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Synthesis and characterization of Fe–Cl boracite
Journal of Alloys and Compounds 329 (2001) 278–284
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Synthesis and characterization of Fe–Cl boracite a a a a a, b b Z.H. Wang , D.Y. Geng , D. Li , X.G. Zhao , Z.D. Zhang *, Y.P. Wang , J.-M. Liu a
Institute of Metal Research, Academic Sinica, Wenhua Road 72, Shenyang 110015, China b Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China Received 29 March 2001; accepted 21 May 2001
Abstract The Fe–Cl boracite is prepared by a procedure, consisting of solution mixing, heating, grinding and H 2 reduction. The high valence metal ion Fe 31 reacts with H 2 to form the low valence metal ion Fe 21 , so that the corresponding boracite can be synthesized. The lattice parameters and the phase transition temperatures are determined by means of X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. The hydrogen not only plays a role in reduction, but also prevents the oxidation of the mixture. The ferroelectric property at room temperature of the sample has been substantially demonstrated. 2001 Elsevier Science B.V. All rights reserved. Keywords: Transition metal compounds; Chemical synthesis; TEM; Ferroelectric
1. Introduction Boracites form a large crystal family with the general formula M 3 B 7 O 13 X, where M stands for divalent metal ions (Mg, Cr, Mn, Fe, Co, Ni, Cu, Zn, or Cd), and X stands for halogen ions (F, Cl, Br, or I), with more than 20 isomorphous compounds. It has been found that in some cases X can be OH, S, Se, or Te and M monovalent Li [1–5]. Boracites possess peculiar properties that have attracted attention not only of crystallographers but also of physicists and materials scientists. Though the detailed behavior varies with M and X, nearly all the halogen boracites exhibit transitions from a high temperature cubic paraelectric phase to one or more lower symmetry phases at low temperatures. The low temperature phase with low symmetry is usually ferroelectric, which has potential for application. The exceptions are Cr–Br, Cr–I and Cu–I boracites, which remain cubic down to 4 K [6,7]. The Cr–Cl boracite undergoes a phase transformation to a tetragonal non-polar 4¯ 2 m phase on cooling, which is so far a unique one in the boracite family [8–11]. A phase transition between the cubic and orthorhombic phase occurs in some boracite single crystals [12]. The structures
*Corresponding author. Tel.: 186-24-2384-3531; fax: 186-24-23891320. E-mail address: [email protected] (Z.D. Zhang).
of high- and low-boracites forming above and below the a–b inversion temperature 2658C are essentially the same, both containing the same boron–oxygen network and differing only in the position of M and halogen atoms [13]. The transition temperature of the boracites varies widely (60–800 K), but for any given metal it falls in the sequence X5Cl→Br→I [1,14]. For instance, when M stands for Fe, and X5Cl, Br, I, the cubic–orthorhombic transition temperatures are about 265, 132 and 728C, respectively [15–19]. The temperature dependence of spontaneous birefringence indicated that three types of phases sequentially occur for the ferroelectric phases of various boracites [20]. The ferroelectric phase transition in Fe–X (X5Br, I) boracites were investigated by EXAFS spectroscopy and special X-ray diffraction technique [15,17]. Linear and quadratic magnetoelectric (ME) effects in the boracite were studied [5,21]. The lattice parameters and the lattice strains of halogen boracites were measured [18,22]. The structure, phase transition temperature and constituents of iron–boracite (Fe, Mg, Mn) 3 B 7 O 13 C1 found in nature were investigated [16]. The chemical vapor transport method is currently the most successful and widely used technique for the synthesis of boracites in single crystal form and permits the preparation of a larger number of boracites [3–5,9–12,21– 24]. With the starting constituents MO, B 2 O 3 , and metal halides, the global reactions for synthesis of nearly all boracites can be written:
0925-8388 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01613-9
Z.H. Wang et al. / Journal of Alloys and Compounds 329 (2001) 278 – 284
MO 1 2HX(g) 5 MX 2 (g) 1 H 2 O(g) 7B 2 O 3 1 6MX 2 1 5H 2 O → 2M 3 B 7 O 13 X 1 10HX The solid solutions of (M, M9)–X and M–(X, X9) boracites have been grown by this method [23]. Many boracites become antiferromagnetic and weakly ferromagnetic at temperatures below 60 K. Magnetic measurements were performed for Fe–X (X5Cl, Br, I) boracites in the temperature range of 3.6–300 K [11,21]. Temperature dependence of the magnetization and magnetic susceptibility for all compounds show the existence of antiferromagnetism, accompanied by a weak ferromagnetic component [11,21]. In this work, a new method is developed for preparing iron–chlorine boracite (Fe 3 B 7 O 13 Cl) powders. The process consists in solution mixing, heating, grinding and H 2 reduction. The powders obtained by this method are a mixture of a-Fe and Fe 3 B 7 O 13 Cl. The Fe 3 B 7 O 13 Cl can be separated by dissolving a-Fe in dilute HCl. The structure of the Fe–Cl boracite has been identified by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The temperatures of the structural phase transitions have been verified by differential scanning calorimetry (DSC). The ferroelectric properties at room temperature of the Fe 3 B 7 O 13 Cl are measured using HVS6000RT standard ferroelectric testing unit under a virtual ground mode. For comparison, the structure of the phases in the powders, prepared by the process of solution mixing, heating, grinding and heating in air, has been studied also.
2. Experiments Fe–Cl boracites crystals were prepared by solution mixing, heating and H 2 reduction in the temperature range from 360 to 10008C, with the starting constituents boric acid (H 3 BO 3 ) (.99.5%) and iron chloride hydrous (FeCl 3 ?6H 2 O) (.99.0%). Synthesizing the Fe–Cl boracite powder consisted in the following steps. The starting raw materials H 3 BO 3 and FeCl 3 ?6H 2 O were dissolved in appropriate de-ionized (DI) water. Their weight ratio was 3:5. Their solutions were mixed together in a beaker, and then heated on an electric stove. The color of the solution became deep when the temperature reached 708C. The solution was boiled until most fluffy blocks appeared. Cooling the blocks down to room temperature changed their color to red–brown. The dried precursors were obtained by grinding the blocks into a fine powder in a mortar. The dried precursors were hydrogenated in hydrogen atmosphere in quartz tubes for half an hour at 360, 500, 600, 700, 800, 900 and 10008C, respectively. The reacted compacts were crushed in a mortar protected by petroleum ether. Then the mixture of a-Fe and Fe 3 B 7 O 13 Cl was obtained and dried in air. Almost pure
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Fe 3 B 7 O 13 Cl was obtained by the following separation procedure. The mixture was dissolved in dilute 9% HCl (hydrochloric acid) for about 10 min, then filtered and washed with DI water several times, and the residue was dried in air. In order to determine the role of the H 2 , the dried precursors were heated in Ar at 8008C for about 30 min, then cooled to room temperature, and the products were crushed into powders in mortar. For comparison, the dried precursors were heated in air for half an hour at 250, 550, 900 and 10008C, respectively. The phases of the powders synthesized by each step were identified using Cu Ka X-ray diffraction at room temperature in a D/ max-gA diffractometer with a pyrolytic monochromator. The grain size and the lattice parameters of the samples were determined using a Philips EM 420 transmission electron microscope with the accelerating voltage of 100 kV. The phase transition temperatures were determined by using a Perkin Elmer DSC-7 differential scanning calorimeter with heating rate of 208C min 21 . In order to estimate the content of metallic iron, the magnetization of the samples were measured with a PAR155type vibrating sample magnetometer. The samples for ferroelectric measurements were simply pressed under a pressure of 1.5?10 4 kg cm 22 . The density of the sample measured is 2.60 g cm 23 in good agreement with the theoretical value of 2.67 g cm 23 . The resistivity measured for the pressed sample is infinite. The ferroelectric properties of the samples were measured using HVS6000RT standard ferroelectric testing unit (Radiant Inc., New Mexico, USA) under a virtual ground mode at room temperature. A capacitor was aligned in parallel to the sample in order to compensate the capacitance loss. The bottom and top electrodes were (111)-oriented Pt layers of |200 nm in thickness, which were fabricated using sputtering technique. In the measurement, five voltage pulses, one for pre-polarization, two for negative poling and two for positive poling, were generated and a charge-integrator was coupled to count the as-generated charge. The total time interval for the five pulses was 108.80 ms as the HVS6000RT standard ferroelectric testing unit did.
3. Results and discussion After treatment in hydrogen at 360, 500, 600, 700 and 8008C, the products are grey and fluffy blocks, with weak hardness, which can be crushed softly into powders in a mortar. The products obtained at / above 9008C are grey blocks with strong hardness, which must be smashed roughly before crushing into powders in the mortar protected by petroleum ether. The color of the powders obtained below 6008C becomes red–brown after aging in air for more than 2 days. It is possible that there is amorphous FeCl 3 (iron chloride) in the powders, which
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changes color when absorbing water in air. We can identify by X-ray diffraction that the constituents of the powders are not altered before and after the color changes. But the powders obtained at temperatures higher than 7008C do not show such change of color. In order to identify the products synthesized by each step of the process, XRD was performed on the powders obtained from the first step to the final step. XRD patterns are shown in Fig. 1(a)–(h) for each step. One can see that the dried precursor (Fig. 1(a)) is in the amorphous state. There is a peak at about 2u 5288, which may correspond to H 3 BO 3 in raw materials. This peak is sometimes obvious, sometimes obscure, depending on the content of H 3 BO 3 (Fig. 1(b)–(e)). The amorphous precursor crystallizes into FeCl 2 ?4H 2 O and H 3 BO 3 by reducing in H 2 at 3608C, as shown in Fig. 1(b). The possible global reaction is: 2FeCl 3 ? 6H 2 O 1 H 2 → 2FeCl 2 ? 4H 2 O 1 2HCl ↑ 1 4H 2 O↑ H 3 BO 3 does not react with other compounds at this temperature. When the temperature rises, the new components appear, while the former ones disappear. We can
Fig. 1. XRD patterns of (a) the dried precursors with FeCl 3 ?6H 2 O and H 3 BO 3 as the starting raw materials, the powders obtained in H 2 atmosphere at (b) 3608C, (c) 5008C, (d) 6008C, (e) 7008C, (f) 8008C, (g) 9008C and (h) 10008C.
see from Fig. 1(c)–(h), that FeCl 2 ?4H 2 O and H 3 BO 3 disappear gradually, while the mixture of a-Fe and Fe 3 B 7 O 13 Cl emerges at 5008C. The amount of the mixture increases with increasing temperature. We can infer the possible reaction in the temperature range from 500 to 8008C: 4FeCl 2 ? 4H 2 O 1 7H 3 BO 3 1 H 2 → Fe 3 B 7 O 13 Cl 1 Fe 1 24H 2 O ↑ 1 7HCl ↑ It is shown in Fig. 1(f)–(h) that the content of Fe in the mixture increases, but that of Fe 3 B 7 O 13 Cl decreases with increasing temperature. As shown in Fig. 1(c)–(h), the amount of Fe–Cl boracite in the compact after treatment in H 2 at 700 and 8008C is higher than that treated at 9008C. This indicates that the Fe 3 B 7 O 13 Cl is decomposed in H 2 atmosphere when heated above 7008C. Finally it disappears at 10008C (see Fig. 1(h)), following the thermal decomposition process described in Ref. [25]. The microstructure of the powders hydrogenated at 7008C was studied by TEM observations. The bright field TEM photograph shown in Fig. 2(a) indicates that the mean size of the powders is of the order of the micron. Fig. 2(b) gives the corresponding selected area diffraction pattern of a grain of Fe–Cl boracite. The interplanar ˚ which is consistent spacing for (110) or (002) is 6.12 A, ˚ [26]. The value correwith the reference data (6.11 A) ˚ b58.65 A, ˚ sponds to the lattice parameters of a58.65 A, ˚ as shown in Table 1. and c512.24 A, The final powders, dissolved in dilute HCl with different duration, were studied. The results show that the content of a-Fe is minimum when the mixture is treated in dilute HCl for about 10 min. The color of the mixture of the Fe–Cl boracite with a little amount of a-Fe is light green, and the color becomes deeper when the Fe content increases. The X-ray diffraction pattern of the Fe–Cl boracite after the separation process is shown in Fig. 3. It is clear that the main phase in the powders after separation is Fe–Cl boracite, with a little amount of a-Fe. In order to determine the content of a-Fe in the mixtures, we measured the magnetization of the mixtures. Fig. 4 shows the magnetization versus magnetic field at room temperature of the mixture, from which we can estimate that the content of a-Fe is about 15 and 0.7 wt.% before and after separating, respectively. It is hard to see the loop from Fig. 4 because the coercive field is extremely low. Although it seems difficult to eliminate all traces of metallic iron by dissolving the crushed material in HCl, without dissolving too much of the boracite itself, such a material may for example be used as the boracite starting material in a purifying vapour transport process [27]. The changes of lattice parameters of M–X boracites were studied previously [12,14,16,23]. The lattice parameters determined by XRD and TEM in this work are very close to those of Ref. [26], as shown in Table 1. DSC measurement was performed on the powders hydrogenated
Z.H. Wang et al. / Journal of Alloys and Compounds 329 (2001) 278 – 284
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Fig. 2. (a) Bright field TEM graph and (b) corresponding selected area diffraction pattern of Fe–Cl boracite treated in H 2 at 7008C.
Table 1 ˚ of Fe–Cl boracite Lattice parameters [A] a
b
c
Technique
Reference
8.62 8.60 8.65
8.62 8.60 8.65
12.23 12.17 12.24
XRD XRD TEM
[21] This work This work
at 8008C and after separation. The DSC curves shown in Fig. 5 reveal that the phase transitions of Fe–Cl boracite occur both upon heating and cooling. The phase transition temperatures of the Fe–Cl boracites are consistent with the
Fig. 3. X-ray diffraction pattern of Fe–Cl boracite treated in H 2 at 7008C.
data studied by spontaneous birefringence (Dn i ) [20]. The data are compared in Table 2. The differences between the data are within the error bars for the different sample preparations and detection techniques. Because the second phase transition at lower temperature is not sensitive to heat, it cannot be distinguished clearly by the DSC. Fig. 6 presents several ferroelectric hysteresis loops at room temperature of the Fe–Cl boracite as measured by different maximum applied electric fields. The sample’s ferroelectricity is demonstrated by these loops. It is seen that the polarization is not saturated under the maximum applied electric field of 25.0 kV cm 21 . The remnant polarization as evaluated from the largest loop is about 0.92 mC cm 22 , whilst the coercivity is about 18.0 kV cm 21 . Maybe due to the metallic ion doping, the sample leakage current may be a little high so that the as-measured loop is shown to be fat. However, the measured coercivity (18.0 kV cm 21 ) is not really the static coercivity. It has been shown that the quasi-static ferroelectric coercivity field of highly insulating single crystals of Fe–Cl boracite at room temperature is extremely high, i.e. beginning of switching occurs at about 320 kV cm 21 and saturation at 800 kV cm 21 [28]. These measurements were done quasi-statically with liquid electrodes, which do not allow charge injection into the sample and herewith avoid the artificial increase of the coercivity field. In our case, we obtain a much lower coercivity so that the charge injection is not serious. The as-measured hysteresis in this work does not show a fully saturated electrical polarization because the static coercivity as reported earlier is quite high [28] and the HVS6000RT test unit cannot apply a voltage higher than 4.0 kV to the present thick sample. Furthermore, the polycrystalline
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Fig. 4. Magnetization versus magnetic field at room temperature (a) before and (b) after the separation process.
sample definitely shows a much lower coercivity than the single crystal. Usually, the coercivity of the polycrystalline should be at least several times lower than the single crystal, mainly because the rotation or reversal of the grain or domain could be much easier in polycrystalline than in single crystal. Nevertheless, the ferroelectric property of the sample has been substantially demonstrated. For the purpose of comparison, the dried precursors were heated in air. The XRD patterns (shown in Fig. 7) indicate that all powders, except for those obtained at 2508C, contain the stable Fe 2 O 3 phase. Compared with the raw powders, those obtained at 2508C are not changed very much. A kind of unstable Fe 2 O 3 (i) phase appears at 5508C, but disappears when the temperature rises up to 6008C. This suggests that the H 2 reduction process is very important for the formation of Fe–Cl boracite.
Fig. 5. DSC graph of Fe–Cl boracite with (a) increasing and (b) decreasing temperature.
In order to determine the role of the H 2 , the dried precursors were heated in Ar at 8008C for about 30 min, then cooled to room temperature, and the products were Table 2 Phase transition temperatures [8C] of Fe–Cl boracite 4¯ 3m↔mm2
mm2↔m
m↔3m
Technique
Reference
336 332
270 274
255 252
Dn i DSC (heating)
[10] This work
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283
Fig. 6. Ferroelectric hysteresis loops at room temperature of Fe–Cl boracite. The loops denoted by P1–P6 correspond to those measured at different maximum applied electric fields.
crushed into powders in a mortar. The XRD patterns in Fig. 8 show that the final powders consisted of a mixture of Fe–Cl boracite, a-Fe, Fe 2 O 3 and Fe 3 O 4 . Furthermore, the dried precursors were made by the
Fig. 8. XRD patterns of the dried precursors (with starting raw materials H 3 BO 3 and FeCl 3 ?6H 2 O) heated in Ar at 8008C.
same process, but with FeCl 2 ?4H 2 O and H 3 BO 3 as starting raw materials. The dried precursors were heated in Ar at 8008C for about 30 min, and cooled to room temperature. The products are blocks with the color of ashen, which were crushed into powders. The structures of the powders were determined by XRD. Fig. 9 shows that the final powders consist of Fe 3 B 7 O 13 Cl and a small amount of a-Fe. The results above suggest that the H 2 reduction process is indispensable for the formation of Fe–Cl boracite, when FeCl 3 ?6H 2 O and H 3 BO 3 are used as the starting raw materials. The high valence metal ion Fe 31 reacts with H 2 to form the low valence state so that the corresponding boracite can be synthesized. It is evident that the hydrogen not only plays a role in reduction, but also prevents the mixture being oxidized. The present synthesis is related to the method described in Ref. [29], where Fe–Cl boracite is also obtained in a hydrogen atmosphere, but starting with Fe 2 O 3 and BCl 3 , leading at high temperatures to a gas phase of similar composition. In this work, Fe 2 O 3 is formed as an intermediate phase.
4. Conclusion
Fig. 7. XRD patterns of the dried precursors (with FeCl 3 ?6H 2 O and H 3 BO 3 as the starting raw materials) heated in air at (a) 10008C, (b) 9008C, (c) 5508C and (d) 2508C together with (e) the dried precursors. Fe 2 O 3 (i) is unstable phase, while Fe 2 O 3 is stable one.
Mixtures of a-Fe and Fe–Cl boracite have been synthesized by a chemical process including solution mixing and heating, grinding and reduction by H 2 , with FeCl 3 ? 6H 2 O and H 3 BO 3 used as the starting materials. Nearly pure Fe–Cl boracite was obtained by separating a-Fe from
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References
Fig. 9. XRD patterns of the dried precursors (with FeCl 2 ?4H 2 O and H 3 BO 3 as the starting raw materials) heated in Ar at 8008C.
the mixture using diluted HCl. The phase transition temperatures and the lattice parameters of Fe–Cl boracite determined by means of DSC, XRD, TEM are consistent with previous reports. The high valence metal ion Fe 31 reacts with H 2 to form the low valence state Fe 21 so that the corresponding boracites may be synthesized. The hydrogen not only plays a role in reduction, but also protects the mixture from oxidation. Furthermore, other high valence states of transition metals can be also reduced by H 2 to their low valence state. Therefore the described method may be extended to the synthesis of other boracite compositions. It has been demonstrated that the as-prepared Fe–Cl boracite is ferroelectric at room temperature.
Acknowledgements This work has been supported by the project No. 59725103 of the National Natural Sciences Foundation of China and by the Science and Technology Commission of Shenyang and Liaoning.
[1] R.J. Nelmes, J. Phys. C: Solid State Phys. 7 (1974) 3840. [2] T. Nagase, K. Sakane, H. Wada, J. Sol–Gel Sci. Technol. 13 (1998) 223. ¨ [3] K.-H. Hellwege, A.M. Hellwege (Eds.), Landolt-Borntein, New Series, Group III: Crystal and Solid State Physics, Vol. 16a, Springer Verlag, Berlin, 1981, pp. 261–623. ¨ [4] O. Madelung (Ed.), Landolt-Bornstein, New Series, Group III: Crystal and Solid State Physics, Vol. 28, Springer Verlag, Berlin, 1990. ¨ [5] E. Burzo, in: H.P.J. Wijn (Ed.), Landolt-Bornstein, New Series, Group III: Crystal and Solid State Physics, Vol. 27h, Springer Verlag, Berlin, 1993, pp. 128–204. [6] P. Toledano, H. Schmid, M. Clin, J.P. Rivera, Phys. Rev. B32 (1985) 6006. [7] H. Schmid, Ferroelectrics 162 (1994) 317. [8] V.Yu. Topolov, Z.G. Ye, J. Phys: Condens. Matter 8 (1996) 6087. [9] Z.G. Ye, J.P. Rivera, H. Schmid, Phase Transition 33 (1991) 43. [10] S.Y. Mao, F. Kubel, H. Schmid, K. Yvon, Acta Crystallogr. B47 (1991) 692. [11] Z.G. Ye, A.M. Janner, H. Schmid, J. Phys. Condens. Matter 9 (1997) 2607. [12] J. Campa-Molina, A.G. Castellanos-Guzman, M. Barcena-Soto, J. Reyes-Gomez, Solid State Commun. 89 (1994) 963. [13] T. Ito, N. Morimoto, R. Sadanaga, Acta Crystallogr. 4 (1951) 310. [14] D. Andreica, J.P. Rivera, S. Gentil, Z.G. Ye, M.S. Kumar, H. Schmid, Ferroelectrics 204 (1997) 73. [15] A.T. Shuvaev, I.V. Pirog, I.A. Zarubin, Physica B 208 (1995) 627. [16] J.K. Milne, M.J. Saundeys, P.J.E. Woods, Short Communications, Mineral. Mag. 41 (1977) 404. [17] J. Kobayashi, Y. Sato, H. Schmid, Phys. State Solid (a) 10 (1972) 259. [18] J. Kobayashi, I. Mizutani, H. Schmid, H. Schachner, Phys. Rev. B 1 (1970) 3801. [19] J. Kobayashi, Y. Uesu, H. Kumomi, Phase Transitions 4 (1984) 255. [20] H. Schmid, H. Tippmann, Ferroelectrics 20 (1978) 21. [21] J.P. Rivera, H. Schmid, J. Appl. Phys. 70 (1991) 6410. [22] L.G. Wood, Y. Qing-Rul, R.W. Whatmore, Phase Transitions 2 (1981) 85. [23] H. Schmid, J. Phys. Chem. Solids 26 (1965) 973. [24] T. Takahashi, O. Yamada, J. Appl. Phys. 48 (1977) 1258. [25] P. Tissot et al., Thermochim. Acta 56 (1982) 359. [26] V.I. Bugakov, V.S. Zasimov, R.N. Kuz’min, A.V. Pechennikov, A.I. Firov, Vestn. Mosk. Univ. Geol. 27 (1972) 95. [27] H. Schmid, H. Tippmann, J. Crystal Growth 46 (1979) 723. [28] H. Schmid, Phys. Stat. Sol. 37 (1970) 209. ´ [29] W.-J. Deiss, P. Blum, Comptes rendus de l’Academie des Sciences, (Paris) 244 (1957) 464.
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Synthesis and characterization of Fe–Cl boracite a a a a a, b b Z.H. Wang , D.Y. Geng , D. Li , X.G. Zhao , Z.D. Zhang *, Y.P. Wang , J.-M. Liu a
Institute of Metal Research, Academic Sinica, Wenhua Road 72, Shenyang 110015, China b Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China Received 29 March 2001; accepted 21 May 2001
Abstract The Fe–Cl boracite is prepared by a procedure, consisting of solution mixing, heating, grinding and H 2 reduction. The high valence metal ion Fe 31 reacts with H 2 to form the low valence metal ion Fe 21 , so that the corresponding boracite can be synthesized. The lattice parameters and the phase transition temperatures are determined by means of X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. The hydrogen not only plays a role in reduction, but also prevents the oxidation of the mixture. The ferroelectric property at room temperature of the sample has been substantially demonstrated. 2001 Elsevier Science B.V. All rights reserved. Keywords: Transition metal compounds; Chemical synthesis; TEM; Ferroelectric
1. Introduction Boracites form a large crystal family with the general formula M 3 B 7 O 13 X, where M stands for divalent metal ions (Mg, Cr, Mn, Fe, Co, Ni, Cu, Zn, or Cd), and X stands for halogen ions (F, Cl, Br, or I), with more than 20 isomorphous compounds. It has been found that in some cases X can be OH, S, Se, or Te and M monovalent Li [1–5]. Boracites possess peculiar properties that have attracted attention not only of crystallographers but also of physicists and materials scientists. Though the detailed behavior varies with M and X, nearly all the halogen boracites exhibit transitions from a high temperature cubic paraelectric phase to one or more lower symmetry phases at low temperatures. The low temperature phase with low symmetry is usually ferroelectric, which has potential for application. The exceptions are Cr–Br, Cr–I and Cu–I boracites, which remain cubic down to 4 K [6,7]. The Cr–Cl boracite undergoes a phase transformation to a tetragonal non-polar 4¯ 2 m phase on cooling, which is so far a unique one in the boracite family [8–11]. A phase transition between the cubic and orthorhombic phase occurs in some boracite single crystals [12]. The structures
*Corresponding author. Tel.: 186-24-2384-3531; fax: 186-24-23891320. E-mail address: [email protected] (Z.D. Zhang).
of high- and low-boracites forming above and below the a–b inversion temperature 2658C are essentially the same, both containing the same boron–oxygen network and differing only in the position of M and halogen atoms [13]. The transition temperature of the boracites varies widely (60–800 K), but for any given metal it falls in the sequence X5Cl→Br→I [1,14]. For instance, when M stands for Fe, and X5Cl, Br, I, the cubic–orthorhombic transition temperatures are about 265, 132 and 728C, respectively [15–19]. The temperature dependence of spontaneous birefringence indicated that three types of phases sequentially occur for the ferroelectric phases of various boracites [20]. The ferroelectric phase transition in Fe–X (X5Br, I) boracites were investigated by EXAFS spectroscopy and special X-ray diffraction technique [15,17]. Linear and quadratic magnetoelectric (ME) effects in the boracite were studied [5,21]. The lattice parameters and the lattice strains of halogen boracites were measured [18,22]. The structure, phase transition temperature and constituents of iron–boracite (Fe, Mg, Mn) 3 B 7 O 13 C1 found in nature were investigated [16]. The chemical vapor transport method is currently the most successful and widely used technique for the synthesis of boracites in single crystal form and permits the preparation of a larger number of boracites [3–5,9–12,21– 24]. With the starting constituents MO, B 2 O 3 , and metal halides, the global reactions for synthesis of nearly all boracites can be written:
0925-8388 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01613-9
Z.H. Wang et al. / Journal of Alloys and Compounds 329 (2001) 278 – 284
MO 1 2HX(g) 5 MX 2 (g) 1 H 2 O(g) 7B 2 O 3 1 6MX 2 1 5H 2 O → 2M 3 B 7 O 13 X 1 10HX The solid solutions of (M, M9)–X and M–(X, X9) boracites have been grown by this method [23]. Many boracites become antiferromagnetic and weakly ferromagnetic at temperatures below 60 K. Magnetic measurements were performed for Fe–X (X5Cl, Br, I) boracites in the temperature range of 3.6–300 K [11,21]. Temperature dependence of the magnetization and magnetic susceptibility for all compounds show the existence of antiferromagnetism, accompanied by a weak ferromagnetic component [11,21]. In this work, a new method is developed for preparing iron–chlorine boracite (Fe 3 B 7 O 13 Cl) powders. The process consists in solution mixing, heating, grinding and H 2 reduction. The powders obtained by this method are a mixture of a-Fe and Fe 3 B 7 O 13 Cl. The Fe 3 B 7 O 13 Cl can be separated by dissolving a-Fe in dilute HCl. The structure of the Fe–Cl boracite has been identified by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The temperatures of the structural phase transitions have been verified by differential scanning calorimetry (DSC). The ferroelectric properties at room temperature of the Fe 3 B 7 O 13 Cl are measured using HVS6000RT standard ferroelectric testing unit under a virtual ground mode. For comparison, the structure of the phases in the powders, prepared by the process of solution mixing, heating, grinding and heating in air, has been studied also.
2. Experiments Fe–Cl boracites crystals were prepared by solution mixing, heating and H 2 reduction in the temperature range from 360 to 10008C, with the starting constituents boric acid (H 3 BO 3 ) (.99.5%) and iron chloride hydrous (FeCl 3 ?6H 2 O) (.99.0%). Synthesizing the Fe–Cl boracite powder consisted in the following steps. The starting raw materials H 3 BO 3 and FeCl 3 ?6H 2 O were dissolved in appropriate de-ionized (DI) water. Their weight ratio was 3:5. Their solutions were mixed together in a beaker, and then heated on an electric stove. The color of the solution became deep when the temperature reached 708C. The solution was boiled until most fluffy blocks appeared. Cooling the blocks down to room temperature changed their color to red–brown. The dried precursors were obtained by grinding the blocks into a fine powder in a mortar. The dried precursors were hydrogenated in hydrogen atmosphere in quartz tubes for half an hour at 360, 500, 600, 700, 800, 900 and 10008C, respectively. The reacted compacts were crushed in a mortar protected by petroleum ether. Then the mixture of a-Fe and Fe 3 B 7 O 13 Cl was obtained and dried in air. Almost pure
279
Fe 3 B 7 O 13 Cl was obtained by the following separation procedure. The mixture was dissolved in dilute 9% HCl (hydrochloric acid) for about 10 min, then filtered and washed with DI water several times, and the residue was dried in air. In order to determine the role of the H 2 , the dried precursors were heated in Ar at 8008C for about 30 min, then cooled to room temperature, and the products were crushed into powders in mortar. For comparison, the dried precursors were heated in air for half an hour at 250, 550, 900 and 10008C, respectively. The phases of the powders synthesized by each step were identified using Cu Ka X-ray diffraction at room temperature in a D/ max-gA diffractometer with a pyrolytic monochromator. The grain size and the lattice parameters of the samples were determined using a Philips EM 420 transmission electron microscope with the accelerating voltage of 100 kV. The phase transition temperatures were determined by using a Perkin Elmer DSC-7 differential scanning calorimeter with heating rate of 208C min 21 . In order to estimate the content of metallic iron, the magnetization of the samples were measured with a PAR155type vibrating sample magnetometer. The samples for ferroelectric measurements were simply pressed under a pressure of 1.5?10 4 kg cm 22 . The density of the sample measured is 2.60 g cm 23 in good agreement with the theoretical value of 2.67 g cm 23 . The resistivity measured for the pressed sample is infinite. The ferroelectric properties of the samples were measured using HVS6000RT standard ferroelectric testing unit (Radiant Inc., New Mexico, USA) under a virtual ground mode at room temperature. A capacitor was aligned in parallel to the sample in order to compensate the capacitance loss. The bottom and top electrodes were (111)-oriented Pt layers of |200 nm in thickness, which were fabricated using sputtering technique. In the measurement, five voltage pulses, one for pre-polarization, two for negative poling and two for positive poling, were generated and a charge-integrator was coupled to count the as-generated charge. The total time interval for the five pulses was 108.80 ms as the HVS6000RT standard ferroelectric testing unit did.
3. Results and discussion After treatment in hydrogen at 360, 500, 600, 700 and 8008C, the products are grey and fluffy blocks, with weak hardness, which can be crushed softly into powders in a mortar. The products obtained at / above 9008C are grey blocks with strong hardness, which must be smashed roughly before crushing into powders in the mortar protected by petroleum ether. The color of the powders obtained below 6008C becomes red–brown after aging in air for more than 2 days. It is possible that there is amorphous FeCl 3 (iron chloride) in the powders, which
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changes color when absorbing water in air. We can identify by X-ray diffraction that the constituents of the powders are not altered before and after the color changes. But the powders obtained at temperatures higher than 7008C do not show such change of color. In order to identify the products synthesized by each step of the process, XRD was performed on the powders obtained from the first step to the final step. XRD patterns are shown in Fig. 1(a)–(h) for each step. One can see that the dried precursor (Fig. 1(a)) is in the amorphous state. There is a peak at about 2u 5288, which may correspond to H 3 BO 3 in raw materials. This peak is sometimes obvious, sometimes obscure, depending on the content of H 3 BO 3 (Fig. 1(b)–(e)). The amorphous precursor crystallizes into FeCl 2 ?4H 2 O and H 3 BO 3 by reducing in H 2 at 3608C, as shown in Fig. 1(b). The possible global reaction is: 2FeCl 3 ? 6H 2 O 1 H 2 → 2FeCl 2 ? 4H 2 O 1 2HCl ↑ 1 4H 2 O↑ H 3 BO 3 does not react with other compounds at this temperature. When the temperature rises, the new components appear, while the former ones disappear. We can
Fig. 1. XRD patterns of (a) the dried precursors with FeCl 3 ?6H 2 O and H 3 BO 3 as the starting raw materials, the powders obtained in H 2 atmosphere at (b) 3608C, (c) 5008C, (d) 6008C, (e) 7008C, (f) 8008C, (g) 9008C and (h) 10008C.
see from Fig. 1(c)–(h), that FeCl 2 ?4H 2 O and H 3 BO 3 disappear gradually, while the mixture of a-Fe and Fe 3 B 7 O 13 Cl emerges at 5008C. The amount of the mixture increases with increasing temperature. We can infer the possible reaction in the temperature range from 500 to 8008C: 4FeCl 2 ? 4H 2 O 1 7H 3 BO 3 1 H 2 → Fe 3 B 7 O 13 Cl 1 Fe 1 24H 2 O ↑ 1 7HCl ↑ It is shown in Fig. 1(f)–(h) that the content of Fe in the mixture increases, but that of Fe 3 B 7 O 13 Cl decreases with increasing temperature. As shown in Fig. 1(c)–(h), the amount of Fe–Cl boracite in the compact after treatment in H 2 at 700 and 8008C is higher than that treated at 9008C. This indicates that the Fe 3 B 7 O 13 Cl is decomposed in H 2 atmosphere when heated above 7008C. Finally it disappears at 10008C (see Fig. 1(h)), following the thermal decomposition process described in Ref. [25]. The microstructure of the powders hydrogenated at 7008C was studied by TEM observations. The bright field TEM photograph shown in Fig. 2(a) indicates that the mean size of the powders is of the order of the micron. Fig. 2(b) gives the corresponding selected area diffraction pattern of a grain of Fe–Cl boracite. The interplanar ˚ which is consistent spacing for (110) or (002) is 6.12 A, ˚ [26]. The value correwith the reference data (6.11 A) ˚ b58.65 A, ˚ sponds to the lattice parameters of a58.65 A, ˚ as shown in Table 1. and c512.24 A, The final powders, dissolved in dilute HCl with different duration, were studied. The results show that the content of a-Fe is minimum when the mixture is treated in dilute HCl for about 10 min. The color of the mixture of the Fe–Cl boracite with a little amount of a-Fe is light green, and the color becomes deeper when the Fe content increases. The X-ray diffraction pattern of the Fe–Cl boracite after the separation process is shown in Fig. 3. It is clear that the main phase in the powders after separation is Fe–Cl boracite, with a little amount of a-Fe. In order to determine the content of a-Fe in the mixtures, we measured the magnetization of the mixtures. Fig. 4 shows the magnetization versus magnetic field at room temperature of the mixture, from which we can estimate that the content of a-Fe is about 15 and 0.7 wt.% before and after separating, respectively. It is hard to see the loop from Fig. 4 because the coercive field is extremely low. Although it seems difficult to eliminate all traces of metallic iron by dissolving the crushed material in HCl, without dissolving too much of the boracite itself, such a material may for example be used as the boracite starting material in a purifying vapour transport process [27]. The changes of lattice parameters of M–X boracites were studied previously [12,14,16,23]. The lattice parameters determined by XRD and TEM in this work are very close to those of Ref. [26], as shown in Table 1. DSC measurement was performed on the powders hydrogenated
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Fig. 2. (a) Bright field TEM graph and (b) corresponding selected area diffraction pattern of Fe–Cl boracite treated in H 2 at 7008C.
Table 1 ˚ of Fe–Cl boracite Lattice parameters [A] a
b
c
Technique
Reference
8.62 8.60 8.65
8.62 8.60 8.65
12.23 12.17 12.24
XRD XRD TEM
[21] This work This work
at 8008C and after separation. The DSC curves shown in Fig. 5 reveal that the phase transitions of Fe–Cl boracite occur both upon heating and cooling. The phase transition temperatures of the Fe–Cl boracites are consistent with the
Fig. 3. X-ray diffraction pattern of Fe–Cl boracite treated in H 2 at 7008C.
data studied by spontaneous birefringence (Dn i ) [20]. The data are compared in Table 2. The differences between the data are within the error bars for the different sample preparations and detection techniques. Because the second phase transition at lower temperature is not sensitive to heat, it cannot be distinguished clearly by the DSC. Fig. 6 presents several ferroelectric hysteresis loops at room temperature of the Fe–Cl boracite as measured by different maximum applied electric fields. The sample’s ferroelectricity is demonstrated by these loops. It is seen that the polarization is not saturated under the maximum applied electric field of 25.0 kV cm 21 . The remnant polarization as evaluated from the largest loop is about 0.92 mC cm 22 , whilst the coercivity is about 18.0 kV cm 21 . Maybe due to the metallic ion doping, the sample leakage current may be a little high so that the as-measured loop is shown to be fat. However, the measured coercivity (18.0 kV cm 21 ) is not really the static coercivity. It has been shown that the quasi-static ferroelectric coercivity field of highly insulating single crystals of Fe–Cl boracite at room temperature is extremely high, i.e. beginning of switching occurs at about 320 kV cm 21 and saturation at 800 kV cm 21 [28]. These measurements were done quasi-statically with liquid electrodes, which do not allow charge injection into the sample and herewith avoid the artificial increase of the coercivity field. In our case, we obtain a much lower coercivity so that the charge injection is not serious. The as-measured hysteresis in this work does not show a fully saturated electrical polarization because the static coercivity as reported earlier is quite high [28] and the HVS6000RT test unit cannot apply a voltage higher than 4.0 kV to the present thick sample. Furthermore, the polycrystalline
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Fig. 4. Magnetization versus magnetic field at room temperature (a) before and (b) after the separation process.
sample definitely shows a much lower coercivity than the single crystal. Usually, the coercivity of the polycrystalline should be at least several times lower than the single crystal, mainly because the rotation or reversal of the grain or domain could be much easier in polycrystalline than in single crystal. Nevertheless, the ferroelectric property of the sample has been substantially demonstrated. For the purpose of comparison, the dried precursors were heated in air. The XRD patterns (shown in Fig. 7) indicate that all powders, except for those obtained at 2508C, contain the stable Fe 2 O 3 phase. Compared with the raw powders, those obtained at 2508C are not changed very much. A kind of unstable Fe 2 O 3 (i) phase appears at 5508C, but disappears when the temperature rises up to 6008C. This suggests that the H 2 reduction process is very important for the formation of Fe–Cl boracite.
Fig. 5. DSC graph of Fe–Cl boracite with (a) increasing and (b) decreasing temperature.
In order to determine the role of the H 2 , the dried precursors were heated in Ar at 8008C for about 30 min, then cooled to room temperature, and the products were Table 2 Phase transition temperatures [8C] of Fe–Cl boracite 4¯ 3m↔mm2
mm2↔m
m↔3m
Technique
Reference
336 332
270 274
255 252
Dn i DSC (heating)
[10] This work
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Fig. 6. Ferroelectric hysteresis loops at room temperature of Fe–Cl boracite. The loops denoted by P1–P6 correspond to those measured at different maximum applied electric fields.
crushed into powders in a mortar. The XRD patterns in Fig. 8 show that the final powders consisted of a mixture of Fe–Cl boracite, a-Fe, Fe 2 O 3 and Fe 3 O 4 . Furthermore, the dried precursors were made by the
Fig. 8. XRD patterns of the dried precursors (with starting raw materials H 3 BO 3 and FeCl 3 ?6H 2 O) heated in Ar at 8008C.
same process, but with FeCl 2 ?4H 2 O and H 3 BO 3 as starting raw materials. The dried precursors were heated in Ar at 8008C for about 30 min, and cooled to room temperature. The products are blocks with the color of ashen, which were crushed into powders. The structures of the powders were determined by XRD. Fig. 9 shows that the final powders consist of Fe 3 B 7 O 13 Cl and a small amount of a-Fe. The results above suggest that the H 2 reduction process is indispensable for the formation of Fe–Cl boracite, when FeCl 3 ?6H 2 O and H 3 BO 3 are used as the starting raw materials. The high valence metal ion Fe 31 reacts with H 2 to form the low valence state so that the corresponding boracite can be synthesized. It is evident that the hydrogen not only plays a role in reduction, but also prevents the mixture being oxidized. The present synthesis is related to the method described in Ref. [29], where Fe–Cl boracite is also obtained in a hydrogen atmosphere, but starting with Fe 2 O 3 and BCl 3 , leading at high temperatures to a gas phase of similar composition. In this work, Fe 2 O 3 is formed as an intermediate phase.
4. Conclusion
Fig. 7. XRD patterns of the dried precursors (with FeCl 3 ?6H 2 O and H 3 BO 3 as the starting raw materials) heated in air at (a) 10008C, (b) 9008C, (c) 5508C and (d) 2508C together with (e) the dried precursors. Fe 2 O 3 (i) is unstable phase, while Fe 2 O 3 is stable one.
Mixtures of a-Fe and Fe–Cl boracite have been synthesized by a chemical process including solution mixing and heating, grinding and reduction by H 2 , with FeCl 3 ? 6H 2 O and H 3 BO 3 used as the starting materials. Nearly pure Fe–Cl boracite was obtained by separating a-Fe from
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References
Fig. 9. XRD patterns of the dried precursors (with FeCl 2 ?4H 2 O and H 3 BO 3 as the starting raw materials) heated in Ar at 8008C.
the mixture using diluted HCl. The phase transition temperatures and the lattice parameters of Fe–Cl boracite determined by means of DSC, XRD, TEM are consistent with previous reports. The high valence metal ion Fe 31 reacts with H 2 to form the low valence state Fe 21 so that the corresponding boracites may be synthesized. The hydrogen not only plays a role in reduction, but also protects the mixture from oxidation. Furthermore, other high valence states of transition metals can be also reduced by H 2 to their low valence state. Therefore the described method may be extended to the synthesis of other boracite compositions. It has been demonstrated that the as-prepared Fe–Cl boracite is ferroelectric at room temperature.
Acknowledgements This work has been supported by the project No. 59725103 of the National Natural Sciences Foundation of China and by the Science and Technology Commission of Shenyang and Liaoning.
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