixture was 1,and ethylene gloycol was used as solvent, the pure and well-crystallized product wasprepared. However, when the mole ratio Li/Ba was 0.5 (or 2), impurity phases BaF2 (orLiF) appeared. Other solvents were employed to the synthesis of LiBaF3, such as n-butanol, ethylenediamine, pyridine and phenol etc, all of these solvents could notachieve the purity of the products.Crystallization temperature was also important factor for an effective synthesis.For instance, in the LiF-BaF 2-ethylene gloycol system, LiBaF; was obtained after 1 dayat 180°C, but at 150°C after 7 days could not form the pure crystals of LiBaF3. Thesynthesis reaction can be formulated as follows:LiF + BaF2_ etbylene glycol 180°C_，LiBaF;The XRD pattern of the LiBaF3 is shown in Figure 1 and can be indexed in cubicsystems. The unit-cell parameters for LiBaF3 is a=3.9953 Aand the value for LiBaF; is .similar to that of the corresponding LiBaF; synthesized by solid-state reaction (a=3.9950A [JCPDS Card 18-0715]. The powder XRD patterns show the products are pure.The SEM observation of the product ( Figure 2 ) shows that the crystallites haveregular morphology and this implies that the product is pure and single phase. Thecomplex fluoride LiBaF3 has the shapes of cubic, and the average grain size is ca. 1.5mThe thermal stability of the LiBaF; was studied by TG-DTA analysis in air. Nophase transition or mass loss was observed from 25°C up to 835°C in system. Thisindicates that LiBaF; is not hydrated and stable in air, which was confirmed by IR atroom temperature.中国煤化工MHCNMH G.Synthesis of the Complex Fluoride LiBaF31023Table 1 Solvothermal synthesis conditions for LiBaF3Starting materials a : bSolventReaction Reaction Phases inmole ratiotime/d_ temp./ °C productiF BaF2Ethylene gloycol180LiBaFEthylene gloycol .LiBaFs+LiFLiF BaFzLiBaF;+BaFzLiF BaF280LiBaF,LiF BaF;LiBaFy00LiBaF3iF BaF,150 LiBaF:+BaF，Figure 1 XRD pattern of LiBaF,1400010全1000204000110 1112000210211 20161212220--40 6030 1002 Theta(degree)Figure 2. SEM photograph of LiBaF,PIn summary， a new method for the synthesis of LiBaF3 by solvothermalcrystallization at 180°C is presented. The solvent, mole ratios of initial mixtures andreaction temperature play an important roles in the formation of the products in thesolvothermal process. The products have uniform sizes in grain shapes. The productis stable in air. Compared with traditional high temperature solid-state methods andmild hydrothermal synthesis method, the solvothermal synthesis method to complexfluorides appears advantageous in terms of lower synthesis temperature, simple operation,high purity and nice-crystallization.中国煤化工MHCNMH G.1024Rui Nian HUA et al.AcknowledgmentsThis work was supported by the State Key Project of Foundation Research (G199806 1306) and theNational Natural Science Foundation of China (50072031).References. Y. Tan, C. Shi, J. Solid State Chem., 2000,150,178 and references therein.A. Meijerink, J. Luomin, 199,55,125.G. Horsch, H. Paus, J. Opti. Commun.,1986,60, 69.K. Somaiah, M. V. Narayana, Mater. Chem. Phys, 1990,24,353.Zhao, S. Feng, Z. Chao, C. Shi, R. Xu, J. Ni, Chem. Commun., 1996, 1641.Xun, S. Feng, J. Wang, R. Xu, Chem. Mater. , 1997, 9, 2966Zhao, S. Feng, R. Xu, C. Shi, J. Ni, Chem. Commun, 1997, 945.Xun, s. Feng, R. Xu, Mater. Res. Bull. 1998, 33, 369.、H. Li,Z. Jia, s. Feng, C. Shi, Chem. J. 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