Article pubs.acs.org/crystal
Self-Adjusted Traveling Solvent Floating Zone Growth of Single Crystal CaFe2O4 Rajasree Das,† Sunil Karna,† Yen-Chung Lai,†,‡ and Fang-Cheng Chou*,†,#,§ †
Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan Department of Physics, Tamkang University, Tamsui 25137, Taiwan # National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan § Taiwan Consortium of Emergent Crystalline Materials, National Science Council, Taipei 10622, Taiwan ‡
ABSTRACT: Single crystals of high-quality CaFe2O4 have been grown successfully using a self-adjusted traveling solvent floating zone method with a four-mirror optical furnace in oxygen atmosphere. Although CaFe2O4 has been reported to melt incongruently, the optimal molten zone has been achieved through a self-adjusted cooling following the liquidus line directly using the stoichiometric CaFe2O4 as both the feed and flux rods in the beginning. Details regarding long-term stability of the molten zone and lamp power optimization to obtain the bubble-free molten zone are discussed. The obtained large single crystal of a few centimeters long and ∼0.5 cm in diameter shows homogeneous stoichiometry and crystallinity as verified by the optical microscopy and synchrotron X-ray diffraction structure refinement. Long range ferrimagnetic spin ordering of TN ≈ 180 K is confirmed from the measurement of magnetization as a function of temperature, which shows significant thermal hysteresis and field dependence. Curie−Weiss law fitting of homogeneous susceptibility data above TN in the paramagnetic regime suggests that the localized spins of Fe3+ are close to the high spin state of d5 with S = 5/2 in an octahedral crystal field, and the negative Weiss temperature of Θ ≈ −115 K is consistent to a ferrimagnetic ground state of antiferromagnetically ordered spins in unequal size.
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INTRODUCTION
Recent HP studies of CaFe2O4 revealed many interesting properties of the HP phase and attracted regenerated research interest in this compound. At low pressure (LP), all Fe atoms in CaFe2O4 occupy 4c position of the Pnma space group, but Yamanaka et al. have shown the existence of a new HP phase with an increase in density by 9.4% above 51.4 GPa via a potential martensitic transition generated by a cooperative atomic displacement.13 In the HP phase, only four Fe atoms are at 4c positions, and the other eight Fe ions occupy 8d positions. Both LP and HP phases belong to the same Pnma space group; however, the HP phase has a six-cation layer along the c-axis in contrast with the two-cation layer of Fe and Ca in the LP phase. It is interesting to compare the crystal structure of the series of AFe2O4 (A = Mg, Ca, Sr, Ba), which shows drastically different arrangements of the FeO4 and FeO6 polyhedra depending on the A site ion size (Ba2+(VI) = 1.35 Å, Sr2+(VI) = 1.16 Å, Ca2+ (VI) = 1.0 Å and Mg2+(VI) = 0.72 Å). MgFe2O4 with the smallest ion size is a semiconducting soft ferromagnetic material with spinel structure,14 where Mg and Fe occupy both the tetrahedral and octahedral sites. The CaFe2O4 structure is composed of double rutile chain and undergoes an isostructural insulator−metal (IM) transition at
Among the chemicals in AB2O4 formula with A = (Li, Na, Mg, Ca, Sr) and B = (Fe, Co, Mn, Al), rich structural variations from the typical spinels, to spineloids of mixed vacancy layers, to the complete octahedral layer and tunnel structures have been examined thoroughly in solid state chemistry.1 These compounds containing transition metal cations of mixed valence are getting great attention for their correlated structural, magnetic and electronic properties, especially on the study of the spin crossover and metal−insulator transition driven by ultrahigh pressure (HP) at high temperature.2−5 Orthorhombic CaFe2O4 has attracted special interest when many other spinels like MgAl2O4 and LiMn2O4 can be transformed into this structure after the HP treatment.6−8 The crystal structure of CaFe2O4 can be described as a zigzag chain of edge-sharing FeO6 octahedra, and these chains are linked via oxygen corner sharing, with Ca2+ ions sitting in the tunnels formed by the Fe− O network.1,6 The orthorhombic crystal structure has been refined using either space group Pnma or Pbnm depending on the choice of principal axis; for example, Zouari et al. obtained a = 9.221, b = 3.0167, and c = 10.689 Å in Pnma, and Merlini et al. obtained a = 10.697, b = 9.302, and c = 2.992 Å in Pbnm.9−12 In the following, we use Pnma to index the XRD pattern following the official International Tables for Crystallography because Pbnm is a nonstandard setting. © XXXX American Chemical Society
Received: October 29, 2015 Revised: November 24, 2015
A
DOI: 10.1021/acs.cgd.5b01529 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 1. (a) Schematic plot of the traveling solvent floating zone (TSFZ) crystal growth process. (b) Image of the as-prepared CaFe2O4 single crystal grown using the TSFZ method, and (c) shows the image of the steady state of the TSFZ growth process at 42% power of the four 500 W lamps.
Figure 2. (a) Partial phase diagram for the CaO−Fe2O3 binary system in the air, adapted from ref 19, and (b) DTA cooling curves of the polycrystalline feed, crushed single crystal, and the solidified molten zone after the stable growth. DTA data were collected at a heating (not shown) and cooling rate of 10 °C/min in Ar flow.
high pressure.5 BaFe2O4 has a very different structure of cornershared FeO4 tetrahedra only in space group Bb21m.15 On the other hand, metastable SrFe2O4 of distorted FeO4 tetrahedra was first synthesized by Berthet et al. with reported space groups of Pbc21 or P1121/n.16,17 It is clear that the electronic and magnetic properties of the AFe2O4 compounds are intimately and sensitively tunable by the Fe−O coordination. For better understanding of the exotic physical properties of CaFe2O4, single crystal samples with high-quality and welldefined composition are desirable. Most early studies on CaFe2O4 were carried out with polycrystalline samples prepared using zirconia or platinum crucible.5,10,13 Attempted crystal growth was accomplished by Kolev et al. using a high temperature solution growth method with the CaCl2 flux.18 On the basis of the reported phase diagram of CaO-Fe2O3,19 the challenge for large crystal growth of CaFe2O4 is its extremely narrow temperature difference between the reported incongruent melt temperature ∼1216 °C and the eutectic temperature ∼1205 °C. Interestingly, although Merlini et al. used direct melt of CaO-Fe2O3 in the Pt crucible and obtained crystals via slow cooling for the compound that is supposed to be incongruent melt,11 the obtained single crystals were needlelike in small size and must be coming from the vapor phase deposition. In addition, one unavoidable drawback of using a
crucible is the potential contamination from the crucible material during high temperature soaking and the long slow cooling process. In contrast, crucible-free optical floating zone technique is most suitable to grow pure and large crystals with specific orientation. Here we report the successful growth of high quality large size CaFe2O4 single crystal using an optical floating zone furnace for the first time. Preliminary susceptibility measurement of the grown single crystal sample indicates that the compound has a ferrimagnetic transition near ∼180 K.
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EXPERIMENTAL DETAILS
As the first step of the crystal synthesis, a polycrystalline powder sample was prepared by a solid-state reaction using high purity CaCO3 and Fe2O3 in a 1:1 molar ratio as the precursor. Obtained powder was then calcined and reground several times at 900 °C for 12 h, at 950 °C for 10 h, and at 1000 °C for 24 h. Finely grounded calcined powder was packed in a rubber sleeve, shaped into a cylindrical shape about 5 mm in diameter and 80 mm long, and hydrostatically pressed at 60 MPa to obtain the feed and seed rods. The prepared rods were then sintered at 980 °C for 40 h in the air to obtain very high density for the floating zone technique, as the crystal growth quality is highly influenced by the density and homogeneity of the feed and seed rods.20 A schematic plot of the TSFZ growth set up is shown in Figure 1a, where the feed rod has also been used as the flux in the initial stage before self-adjusted stable molten zone of correct flux ratio is achieved. B
DOI: 10.1021/acs.cgd.5b01529 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Thermal analysis of the polycrystalline feed rod, single crystal, and the solidified molten zone after the growth was performed using a TGDTA system (STA 449F3, NETZSCH) at a constant heating and cooling rate (10 °C/min) in Ar flow of 50 mL/min. Phase purity and crystal structure of the shiny black crystal were investigated using synchrotron X-ray (18 keV energy and 0.6889 Å wavelength) powder diffraction (SXRD) collected at SPring-8 and BRUKER D2 Phaser Xray diffractometer (XRD) equipped with Cu tube (λ = 1.5418 Å). Room temperature (RT) SXRD pattern was analyzed with the Rietveld method using the General Structure Analysis System (GSAS) software package. Chemical composition of the obtained crystal was analyzed using an electron probe microanalyser (EPMA). Magnetic properties were measured using SQUID VSM (Quantum Design, USA).
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curves of the DTA measurements are shown in Figure 2b, which indicates exothermic reaction near eutectic point (∼1193 °C) for all three samples. For the polycrystalline sample, the first exothermic reaction observed at 1266 °C (labeled as point A in the phase diagram) is due to the incongruent solidification of the CaFe2O4 phase before reaching the eutectic temperature, which is labeled as point C in the phase diagram. Crushed crystal shows a similar behavior except that the exothermic peak corresponding to the point A solidification is weaker. The growth range has been demonstrated clearly by the final flux that shows an incongruent solidification peak right before the eutectic temperature, as shown in the third part of Figure 2c. On the basis of the DTA study results, we estimate that the actual final flux has a CaO:Fe2O3 molar ratio very close to 1:1.2. To achieve a stable molten zone throughout the complete growth process, the initial power adjustment is crucial. A constant power of the four lamps was maintained to keep the molten zone undisturbed in a stable balance among viscosity, surface tension, and gravity. Figure 3a shows the 500 W
RESULTS AND DISCUSSION
Crystal Growth. On the basis of the reported CaO-Fe2O3 binary phase diagram,19 CaFe2O4 melts incongruently near ∼1216 °C and with an eutectic temperature near ∼1205 °C, which suggests that single crystal can be grown using flux with a CaO:Fe2O3 molar ratio approximately between 1:1.1 and 1:1.4. In the present study, no predetermined flux rod with CaO:Fe2O3 molar ratio in the expected range was used; instead, stable molten zone has been obtained using the same stoichiometric feed rod in the beginning and self-adjusted through fine-tuning of the lamp power in the slow pulling process. Several different types of gas, pressures, and the lamp powers were tried. The optimal condition was found for the growth carried out in a quartz tube maintained in oxygen atmosphere at a pressure near ∼0.15−0.25 MPa. The growth rate was 0.5−1 mm/h, and the rotation rate of the seed and feed rods was fixed at 20 rpm in opposite directions. Compared to the growth of Ca2Fe2O5 which melts congruently near 1454 °C,19,21,22 the incongruent melt of CaFe2O4 with a small difference between the incongruently melt point ∼1216 °C and the eutectic temperature ∼1205 °C is a challenge. A small fluctuation in growth temperature could introduce additional phases as illustrated in the phase diagram of Figure 2a.19,23 CaFe2O4 can only be grown in a very narrow temperature range, and the optimal molten zone is expected to have the stoichiometry very close to the top of the peritectic line between ∼1216−1205 °C.19 To achieve the optimal condition required by the phase diagram, it is easier to use the stoichiometric feed as the initial flux and allow the molten zone concentration self-adjusted with the help of power adjustment during constant pulling, instead of preparing a flux rod separately. In order to adjust the molten zone stoichiometry from the initial incongruently melted stoichiometric feed rod and stabilize into the proper expected range close to 1:1.2 as predicted from the phase diagram, the preliminary power must reach a temperature above ∼1250 °C first and then cool down slowly following the peritectic line, and finally a stable growth can be achieved only when the molten zone has a CaO:Fe2O3 molar ratio that falls into the range close to 1:1.2 before reaching the eutectic point. The molten zone ratio has been self-adjusted through the initial heating and cooling process with constant pulling simultaneously. After a stable molten zone is achieved, stable growth of single crystal can be maintained throughout the whole process for at least 4−5 days. Figure 1c illustrates the molted zone during the stable growth. To understand the growth mechanism better, DTA analysis was performed on the polycrystalline feed rod, the as-grown single crystal, and the solidified flux after the growth. Cooling
Figure 3. (a) Plot of heating lamp power vs time and the corresponding molten zone in the TSFZ growth experiment and (b) optical image of the initial stage of the growth.
halogen lamp power as a function of time. The lamp power was programmed to increase linearly to reach the set point and was kept constant for the complete growth period. The stable and uniform molten zone can only be maintained with a low power near 42%. Most importantly, the growth of CaFe2O4 is very sensitive with the power variation due to narrow temperature range between the incongruent melt temperature and the eutectic temperature for the whole growth period of more than 50 h. Since the oxygen loss at high temperature is not recoverable upon cooling efficiently, the crystal was grown at a constant oxygen pressure. The as-grown CaFe2O4 single crystal has a preferred orientation of easy cleavage character, which follows the parallel lines shown in Figure 3b in the initial growth period. It has been confirmed that the parallel lines represent the easy cleavage planes parallel to (h 0 0) from the XRD pattern (inset of Figure 4) showing preferred orientation. Crystal Structure. The rod shaped crystals grown at 0.18 and 0.25 MPa oxygen pressure were cut and polished from both end of the rods to determine the elemental composition and homogeneity using EPMA chemical analysis. Oxide stoichiometry is found to be identical throughout the rod growth length, but both crystals show oxygen deficiency. For a 10 point average per scan within the EPMA resolution for oxygen (±0.02), the single crystals prepared at 0.18 and 0.25 MPa oxygen pressure show average compositions of CaFe2O3.76 and CaFe2O3.81, respectively. These results suggest the earlier claim of superoxygenated CaFe2O4+δ reported by Lobanovskii et al. before should be re-examined.24 C
DOI: 10.1021/acs.cgd.5b01529 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 4. Experimental and refined powder X-ray diffraction pattern of CaFe2O4 crystal at room temperature, collected using synchrotron Xray source in NSRRC Taiwan. Inset shows diffraction pattern of the cleaved surface with preferred orientation from planes parallel to (h 0 0).
Crystal phase purity and structure of the as-grown single crystals were examined by synchrotron X-ray (SXRD) at room temperature. To check structural consistency throughout the grown boul, SXRD patterns using crushed crystal specimen cut from both ends were recorded to showed identical results. Figure 4 shows the SXRD diffraction pattern together with the refined pattern, and the inset shows XRD diffraction pattern of the cleaved single crystal with preferred orientation. XRD pattern of surface normal to the easily cleaved face exhibits preferred orientation parallel to (400) planes. SXRD pattern is refined using GSAS Rietveld refinement software with the space group Pnma, and the orthorhombic phase at 300 K shows lattice constants of a = 9.2297(3) Å, b = 3.0192(1) Å, and c = 10.7028(4) Å. The refined parameters are summarized in Table 1 to be in fair agreement with those reported in the literature.11
Figure 5. (a, b) Crystal structure of CaFe2O4, where two zigzag chains formed with Fe(1)O6 and Fe(2)O6 octahedra are shown in different colors. Bond length, bond angle, and the superexchange route of Fe− O−Fe for (c) Fe(1)O6 and (d) Fe(2)O6 octahedra.
should affect the magnetic ground state at low temperature as a result of the difference in the Fe−O−Fe superexchange paths. Magnetic Properties. Since the behavior of a ferrimagnetic transition of TN ≈ 180 K has been observed by Lobanovskii et al. before,24 the magnetic property of the as-grown crystal has been checked using crushed crystal to separate the complication from crystal anisotropy on the data analysis. Figure 6 shows the magnetization (M) as a function of
Table 1. List of Parameters Obtained from Rietveld Refinement of SXRD Pattern sample name CaFe2O4 crystal structure orthorhombic space group Pnma a 9.2297(3) Å χ2 b 3.0192(1) Å Rp c 10.7028(4) Å Rwp atom x y z Fe1 Fe2 Ca O1 O2 O3 O4
0.0816(2) 0.0669(2) 0.2438(3) 0.2944(8) 0.3821(9) 0.4750(9) 0.0790(8)
0.25 0.25 0.25 0.25 0.25 0.25 0.25
0.6051(3) 0.1121(3) 0.3463(5) 0.6602(8) −0.0236(9) 0.2134(8) −0.0733(8)
3.609 4.86% 5.74% occupancy 1 1 1 1 1 1 1
Figure 6. Temperature dependence of the magnetization of the asgrown CaFe2O4 single crystal measured in applied fields of (a) 100 Oe and (b) 2 T.
Figure 5 shows crystal structure of CaFe2O4. On the basis of the refined Fe−O bond lengths for each FeO6 octahedron, CaFe2O4 could be viewed as a compound composed of cornershared zigzag chains, but the neighboring zigzag chains could be distinguished as Fe(1)O6 and Fe(2)O6 of slightly different octahedral distortion (Figure 5a). The bond lengths and bond angles of Fe(1)O6 and Fe(2)O6 octahedra are summarized in Figure 5b. It is expected that the spin size and/or orientation of Fe spins are different in two different zigzag chains, which
temperature (T) for the CaFe2O4 crushed crystal sample at low field of H = 100 Oe and high field of 2 T. Strong field dependence and thermal hysteresis has been observed, as expected for a sample with a ferrimagnetic spin ordering at low temperature. At low magnetic field, M(T) shows (Figure 6a) strong temperature and field dependence from the zero field cooled (ZFC) and field cooled (FC) scans. The onset of the D
DOI: 10.1021/acs.cgd.5b01529 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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ferrimagnetic transition is near ∼180 K. The ferrimagnetism is confirmed from the peculiar thermal history dependent ZFC scan, which could be negative at a certain temperature range, mostly coming from the incomplete cancellation of the antiferromagnetically aligned spins. At a higher field of 2 T, a totally different ZFC/FC hysteresis is observed (Figure 6b), where the same onset is observed but with a significantly different ZFC scan compared to that measured at low field. For a sample with ferrimagnetic ordering, measurement history plays a significant role in the magnetic measurement, and the multivalent Fe ions can easily form many other iron oxides with ferromagnetic ordering; thus we believe all inconsistent results on the onset of TN and ZFC/FC hysteresis of the nominal CaFe2O4 in the literature could be coming from the impurity phase and/or different measurement history.24,25 The ferrimagnetic ordering is supported by the Curie−Weiss law fitting in the high temperature paramagnetic regime. By using the homogeneous susceptibility data χ = M/H at 2 T around 2TN ≈ 320 K in the paramagnetic state, the fitting of Curie−Weiss law as χ(T) = C/(T − Θ) returns values of Curie constant C = 3.19 cm3 K/mol and Weiss temperature Θ = −115 K. The calculated average effective magnetic moment μeff = 4.92 μB per Fe ion is close but lower than the theoretical value (5.92 μB) assuming all Fe3+ spins are in the high spin state of S = 5/2. Our result of μeff = 4.92 μB is higher than that of 2.72 μB obtained by Zouari et al.,26 which could be due to the different Fe3+/Fe2+ ratio induced by the oxygen deficiency. A complete magnetic property study for single crystals of various oxygen deficiency levels are in progress and will be reported separately.
(2) Antao, S. M.; Hassan, I.; Crichton, W. A.; Parise, J. B. Am. Mineral. 2005, 90, 1500−1505. (3) Wang, Z.; Lazor, P.; Saxena, S. K.; Artioli, G. J. Solid State Chem. 2002, 165, 165−170. (4) Fei, Y.; Frost, D. J.; Mao, H. K.; Prewitt, C. T.; Hausermann, D. Am. Mineral. 1999, 84, 203. (5) Greenberg, E.; Rozenberg, G. Kh.; Xu, M. W.; Pasternak, M. P.; McCammon, C.; Glazyrin, K.; Dubrovinsky, L. S. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 88, 214109. (6) Irifune, T.; Fujino, K.; Ohtani, E. Nature (London, U. K.) 1991, 349, 409. (7) Yutani, M.; Yagi, T.; Yusa, H.; Irifune, T. Phys. Chem. Miner. 1997, 24, 340. (8) Wang, Z.; Downs, R. T.; Pischedda, V.; Shetty, R.; Saxena, S. K.; Zha, C. S.; Zhao, Y. S.; Schiferl, D.; Waskowska, A. Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 68, 094101. (9) Zouari, S.; Ranno, L.; Cheikh-Rouhou, A.; Pernet, M.; Strobel, P. J. Mater. Chem. 2003, 13, 951−956. (10) Decker, B. F.; Kasper, J. S. Acta Crystallogr. 1957, 10, 332. (11) Merlini, M.; Hanfland, M.; Gemmi, M.; Huotari, S.; Simonelli, L.; Strobel, P. Am. Mineral. 2010, 95, 200. (12) Gemmi, M.; Oleynikov, P. Z. Kristallogr. - Cryst. Mater. 2013, 228, 51. (13) Yamanaka, T.; Uchida, A.; Nakamoto, Y. Am. Mineral. 2008, 93, 1874. (14) Willey, R. J.; Noirclerc, P.; Busca, G. Chem. Eng. Commun. 1993, 123, 1−16. (15) Mitsuda, H.; Mori, S.; Okazaki, C. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1971, 27, 1263. (16) Berthet, P.; Berthon, J.; Heger, G.; Revcolevschi, A. Mater. Res. Bull. 1992, 27, 919−924. (17) Kahlenberg, V.; Fischer, R. X. Solid State Sci. 2001, 3, 433−439. (18) Kolev, N.; Iliev, M. N.; Popov, V. N.; Gospodinov, M. Solid State Commun. 2003, 128, 153−155. (19) Phillips, B.; Muan, A. J. Am. Ceram. Soc. 1958, 41, 445. (20) Pless, J. D.; Erdman, N.; Ko, D.; Marks, L. D.; Stair, P. C.; Poeppelmeier, K. R. Cryst. Growth Des. 2003, 3, 615−619. (21) Maljuk, A.; Strempfer, J.; Lin, C. T. J. Cryst. Growth 2003, 258, 435−440. (22) Kahlenberg, V.; Fischer, R. X. Eur. J. Mineral. 2000, 12, 129. (23) Kimura, S.; Muan, A. Am. Mineral. 1971, 56, 1332. (24) Lobanovskii, L. C.; Truhanov, S. V. Crystallogr. Rep. 2011, 56, 510−513. (25) Samariya, A.; Dolia, S. N.; Prasad, A. S.; Sharma, P. K.; Pareek, S. P.; Dhawan, M. S.; Kumar, S. Current Appl. Phys. 2013, 13, 830− 835. (26) Zouari, S.; Ranno, L.; Cheikhrouhou, A.; Isnard, O.; Wolfers, P.; Bordet, P.; Strobel, P. J. Alloys Compd. 2008, 452, 234−240.
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CONCLUSION In conclusion, high quality large single crystal of CaFe2O4 were grown by the floating zone technique for the first time. Details on how to establish a stable molten zone which allows the growth of an incongruent melt crystal are presented with proposed post thermal analysis to explain the related growth mechanism. The SXRD pattern for the CaFe2O4 single crystal sample has been refined with GSAS refinement to demonstrate the impurity-free crystal quality. The oxygen content of the asgrown CaFe2O4−δ single crystal prepared under various oxygen pressures is suggested to show slight oxygen deficiency within EPMA resolution. Preliminary magnetic characterization of the as-grown crystal suggests that CaFe2O4 has a ferrimagnetic transition below ∼180 K.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected](FCC). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS F.C.C. acknowledges support from Ministry of Science and Technology in Taiwan under Project Number MOST-1022119-M-002-004. R.D. acknowledges support from National Taiwan University postdoctoral research fellowship under project number 104R4000.
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REFERENCES
(1) Muller-Buschbaum, Hk. J. Alloys Compd. 2003, 349, 49. E
DOI: 10.1021/acs.cgd.5b01529 Cryst. Growth Des. XXXX, XXX, XXX−XXX
