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Growth of Sr3Fe2O7−x single crystals by the floating zone method
ARTICLE IN PRESS
Journal of Crystal Growth 273 (2004) 207–212 www.elsevier.com/locate/jcrysgro
Growth of Sr3Fe2O7x single crystals by the floating zone method A. Maljuk, J. Strempfer, C. Ulrich, M. Sofin, L. Capogna, C.T. Lin, B. Keimer Max-Planck-Institut fu¨r Festko¨rperforschung, Heisenbergstrasse 1, D-70569, Stuttgart, Germany Received 10 July 2004; accepted 28 July 2004 Communicated by M. Schieber Available online 30 September 2004
Abstract We report the successful growth of high-quality Sr3Fe2O7x single crystals with volumes of up to 0.3 cm3 by using the floating zone method. Compositional homogeneity of the crystals was verified by powder X-ray diffraction, optical and electron microscope measurements. The X-ray rocking curves of the Sr3Fe2O7x single crystal gave a full-width at halfmaximum (FWHM) of Bragg reflections of 0.02–0.031. The influence of the growth rate on the crystal size and quality was demonstrated. The effect of oxygen annealing was studied by magnetic susceptibility and Raman light scattering experiments. The Fe(+3)/Fe(+4) ratio was determined both from the magnetization and thermo-gravimetric data. r 2004 Elsevier B.V. All rights reserved. PACS: 81.10.Fq; 81.30.Dz Keywords: A2. Floating zone technique; B1. Oxides
1. Introduction It is well known that iron ions in compounds are usually divalent or trivalent. In combination with strontium several compounds have been synthesized with iron in the tetravalent state: SrFeO3x, Sr3Fe2O7x and Sr2FeO4x [1,2]. All of them demonstrated exciting physical properties like a Corresponding author. Tel.: +49-711-689-1405; fax: +49-
711-689-1093. E-mail address: [email protected] (A. Maljuk).
giant magnetoresistance associated with the ordering of spin and charge degrees of freedom [3,4]. Since the growth of single crystals of tetravalent iron oxide is quite difficult, these compounds are predominantly studied in form of powder samples [1–6]. Thus, high-quality and large-size single crystals are in great demand for reliable physical and structural measurements. Sr3Fe2O7 consists of isolated double sheets of FeO6 octahedra, which is called a layered perovskite structure. Preparation of Sr3Fe2O7x crystals is not obvious due to the question of the
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.07.091
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phase stability limit of this compound in air [5,6]. The crystal structure of the Sr3Fe2O7x phase depends on the actual oxygen content [5], which widely varies (0:0pX p1:0) with temperature and oxygen pressure. According to the published SrO–Fe2O3 phase diagram [7] the Sr3Fe2O7x compound melts incongruently at 1520 1C in air: Sr3 Fe2 O7x ! SrFeO3x þ liquid: The last reaction is not correct because, as it was found in Ref. [8], SrFeO3x phase melts at 1435 1C in air. Therefore, the question of the melting point and behavior of the Sr3Fe2O7x phase is still open. It was reported that Sr3Fe2O7x single crystalline samples can be prepared by floating zone melting [9], but no data concerning the crystal size and crystal quality were provided. It must be mentioned that the reported growth rate of 10 mm/h [9] is unusually high for the incongruently melted compound. To the best of our knowledge no other attempts were performed to grow Sr3Fe2O7x single crystals. This is probably due to the complex structural and melting behavior of this phase, as mentioned above. In this paper, we report the successful growth of large-size and high-quality Sr3Fe2O7x single crystals with volumes of up to 0.3 cm3 by the floating zone method. The crystals were examined by X-ray diffraction, electron and optical microscope analysis. The oxygen content was measured by the thermo-gravimetric method. Magnetic susceptibility and Raman light scattering experiments were performed on as-grown and oxygen annealed crystals.
2. Experimental procedure The starting materials for the preparation of feed rods were SrCO3 (99.995%) and Fe2O3 (99.99%) powders. Before mixing, the SrCO3 compound was dried by heating at 150 1C for 5–6 h in air. The raw materials with stoichiometric amounts were manually mixed in presence of ethanol, dried, and calcined at 1000 1C for 2 24 h in air with an intermittent manual grinding in air. The pre-heated powder was formed into a cylindrical shape 7–8 mm in diameter and 80–100 mm in length, and then pressed at a
hydrostatic pressure of 300 MPa.The rods were sintered at 1250–1350 1C for 24 h in flowing oxygen. The heating and cooling rates were 100 1C/h. It was reported that ceramic samples of the Sr3Fe2O7x compound, prepared at 1250–1300 1C, were very sensitive to moisture [5,6]. Our experiments have shown that this sensitivity causes an additional difficulty for following operations with rods after sintering. For example, it leads to the rods crushing after cutting and grinding in air. Therefore, the rods should be kept in an argon box after sintering. It was found that synthesis at 1350–1370 1C in oxygen flow leads to more dense and stable feed rods in air. The best results were achieved when feed rods were prepared by a 2-step synthesis: (1) high-temperature treatment at 1300 1C for 24 h in oxygen flow; (2) low-temperature annealing at 550 1C for 2–3 days in oxygen flow. These rods were the most stable in air and could be handled without serious problems. The apparatus used for crystal growth was a 4mirror type infrared image furnace (Crystal System Inc. FZ-T-10000-H-III-VPR) equipped with four 1000 W halogen lamps as heat source. The growth conditions were as follows: the seed and feed shafts were rotated in opposite directions at rates of 10–15 rpm; the traveling rate was 1, 2, 3 and 5 mm/h. Oxygen pressures of 0.2–3.0 bar were applied during growth. The grown ingots were cut into wafers perpendicular to the growth direction, and both surfaces of wafers were polished to the mirror finish. Then polished specimens were characterized using a polarizing microscope to examine the presence of sub-grains and inclusions. A compositional analysis of the as-grown crystals was carried out by powder X-ray diffraction (XRD, ‘‘PHILIPS’’ PW3710), scanning electron microscope (SEM, ‘‘TESCAN’’ TS-5130MM) and energy dispersive X-ray analysis (EDX, ‘‘RO¨NTEC’’). A CuKa X-ray source was used for powder XRD measurements to check the phase purity and the crystal structure. A precise lattice parameter determination was performed using MoKa X-ray radiation generated by a rotating anode equipped with a CCD detector. The oxygen content in the as-grown Sr3Fe2O7x single crystals was measured by
ARTICLE IN PRESS A. Maljuk et al. / Journal of Crystal Growth 273 (2004) 207–212
3. Results and discussion Fig. 1 presents a typical Sr3Fe2O7x boule, grown at a traveling rate of 1 mm/h under an oxygen pressure of 3 bar. The boule was black with a metallic luster, and had 6 mm in diameter and 40 mm in length. Optical polarized microscope (‘‘Olympus’’ MS-11) and EDX study confirmed that the end-growth part of the ingot of about 15 mm consists of two single-crystalline and inclusion-free grains. The largest grain with the volume of 0.3 cm3 has a growth direction close to the [1 2 0]-axis, as found from a Laue picture. Fig.
Fig. 1. As-grown Sr3Fe2O7x boule, pulling rate 1 mm/h.
Sr3Fe2O7-xcrystal. (0010)
(105)
4000
10
20
30
40
(208)
(0012), (206) (118) (204) 50
(1011)
(200)
0
(116)
(114) (107)
(008) (112)
(101)
1000
(211)
2000
(1110) (215)
(110)
(006)
3000
(004)
Intensity (arb. units)
heating small amounts of Sr3Fe2O7x crystals in Ar/H2 flow using a DTA-TG apparatus (NETZSCH STA-449C). Raman light scattering experiments were performed using a Dilor-XY triple spectrometer in a close to true backscattering geometry. The spectra were detected using a liquid nitrogen cooled CCD camera. For excitation, the 514 nm line of an Ar+–Kr+ mixed gas laser was used. In order to avoid sample heating, the laser power was kept below 10 mW on the sample position. All spectra were recorded at room temperature on nonoriented crystals without polarization analysis. The magnetic susceptibility measurements were carried out by a superconducting quantum interference device magnetometer (‘‘Quantum Design’’, model MPMS 7.0) in the temperature range of 5–300 K. Magnetic field was perpendicular to the c-axis which is the easy magnetization axis for Sr3Fe2O7x [9].
209
60
2θ (degrees)
Fig. 2. Powder XRD pattern of the as-grown Sr3Fe2O7x crystal, pulling rate 1 mm/h (CuKa -radiation).
2 shows the powder X-ray diffraction pattern of the as-grown Sr3Fe2O7x crystal. The crystal was compositionally homogeneous without any trace of secondary phase in the XRD pattern. The crystal structure could be assigned to a tetragonal space group and all peaks corresponded to the pure Sr3Fe2O7x phase. The lattice parameters ( and c ¼ were determined as a ¼ 3:875ð4Þ A ( 20:23ð1Þ A from the single crystal refinement. The data are in good agreement with ceramic samples [6]. No visible iron evaporation was observed during Sr3Fe2O7x crystal growth. It must be mentioned that the Sr3Fe2O7x ingot, prepared at the traveling rate of 5 mm/h, consists of small single crystalline grains with typical dimensions of 2 1.5 0.5 mm3. Powder XRD pattern of Sr3Fe2O7x crystals, mechanically separated by a sharp scalpel from the end-growth part of ingot, showed only the presence of the Sr3Fe2O7x compound. X-ray rocking curves, measured on the as-grown crystal, showed that Bragg reflections have much larger FWHM values of about 1.01, than that observed for samples grown at 2 mm/h (see below). This reflects the effect of the relatively high growth rate, 5 mm/h, on the crystal quality (mosaicity). Single crystalline Sr3Fe2O7x samples with dimensions up to 7 4 3 mm3 were mechanically separated (cleaved) from an as-grown ingot prepared at 2 mm/h. All crystals were inclusion-free, as confirmed by powder XRD, optical and
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Magnetic susceptibility (emu/mol)
0.06
as-grown Sr3Fe2O6.76 crystal, 10 Oe.
0.05
T1=78 K
annealed Sr3Fe2O6.80 crystal, 10 Oe.
0.04 0.03 0.02 0.01
T2=115 K
T3=148 K.
0.00 0
50
100
150 200 Temperature (K)
250
300
Fig. 4. Magnetic susceptibility of the Sr3Fe2O7x crystals (H?c-axis). Fig. 3. X-ray rocking curves for the as-grown Sr3Fe2O7x crystal, pulling rate 2 mm/h (MoKa -radiation).
magnetization (see below) measurements. The Xray rocking curves, measured on the as-grown crystal, are depicted in Fig. 3. The profiles of (0 0 20) and (2 4 0) Bragg-reflections have FWHM values of 0.021 and 0.031, respectively, indicating the excellent quality of the crystal. Thus, our experimental results suggest that high-quality and large-size single crystals can be obtained only at pulling rates of 1–2 mm/h. The oxygen content of the as-grown Sr3Fe2O7x crystal prepared at oxygen pressure of 3 bar is X ¼ 0:24 0:03; as obtained by the thermo-gravimetric method. Oxygen deficient Sr3Fe2O7x crystals are hygroscopic and contain water (up to 1.0 wt%), when kept in air at room temperature. The water is released above 100–150 1C. It is worth mentioning that Sr3Fe2O7x crystals, when stored in air for several days, became very brittle and cleaved easily along the (0 0 l) plane. Our DTA-TG and melting experiments showed that the Sr3Fe2O7x phase melts according to the peritectic reaction: Sr3 Fe2 O7x ! SrO þ liquid at 1470–1480 1C in air. Based on the results of these experiments, we assume the existence of eutectic between Sr3Fe2O7x and SrFeO3x phases. The temperature dependence of the magnetization of the as-grown Sr3Fe2O6.76 single crystal is shown in Fig. 4. The magnetization measurement confirmed that the crystal was free from magnetic inclusions (for example, a-Fe2O3 or Fe3O4) for the
following reasons. It should be taken into account that only the Morin phase transition at 260 K is known for a-Fe2O3 below room temperature [10]. The absence of magnetic anomaly at 260 K (see Fig. 4) means that there is no hematite inclusion. In the case of magnetite only the Verwey phase transition occurs at 120–122 K below room temperature [11]. The absence of magnetic feature on the magnetization curve at this temperature is evident that the Fe3O4 phase does not exist as inclusions in the crystal. We exclude the presence of maghemite because the phase diagram of the SrO–Fe2O3 system [7] does not indicate the existence of g-Fe2O3. A small magnetic anomaly, unknown before, was observed at T 1 ¼ 78 K: The temperature dependence of the magnetization of a Sr3Fe2O6.80 crystal annealed at 550 1C for 250 h in O2 flow showed two magnetic transitions at T 2 ¼ 115 K and T 3 ¼ 148 K: Furthermore, the magnetic anomaly at T 1 ¼ 78 K has nearly vanished. It is well known that the Sr3Fe2O7x phase has an antiferromagnetic transition at T N ¼ 1102120 K for X ¼ 0:0020:05 [1,12,13]. The Neel temperature decreases with decreasing oxygen content and converges to zero degrees for X ¼ 0:50 [1]. Therefore, the magnetic feature at T 2 ¼ 115 K should be associated with this transition. The magnetic transition at T 3 150 K was previously observed for oxygen-deficient Sr3Fe2O7x (X 40:5) ceramic samples [9] both by magnetization and neutron
ARTICLE IN PRESS A. Maljuk et al. / Journal of Crystal Growth 273 (2004) 207–212 450 555
Sr3Fe2O7-x crystal
400
T=300 K.
594 384 173
235
250
460
300
78
annealed (1 atm O2)
200
919.4
Intensity (arb. units)
350
150 100 as-grown 50 0
200
400
600
800
1000
Raman Shift (cm-1)
Fig. 5. Raman spectra of as-grown Sr3Fe2O6.76 and annealed Sr3Fe2O6.80 single crystals (at room temperature). 100000 Sr3Fe2O6.8 single crystal.
80000
Resistivity, Ohm.
diffraction experiments. It was demonstrated [9] that the magnetic structure of Sr3Fe2O6.94 is different from that of Sr3Fe2O6.21. It should be noted that annealing at 550 1C in oxygen flow changes the actual oxygen content only a little, i.e. X ¼ 0:24 0:03 (as-grown) and X ¼ 0:20 0:03 (annealed), respectively. The magnetic susceptibility can be described in the high-temperature range (150–300 K) by the Curie–Weiss law: m ðper FeÞ ¼ 5:5 mB and Y ¼ 15 K: The obtained magnetic moment of 5:5 mB is somewhat higher than for the stoichiometric Sr3Fe2O7 phase ð5:3 mB Þ [13]. This is caused by the presence of Fe+3 (d5) ions having a larger magnetic moment than Fe+4 (d6). Taking as a reference the stoichiometric Sr3Fe2O7 [6] compound, the Fe+3/ Fe+4 ratio in the as-grown crystal can be estimated. Since for Fe+3(d5) the pure spin magnetism is expected, one obtains, that about 30% of Fe+3 lead to the observed increase of the magnetic moment. The oxygen content X=0.24 (as-grown crystal) corresponds to the effective oxidation state of iron ions +3.76. This means that the Fe+3 content is about 24%. This value agrees well with that obtained from the magnetization data. The changes in the Fe+3/Fe+4 ratio induced by annealing in O2 are only minor and thus be reliably determined from the magnetic data. Fig. 5 shows Raman spectra of as-grown Sr3Fe2O6.76 and annealed Sr3Fe2O6.80 single crystals. It is worth mentioning that the Raman spectra were identical for different positions on the sample surface. This proves the good homogeneity and high quality of the Sr3Fe2O7x single crystals. The low-frequency modes at 173 cm1 can be assigned to vibrations of the Sr-ion, whereas the high-energy phonon modes, especially the mode at 555 cm1, originate from cooperative vibrations of the FeO6 octahedra. Our Raman experiments showed that the intensity of the mode at 555 cm1 increased drastically after annealing although the oxygen content increased only a little. At the same time other modes like those at 173 cm1 and 460 cm1 remained almost unchanged. Dann et al. studied the crystal and magnetic structure of Sr3Fe2O7x with different oxygen deficiencies, using neutron powder diffraction [6]. They found a crystallographic transition around
211
zero magnetic field 10 Oe
60000
1T
40000 20000 0 50
100
150
200
250
300
350
400
Temperature, K.
Fig. 6. Temperature dependence of the conductivity of the Sr3Fe2O6.8 crystal (H?c-axis).
X ¼ 0:5: Furthermore, the O(3) site was found to be partly occupied, while the O(1) and O(2) positions were fully occupied for all oxygen contents. Low temperature specific heat measurements performed by Kobayashi et al. [14] have shown that the electronic state of the Sr3Fe2O7x drastically changes around X ¼ 0:2: This transition was not observed in Ref. [6]. Thus, our SQUID and Raman data give the evidence of the oxygen redistribution during the oxygen annealing, which can lead to the electronic state transition reported in Ref. [14]. Preliminary resistivity measurements (4-point technique) show a semi-conducting behavior of the electrical conductivity (annealed sample) in the temperature range of 50–350 K (Fig. 6) This result is in good agreement with the data for ceramic
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samples [12]. No effect of magnetic field on the conductivity was observed up to a field strength of 7 T. The conductivity curve at 7 T is not shown because it is almost the same with the 1 T curve.
4. Conclusions Single crystals of the Sr3Fe2O7x compound were successfully grown with volumes of up to 0.3 cm3 by the floating zone method. All crystals were inclusion-free. The effect of growth rate on the crystal size and quality was shown. The X-ray rocking curves of the Sr3Fe2O7x single crystal have FWHM values of 0.02–0.031, demonstrating the high quality of the crystals. The effect of oxygen annealing was shown by means of magnetic susceptibility and Raman measurements, demonstrating the possibility of oxygen redistribution. The Fe(+3)/Fe(+4) ratio was determined both from the magnetization and thermo-gravimetric data.
Acknowledgements The authors are grateful to Mrs. G. Goetz for X-ray and Mrs. E. Bruecher for SQUID measurements.
References [1] J.B. MacChesney, H.J. Williams, R.C. Sherwood, J.F. Potter, Mater. Res. Bull. 1 (1966) 113. [2] T. Takeda, Y. Yamaguchi, H. Watanabe, J. Phys. Soc. Japan 33 (1972) 967. [3] P. Adler, U. Schwarz, K. Syassen, G.K. Rozenberg, G.Y. Machavariani, A.P. Milner, M.P. Pasternak, M. Hanfland, Phys. Rev. B 60 (1999) 4609. [4] A. Lebon, P. Adler, C. Bernhard, A.V. Boris, A.V. Pimenov, A. Maljuk, C.T. Lin, C. Ulrich, B. Keimer, Phys. Rev. Lett. 92 (2004) 037202. [5] E. Lucchini, D. Minichelli, G. Sloccari, J. Mater. Sci. 9 (1974) 212. [6] S.E. Dann, M.T. Weller, D.B. Currie, J. Solid State Chem. 97 (1992) 179. [7] P. Batti, Ann. Chim. (Rome) 52 (1962) 941. [8] A. Maljuk, J. Strempfer, C. Ulrich, A. Lebon, C.T. Lin, J. Crystal Growth 257 (2003) 427. [9] K. Mori, T. Kamiyama, H. Kobayashi, T. Arima, K. Ohoyama, S. Ikeda, Appl. Phys. A 74 (Suppl) (2002) S914. [10] F.J. Morin, Phys. Rev. 78 (1950) 819. [11] E.J. Verwey, P.W. Haajman, Physica (Utrecht) 8 (1941) 979. [12] K. Kuzushita, S. Morimoto, S. Nasu, S. Nakamura, J. Phys. Soc. Japan 69 (2000) 2767. [13] S.E. Dann, M.T. Weller, D.B. Currie, M.F. Thomas, A.D. Al-Rawwas, J. Mater. Chem. 3 (1993) 1231. [14] H. Kobayashi, M. Kita, H. Onodera, T. Suzuki, T. Kamimura, Physica B 237–238 (1997) 105.
Journal of Crystal Growth 273 (2004) 207–212 www.elsevier.com/locate/jcrysgro
Growth of Sr3Fe2O7x single crystals by the floating zone method A. Maljuk, J. Strempfer, C. Ulrich, M. Sofin, L. Capogna, C.T. Lin, B. Keimer Max-Planck-Institut fu¨r Festko¨rperforschung, Heisenbergstrasse 1, D-70569, Stuttgart, Germany Received 10 July 2004; accepted 28 July 2004 Communicated by M. Schieber Available online 30 September 2004
Abstract We report the successful growth of high-quality Sr3Fe2O7x single crystals with volumes of up to 0.3 cm3 by using the floating zone method. Compositional homogeneity of the crystals was verified by powder X-ray diffraction, optical and electron microscope measurements. The X-ray rocking curves of the Sr3Fe2O7x single crystal gave a full-width at halfmaximum (FWHM) of Bragg reflections of 0.02–0.031. The influence of the growth rate on the crystal size and quality was demonstrated. The effect of oxygen annealing was studied by magnetic susceptibility and Raman light scattering experiments. The Fe(+3)/Fe(+4) ratio was determined both from the magnetization and thermo-gravimetric data. r 2004 Elsevier B.V. All rights reserved. PACS: 81.10.Fq; 81.30.Dz Keywords: A2. Floating zone technique; B1. Oxides
1. Introduction It is well known that iron ions in compounds are usually divalent or trivalent. In combination with strontium several compounds have been synthesized with iron in the tetravalent state: SrFeO3x, Sr3Fe2O7x and Sr2FeO4x [1,2]. All of them demonstrated exciting physical properties like a Corresponding author. Tel.: +49-711-689-1405; fax: +49-
711-689-1093. E-mail address: [email protected] (A. Maljuk).
giant magnetoresistance associated with the ordering of spin and charge degrees of freedom [3,4]. Since the growth of single crystals of tetravalent iron oxide is quite difficult, these compounds are predominantly studied in form of powder samples [1–6]. Thus, high-quality and large-size single crystals are in great demand for reliable physical and structural measurements. Sr3Fe2O7 consists of isolated double sheets of FeO6 octahedra, which is called a layered perovskite structure. Preparation of Sr3Fe2O7x crystals is not obvious due to the question of the
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.07.091
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phase stability limit of this compound in air [5,6]. The crystal structure of the Sr3Fe2O7x phase depends on the actual oxygen content [5], which widely varies (0:0pX p1:0) with temperature and oxygen pressure. According to the published SrO–Fe2O3 phase diagram [7] the Sr3Fe2O7x compound melts incongruently at 1520 1C in air: Sr3 Fe2 O7x ! SrFeO3x þ liquid: The last reaction is not correct because, as it was found in Ref. [8], SrFeO3x phase melts at 1435 1C in air. Therefore, the question of the melting point and behavior of the Sr3Fe2O7x phase is still open. It was reported that Sr3Fe2O7x single crystalline samples can be prepared by floating zone melting [9], but no data concerning the crystal size and crystal quality were provided. It must be mentioned that the reported growth rate of 10 mm/h [9] is unusually high for the incongruently melted compound. To the best of our knowledge no other attempts were performed to grow Sr3Fe2O7x single crystals. This is probably due to the complex structural and melting behavior of this phase, as mentioned above. In this paper, we report the successful growth of large-size and high-quality Sr3Fe2O7x single crystals with volumes of up to 0.3 cm3 by the floating zone method. The crystals were examined by X-ray diffraction, electron and optical microscope analysis. The oxygen content was measured by the thermo-gravimetric method. Magnetic susceptibility and Raman light scattering experiments were performed on as-grown and oxygen annealed crystals.
2. Experimental procedure The starting materials for the preparation of feed rods were SrCO3 (99.995%) and Fe2O3 (99.99%) powders. Before mixing, the SrCO3 compound was dried by heating at 150 1C for 5–6 h in air. The raw materials with stoichiometric amounts were manually mixed in presence of ethanol, dried, and calcined at 1000 1C for 2 24 h in air with an intermittent manual grinding in air. The pre-heated powder was formed into a cylindrical shape 7–8 mm in diameter and 80–100 mm in length, and then pressed at a
hydrostatic pressure of 300 MPa.The rods were sintered at 1250–1350 1C for 24 h in flowing oxygen. The heating and cooling rates were 100 1C/h. It was reported that ceramic samples of the Sr3Fe2O7x compound, prepared at 1250–1300 1C, were very sensitive to moisture [5,6]. Our experiments have shown that this sensitivity causes an additional difficulty for following operations with rods after sintering. For example, it leads to the rods crushing after cutting and grinding in air. Therefore, the rods should be kept in an argon box after sintering. It was found that synthesis at 1350–1370 1C in oxygen flow leads to more dense and stable feed rods in air. The best results were achieved when feed rods were prepared by a 2-step synthesis: (1) high-temperature treatment at 1300 1C for 24 h in oxygen flow; (2) low-temperature annealing at 550 1C for 2–3 days in oxygen flow. These rods were the most stable in air and could be handled without serious problems. The apparatus used for crystal growth was a 4mirror type infrared image furnace (Crystal System Inc. FZ-T-10000-H-III-VPR) equipped with four 1000 W halogen lamps as heat source. The growth conditions were as follows: the seed and feed shafts were rotated in opposite directions at rates of 10–15 rpm; the traveling rate was 1, 2, 3 and 5 mm/h. Oxygen pressures of 0.2–3.0 bar were applied during growth. The grown ingots were cut into wafers perpendicular to the growth direction, and both surfaces of wafers were polished to the mirror finish. Then polished specimens were characterized using a polarizing microscope to examine the presence of sub-grains and inclusions. A compositional analysis of the as-grown crystals was carried out by powder X-ray diffraction (XRD, ‘‘PHILIPS’’ PW3710), scanning electron microscope (SEM, ‘‘TESCAN’’ TS-5130MM) and energy dispersive X-ray analysis (EDX, ‘‘RO¨NTEC’’). A CuKa X-ray source was used for powder XRD measurements to check the phase purity and the crystal structure. A precise lattice parameter determination was performed using MoKa X-ray radiation generated by a rotating anode equipped with a CCD detector. The oxygen content in the as-grown Sr3Fe2O7x single crystals was measured by
ARTICLE IN PRESS A. Maljuk et al. / Journal of Crystal Growth 273 (2004) 207–212
3. Results and discussion Fig. 1 presents a typical Sr3Fe2O7x boule, grown at a traveling rate of 1 mm/h under an oxygen pressure of 3 bar. The boule was black with a metallic luster, and had 6 mm in diameter and 40 mm in length. Optical polarized microscope (‘‘Olympus’’ MS-11) and EDX study confirmed that the end-growth part of the ingot of about 15 mm consists of two single-crystalline and inclusion-free grains. The largest grain with the volume of 0.3 cm3 has a growth direction close to the [1 2 0]-axis, as found from a Laue picture. Fig.
Fig. 1. As-grown Sr3Fe2O7x boule, pulling rate 1 mm/h.
Sr3Fe2O7-xcrystal. (0010)
(105)
4000
10
20
30
40
(208)
(0012), (206) (118) (204) 50
(1011)
(200)
0
(116)
(114) (107)
(008) (112)
(101)
1000
(211)
2000
(1110) (215)
(110)
(006)
3000
(004)
Intensity (arb. units)
heating small amounts of Sr3Fe2O7x crystals in Ar/H2 flow using a DTA-TG apparatus (NETZSCH STA-449C). Raman light scattering experiments were performed using a Dilor-XY triple spectrometer in a close to true backscattering geometry. The spectra were detected using a liquid nitrogen cooled CCD camera. For excitation, the 514 nm line of an Ar+–Kr+ mixed gas laser was used. In order to avoid sample heating, the laser power was kept below 10 mW on the sample position. All spectra were recorded at room temperature on nonoriented crystals without polarization analysis. The magnetic susceptibility measurements were carried out by a superconducting quantum interference device magnetometer (‘‘Quantum Design’’, model MPMS 7.0) in the temperature range of 5–300 K. Magnetic field was perpendicular to the c-axis which is the easy magnetization axis for Sr3Fe2O7x [9].
209
60
2θ (degrees)
Fig. 2. Powder XRD pattern of the as-grown Sr3Fe2O7x crystal, pulling rate 1 mm/h (CuKa -radiation).
2 shows the powder X-ray diffraction pattern of the as-grown Sr3Fe2O7x crystal. The crystal was compositionally homogeneous without any trace of secondary phase in the XRD pattern. The crystal structure could be assigned to a tetragonal space group and all peaks corresponded to the pure Sr3Fe2O7x phase. The lattice parameters ( and c ¼ were determined as a ¼ 3:875ð4Þ A ( 20:23ð1Þ A from the single crystal refinement. The data are in good agreement with ceramic samples [6]. No visible iron evaporation was observed during Sr3Fe2O7x crystal growth. It must be mentioned that the Sr3Fe2O7x ingot, prepared at the traveling rate of 5 mm/h, consists of small single crystalline grains with typical dimensions of 2 1.5 0.5 mm3. Powder XRD pattern of Sr3Fe2O7x crystals, mechanically separated by a sharp scalpel from the end-growth part of ingot, showed only the presence of the Sr3Fe2O7x compound. X-ray rocking curves, measured on the as-grown crystal, showed that Bragg reflections have much larger FWHM values of about 1.01, than that observed for samples grown at 2 mm/h (see below). This reflects the effect of the relatively high growth rate, 5 mm/h, on the crystal quality (mosaicity). Single crystalline Sr3Fe2O7x samples with dimensions up to 7 4 3 mm3 were mechanically separated (cleaved) from an as-grown ingot prepared at 2 mm/h. All crystals were inclusion-free, as confirmed by powder XRD, optical and
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Magnetic susceptibility (emu/mol)
0.06
as-grown Sr3Fe2O6.76 crystal, 10 Oe.
0.05
T1=78 K
annealed Sr3Fe2O6.80 crystal, 10 Oe.
0.04 0.03 0.02 0.01
T2=115 K
T3=148 K.
0.00 0
50
100
150 200 Temperature (K)
250
300
Fig. 4. Magnetic susceptibility of the Sr3Fe2O7x crystals (H?c-axis). Fig. 3. X-ray rocking curves for the as-grown Sr3Fe2O7x crystal, pulling rate 2 mm/h (MoKa -radiation).
magnetization (see below) measurements. The Xray rocking curves, measured on the as-grown crystal, are depicted in Fig. 3. The profiles of (0 0 20) and (2 4 0) Bragg-reflections have FWHM values of 0.021 and 0.031, respectively, indicating the excellent quality of the crystal. Thus, our experimental results suggest that high-quality and large-size single crystals can be obtained only at pulling rates of 1–2 mm/h. The oxygen content of the as-grown Sr3Fe2O7x crystal prepared at oxygen pressure of 3 bar is X ¼ 0:24 0:03; as obtained by the thermo-gravimetric method. Oxygen deficient Sr3Fe2O7x crystals are hygroscopic and contain water (up to 1.0 wt%), when kept in air at room temperature. The water is released above 100–150 1C. It is worth mentioning that Sr3Fe2O7x crystals, when stored in air for several days, became very brittle and cleaved easily along the (0 0 l) plane. Our DTA-TG and melting experiments showed that the Sr3Fe2O7x phase melts according to the peritectic reaction: Sr3 Fe2 O7x ! SrO þ liquid at 1470–1480 1C in air. Based on the results of these experiments, we assume the existence of eutectic between Sr3Fe2O7x and SrFeO3x phases. The temperature dependence of the magnetization of the as-grown Sr3Fe2O6.76 single crystal is shown in Fig. 4. The magnetization measurement confirmed that the crystal was free from magnetic inclusions (for example, a-Fe2O3 or Fe3O4) for the
following reasons. It should be taken into account that only the Morin phase transition at 260 K is known for a-Fe2O3 below room temperature [10]. The absence of magnetic anomaly at 260 K (see Fig. 4) means that there is no hematite inclusion. In the case of magnetite only the Verwey phase transition occurs at 120–122 K below room temperature [11]. The absence of magnetic feature on the magnetization curve at this temperature is evident that the Fe3O4 phase does not exist as inclusions in the crystal. We exclude the presence of maghemite because the phase diagram of the SrO–Fe2O3 system [7] does not indicate the existence of g-Fe2O3. A small magnetic anomaly, unknown before, was observed at T 1 ¼ 78 K: The temperature dependence of the magnetization of a Sr3Fe2O6.80 crystal annealed at 550 1C for 250 h in O2 flow showed two magnetic transitions at T 2 ¼ 115 K and T 3 ¼ 148 K: Furthermore, the magnetic anomaly at T 1 ¼ 78 K has nearly vanished. It is well known that the Sr3Fe2O7x phase has an antiferromagnetic transition at T N ¼ 1102120 K for X ¼ 0:0020:05 [1,12,13]. The Neel temperature decreases with decreasing oxygen content and converges to zero degrees for X ¼ 0:50 [1]. Therefore, the magnetic feature at T 2 ¼ 115 K should be associated with this transition. The magnetic transition at T 3 150 K was previously observed for oxygen-deficient Sr3Fe2O7x (X 40:5) ceramic samples [9] both by magnetization and neutron
ARTICLE IN PRESS A. Maljuk et al. / Journal of Crystal Growth 273 (2004) 207–212 450 555
Sr3Fe2O7-x crystal
400
T=300 K.
594 384 173
235
250
460
300
78
annealed (1 atm O2)
200
919.4
Intensity (arb. units)
350
150 100 as-grown 50 0
200
400
600
800
1000
Raman Shift (cm-1)
Fig. 5. Raman spectra of as-grown Sr3Fe2O6.76 and annealed Sr3Fe2O6.80 single crystals (at room temperature). 100000 Sr3Fe2O6.8 single crystal.
80000
Resistivity, Ohm.
diffraction experiments. It was demonstrated [9] that the magnetic structure of Sr3Fe2O6.94 is different from that of Sr3Fe2O6.21. It should be noted that annealing at 550 1C in oxygen flow changes the actual oxygen content only a little, i.e. X ¼ 0:24 0:03 (as-grown) and X ¼ 0:20 0:03 (annealed), respectively. The magnetic susceptibility can be described in the high-temperature range (150–300 K) by the Curie–Weiss law: m ðper FeÞ ¼ 5:5 mB and Y ¼ 15 K: The obtained magnetic moment of 5:5 mB is somewhat higher than for the stoichiometric Sr3Fe2O7 phase ð5:3 mB Þ [13]. This is caused by the presence of Fe+3 (d5) ions having a larger magnetic moment than Fe+4 (d6). Taking as a reference the stoichiometric Sr3Fe2O7 [6] compound, the Fe+3/ Fe+4 ratio in the as-grown crystal can be estimated. Since for Fe+3(d5) the pure spin magnetism is expected, one obtains, that about 30% of Fe+3 lead to the observed increase of the magnetic moment. The oxygen content X=0.24 (as-grown crystal) corresponds to the effective oxidation state of iron ions +3.76. This means that the Fe+3 content is about 24%. This value agrees well with that obtained from the magnetization data. The changes in the Fe+3/Fe+4 ratio induced by annealing in O2 are only minor and thus be reliably determined from the magnetic data. Fig. 5 shows Raman spectra of as-grown Sr3Fe2O6.76 and annealed Sr3Fe2O6.80 single crystals. It is worth mentioning that the Raman spectra were identical for different positions on the sample surface. This proves the good homogeneity and high quality of the Sr3Fe2O7x single crystals. The low-frequency modes at 173 cm1 can be assigned to vibrations of the Sr-ion, whereas the high-energy phonon modes, especially the mode at 555 cm1, originate from cooperative vibrations of the FeO6 octahedra. Our Raman experiments showed that the intensity of the mode at 555 cm1 increased drastically after annealing although the oxygen content increased only a little. At the same time other modes like those at 173 cm1 and 460 cm1 remained almost unchanged. Dann et al. studied the crystal and magnetic structure of Sr3Fe2O7x with different oxygen deficiencies, using neutron powder diffraction [6]. They found a crystallographic transition around
211
zero magnetic field 10 Oe
60000
1T
40000 20000 0 50
100
150
200
250
300
350
400
Temperature, K.
Fig. 6. Temperature dependence of the conductivity of the Sr3Fe2O6.8 crystal (H?c-axis).
X ¼ 0:5: Furthermore, the O(3) site was found to be partly occupied, while the O(1) and O(2) positions were fully occupied for all oxygen contents. Low temperature specific heat measurements performed by Kobayashi et al. [14] have shown that the electronic state of the Sr3Fe2O7x drastically changes around X ¼ 0:2: This transition was not observed in Ref. [6]. Thus, our SQUID and Raman data give the evidence of the oxygen redistribution during the oxygen annealing, which can lead to the electronic state transition reported in Ref. [14]. Preliminary resistivity measurements (4-point technique) show a semi-conducting behavior of the electrical conductivity (annealed sample) in the temperature range of 50–350 K (Fig. 6) This result is in good agreement with the data for ceramic
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samples [12]. No effect of magnetic field on the conductivity was observed up to a field strength of 7 T. The conductivity curve at 7 T is not shown because it is almost the same with the 1 T curve.
4. Conclusions Single crystals of the Sr3Fe2O7x compound were successfully grown with volumes of up to 0.3 cm3 by the floating zone method. All crystals were inclusion-free. The effect of growth rate on the crystal size and quality was shown. The X-ray rocking curves of the Sr3Fe2O7x single crystal have FWHM values of 0.02–0.031, demonstrating the high quality of the crystals. The effect of oxygen annealing was shown by means of magnetic susceptibility and Raman measurements, demonstrating the possibility of oxygen redistribution. The Fe(+3)/Fe(+4) ratio was determined both from the magnetization and thermo-gravimetric data.
Acknowledgements The authors are grateful to Mrs. G. Goetz for X-ray and Mrs. E. Bruecher for SQUID measurements.
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