- Email: [email protected]
Self-adjusted flux for the traveling solvent floating zone growth of YBaCuFeO5 crystal
Journal of Crystal Growth 413 (2015) 100–104
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Self-adjusted flux for the traveling solvent floating zone growth of YBaCuFeO5 crystal Yen-Chung Lai a,b, Guo-Jiun Shu a, Wei-Tin Chen a, Chao-Hung Du b, Fang-Cheng Chou a,c,d,n a
Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan Department of Physics, Tamkang University, Tamsui 25137, Taiwan c National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan d Taiwan Consortium of Emergent Crystalline Materials, Ministry of Science and Technology, Taipei 10622, Taiwan b
art ic l e i nf o
a b s t r a c t
Article history: Received 19 September 2014 Received in revised form 3 November 2014 Accepted 21 December 2014 Communicated by: R.S. Feigelson Available online 29 December 2014
A modified traveling solvent floating zone (TSFZ) technique was used to successfully grow a large size and high quality single crystal of multiferroic material YBaCuFeO5. This modified TSFZ growth uses a stoichiometric feed rod and pure copper oxide as the initial flux without prior knowledge of the complex phase diagram involving four elements, and the optimal flux for the growth of incongruently melt crystal is self-adjusted after a prolonged stable pulling. The wetting of the feed rod edge that often perturbs the molten zone stability was avoided by adding 2 wt% B2O3. The optimal flux concentration for the YBaCuFeO5 growth can be extracted to be near YBaCuFeO5:CuO¼ 13:87 in molar ratio. The crystal quality was confirmed by the satisfactory refinement of crystal structure of space group P4mm and the two consecutive anisotropic antiferromagnetic phase transitions near 455 K and 170 K. & 2014 Elsevier B.V. All rights reserved.
Keywords: Single crystal growth Floating zone technique Phase diagram X-ray diffraction Magnetic materials
1. Introduction The oxygen-deficient perovskite material YBaCuFeO5 was first identified by Er-Rakho et al. [1] before it was treated as an impurity phase in the Fe-doped superconductor YBa2Cu3O7 x and the related Y2BaCuO5 [2,3]. YBaCuFeO5 has been reported to be multiferroic, with intriguing couplings that enable the material to exhibit ferroelectricity and incommensurate magnetic ordering below 230 K [4]. Note that after the discovery of multiferroicity of CuO at 220 K, [5] YBaCuFeO5 became the first example of a rare oxide material demonstrating a multiferroic transition temperature above 200 K. More attention should be placed on YBaCuFeO5 due to the fascinating phenomena in fundamental physics and the possible technical application of the material. The crystal structure of YBaCuFeO5 can be derived from YBa2Cu3O7 by removing a layer of BaO and the Cu(1)O chain layer and then replacing one-half of the Cu in the Cu(2)O2 layer by Fe, as illustrated in Fig. 1. The tetragonal structure was confirmed by X-ray powder diffraction, with lattice parameters of a ¼b¼ 3.867 Å and c¼ 7.656 Å E2a. It is still under debate whether the Cu and Fe
n Corresponding author at: Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan. Tel.: þ 886 2 33665254; fax: þ 886 2 33663843. E-mail address: [email protected] (F.-C. Chou).
http://dx.doi.org/10.1016/j.jcrysgro.2014.12.020 0022-0248/& 2014 Elsevier B.V. All rights reserved.
cations order or not. Two models have been considered in the earlier crystal structural works, [1,6–10] including (a) a complete disordering between Fe/Cu in the (Fe/Cu)O2 layer (P4/mmm space group) and (b) a fully ordered structure between Fe and Cu ions occupying distinct crystallographic sites (P4mm space group). Mossbauer spectroscopy studies suggested that iron and copper ions are fully ordered and are located in separate layers of the space group P4mm [11,12]. Conflicting reports from neutron powder diffraction were found, where either space group P4mm or P4/mmm was satisfactorily assigned to the crystal and magnetic structures. Although disagreement exists regarding the symmetry space group assignment in Rietveld refinements, no significant difference between the two space groups is shown on the structural parameters. The nature of two magnetic transitions have been reported with TN1 ¼450 K and TN2 ¼190 K based on neutron powder diffraction [10]. Below TN1, the diffraction patterns reveal additional magnetic reflections at (h/2 k/2 l/2) to indicate a magnetic unit cell of amag ¼ 2a and cmag ¼ 2c. Below TN2, two sets of satellite peaks exist surround the (1/2 1/2 1/2) magnetic peak. A tentative magnetic structure model was proposed, based on neutron powder diffraction results so far. A large single crystal sample is desirable to resolve the contradicting results on the crystal and magnetic structures conclusively. Here, using an improved optical floating zone method, we report the successful growth of a high quality YBaCuFeO5 single crystal with a size as large as 5 cm long
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101
3. Results and discussion
Fig. 1. Crystal structures of (a) YBaCuFeO5 (P4mm or P4/mmm) and (b) YBa2Cu3O7 (Pmmm). The space group P4/mmm for YBaCuFeO5 is defined when Fe and Cu atoms in the (Fe/Cu)O2 layer are disordered at Fe/Cu site of half-occupancy.
and 0.5 cm in diameter. In particular, a tentative sectional view of the YBaCuFeO5-CuO pseudo-binary phase diagram is proposed based on the corresponding DTA studies.
2. Experiment Powder sample of YBaCuFeO5 was prepared in the air through solid state reaction. Stoichiometric amounts of Y2O3, BaCO3, Fe2O3, and CuO were mixed and heated at 900 1C in the air. The mixture was ground and fired repeatedly at 1000 1C for 48 h. The obtained powder was packed and subsequently sealed into a thin-rubber tubing to form a cylindrical rod as the feed rod. The solvent rods consisted of CuO with 2 wt% B2O3 added to increase the molten zone viscosity. The typical dimensions of feed and solvent rods were 5 mm in diameter and 50 mm in length after being shaped with a hydraulic press under a pressure of 60 MPa. The feed and solvent rods were sintered in the air at 1150 1C and 900 1C, respectively, for 24 h. Single crystal growth was performed through the traveling solvent floating zone (TSFZ) method using a four-mirror optical furnace in which four 150 Watt halogen lamps were used as the heating source (Crystal System Corporation, Japan). Thermal analysis for YBaCuFeO5 and CuO powder mixtures with various molar ratios and the solvent flux after the crystal growth were performed using a TG-DTA thermal analysis system (STA 449 F3, NETZSCH). The measurements were performed with the heating and cooling rate of 10 1C/min in air flow of 50 ml/min. The as-grown single crystals were annealed at 1000 1C for 12 h in the air. Phase characterization and crystal structure analysis were performed using synchrotron X-ray powder diffraction (SXRD). The SXRD pattern for a pulverized crystal was collected at room temperature using synchrotron X-ray (20 keV energy, wavelength¼ 0.619925 Å) at beam line Bl01C2 of the National Synchrotron Radiation Research Center (NSRRC) in Taiwan. The obtained diffraction patterns were analyzed with the Rietveld method using the General Structure Analysis System (GSAS) software package. The composition of the solvent and single crystal was analyzed using an electron probe micro-analyzer (EPMA). The magnetic susceptibility was measured using a superconducting quantum interference device magnetometer (Quantum Design, VSM) with a 1-T magnetic field perpendicular and parallel to the ab-plane.
The traveling solvent floating zone (TSFZ) method is a widely used technique to grow large size oxide single crystals. Compared to flux growth, the TSFZ method has the advantage that large size single crystal can be grown without contamination from the crucible material. However, there are some challenges when applying the TSFZ method to the growth of YBaCuFeO5. First, YBaCuFeO5 melts incongruently near 1220 1C, and a proper solvent rod is required, with CuO preferably used as one of the constituents. However, there is no reported phase diagram that involves all four elements simultaneously, and constructing such a complex phase diagram would not be practical. A pseudo-binary phase diagram of YBaCuFeO5-CuO is an alternative choice, but the complexity of the diagram remains high. In this work, we used pure CuO as the initial solvent in the molten zone and slowly modified the relative YBaCuFeO5-CuO ratio during the solidification process; single crystal YBaCuFeO5 can be grown after the supersaturation condition is reached naturally without the need of prior knowledge of the phase diagram, i.e., the optimal flux is obtained through a self-adjusted method after more YBaCuFeO5 is dissolved into the initial CuO solvent. The penetration effect, or wetting phenomenon at the edge of the feed rod, is known to be the greatest problem in the TSFZ growth of similar oxide systems [13]. The growth of many high-temperature superconductor materials, such as La2 xSrxCuO4 and YBa2Cu3O7 x, [14] encountered difficulties due to strong penetration effects in the growth process. The solvent in the molten zone tends to penetrate into the feed rod edge during the growth. As a result, the molten zone cannot remain stabilized for the required long time crystal pulling. Furthermore, the CuO flux becomes unstable at high temperature after melting because of not only the copper oxide vapor loss at high temperature but also the oxygen loss, which often creates bubbles in the molten zone from the reaction of CuO(liq.)-Cu2O(liq.)þ O2(gas). The Cu2O raises the melting point of the molten zone, and O2 bubbles collapse the molten zone eventually. We can solve this problem by adding 2 wt% B2O3 into the solvent rod to increase the viscosity of the molten zone and decrease the melting temperature significantly [13]. With the decrease in the molten zone temperature, the bubbles in the molten zone are reduced substantially. Moreover, the higher viscosity of the solvent provides stability to the molten zone, thereby preventing the penetration effect. Considering the small radius of the boron ion, no boron contamination due to substitution is expected. In the TSFZ method of the growth of single crystal YBaCuFeO5, the prepared stoichiometric feed rod was hung at the upper shaft and also used as the seed rod at the lower shaft in the beginning. The feed and seed rods rotate at 25 rpm in opposite directions to form a molten zone, which is composed of pure CuO in the beginning, as shown in Fig. 2. A slow pulling rate of 0.25 mm/h is beneficial for the gradual adjustment of the molten zone composition, and the lamp power is adjusted accordingly during the initial pulling before the single phase sample can be extracted due to supersaturation. A constant power was maintained to keep the molten zone in the liquid state balanced among viscosity, surface tension, and gravity during the entire pulling process for more than a week; as a result, more YBaCuFeO5 solute is dissolved gradually into the molten zone until supersaturation is reached in one day, which allows for only YBaCuFeO5 precipitates at the lower shaft. The evolution of the solidified part has been cut and polished to obtain a backscattered electron image (BEI) and to perform EPMA chemical analysis, as shown in Fig. 2. In the initial stage of growth, Cu2O dominates with a small amount of needle-like YBaCuFeO5 crystals grown. The grain size of YBaCuFeO5 grows until single grain single phase is achieved, which indicates that the YBaCuFeO5 is grown out of the Cu2O in the air atmosphere. Because it takes a long time to ensure that the solute YBaCuFeO5 is dissolved completely in the Cu2O flux, the stability of the molten zone becomes extremely
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Fig. 2. Schematic plot of the modified traveling solvent floating zone technique, and the backscattered electron image (BEI) of the indicated area is shown on the right. EPMA results indicate that the atomic ratio of the Cu2O area (deep gray) is Cu:O¼65.33:34.67, and the YBaCuFeO5 area (light gray) is Y:Ba:Cu:Fe:O¼ 11.03:11.63:11.04:10.52:55.68.
Fig. 3. DTA curves of the YBaCuFeO5/CuO mixtures with molar ratios of (a) 20:80, (b) 15:85, (c) 13:87, (d) 10:90, and (e) 5:95; (f) the solidified flux after the crystal growth. The heating and cooling rate is 10 1C/min in the air atmosphere. The heating curve is shown for (a) only, and only the cooling curves are shown for the rest.
important, and the pulling rate must be as slow as possible to reach single phase and single grain. To determine the crystal growth temperature and flux concentration in the process, DTA analysis was performed on the solidified flux after the growth and compared with the specimens of various YBaCuFeO5-CuO ratios, including YBaCuFeO5:CuO molar ratios of (a) 20:80, (b) 15:85, (c) 13:87, (d) 10:90, and (e) 5:95. The DTA results between 750 1C and 1200 1C are summarized in Fig. 3. All of the samples exhibit endothermic reactions during heating between 1010 and 1100 1C and exothermic reactions during cooling above 805 1C. The reactions at approximately 1020 1C are possibly due to pure copper oxide, and the lowest 805 1C exothermic reaction indicates the eutectic point of the YBaCuFeO5-CuO system. The endothermic reaction at 1100 1C is related to the dissolution of YBaCuFeO5 compound into the CuO melt. The liquidus for various YBaCuFeO5/CuO ratios can be determined from the exothermic reactions between 1126 1C and 1020 1C. The DTA scan for the solidified flux after crystal growth is analyzed for comparison, as shown in the (f) line shown in Fig. 3, which indicates that the first solidification temperature is near 1088 1C and close to that of sample (c); in addition, three more endothermic reactions near 990 1C, 970 1C, and 900 1C are found before reaching the eutectic point near 805 1C. The optimal flux concentration for YBaCuFeO5 growth can thus be extracted to be approximately YBaCuFeO5:CuO¼13:87.
Fig. 4. The proposed pseudo-binary phase diagram of YBaCuFeO5-CuO, which is based on the combined DTA and the modified TSFZ technique shown in Fig. 3.
A partial phase diagram of the YBaCuFeO5-CuO pseudo-binary system is proposed tentatively based on the DTA analysis, as shown in Fig. 4. The peritectic and eutectic lines are proposed tentatively based on the initial and final solidification temperatures for various YBaCuFeO5-CuO ratios, and the analysis of the solidified flux from TSFZ method is used to identify the three possible tie lines. The as-grown YBaCuFeO5 single crystal exhibits a shiny and clean surface in black color. The (0 0 1) plane is easy to cleave, and the (1 0 0) direction is perpendicular to the growth direction, as shown in Fig. 5. The back reflection Laue X-ray diffraction pattern with a 451 tilt is shown in the inset of Fig. 5, which confirms the good crystallinity from the diffraction spots with (0 0 1) orientation. The SXRD pattern of pulverized crystal is shown in Fig. 6. The SXRD patterns for samples taken from the beginning and the end sections of the growth are identical, which confirms that the sample is homogeneous single phase. Due to the similarity of the X-ray scattering cross-sections between Fe and Cu, it is difficult to distinguish the contributions from Fe and Cu, making it difficult to differentiate the order/disorder structure. The obtained SXRD patterns were analyzed using the Rietveld method, and both the reported P4/mmm and P4mm space groups from the literature were examined, [9,10] The resultant weighted profile residual and goodness-of-fit were Rwp ¼5.32% & χ2 ¼8.04 and Rwp ¼5.24% & χ2 ¼ 7.81 for the P4/mmm and P4mm space groups, respectively. Although the difference is not significant between the two models, the Rietveld refinement with the latter model achieves better values; thus, the P4mm space group is considered in the following discussion. A satisfactory refinement for the room temperature SXRD pattern is shown in Fig. 6, and the structural parameters are listed in Table 1. The sample crystallizes into a tetragonal lattice, with a¼ b¼ 3.87166(5) Å and c¼7.65703(9) Å. Because neither impurity phases nor obvious indications of oxygen off-stoichiometry were observed in the refinement, the sample contribution is set to be pure YBaCuFeO5. The temperature dependence of the homogeneous spin susceptibility for single crystal YBaCuFeO5 was measured in a field of 1 T oriented either along or perpendicular to the (0 0 1) direction at temperatures ranging from 2 to 1000 K, as shown in Fig. 7. A near paramagnetic behavior in the entire measurement temperature range is found for the field applied parallel to the c-axis. Two transitions of antiferromagnetic character can be identified with field applied parallel to the ab-plane, namely the antiferromagnetic transition at TN1 ¼455 K and the TN2 ¼170 K, which has been suggested to be a commensurate-incommensurate transition [4,10]. Based on the spin anisotropy implied from the measurement results of both orientations, the copper and iron spins lie in the abplane; a neutron scattering experiment based on a large single crystal grown in this study is proposed for future work.
Y.-C. Lai et al. / Journal of Crystal Growth 413 (2015) 100–104
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Fig. 5. YBaCuFeO5 single crystal grown through the TSFZ method, with the direction identified in the scheme. The Laue X-ray backscattering pattern of the cleavage face (0 0 1) at a 451 tilt angle is shown on the right.
Fig. 6. SXRD pattern at ambient conditions and the result of Rietveld refinement with space groups P4mm. The observed pattern, calculated profile, and the Bragg peak positions are indicated by cross marks, a red curve, and tick marks, respectively. The bottom curve shows the difference between the observed and calculated intensities. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Table 1 Structure parameters derived from the Rietveld refinements of YBaCuFeO5 with the space group of P4mm at room temperature. Atom
Site
x
y
z
Y Ba Fe Cu O1 O2 O3
1a 1a 1b 1b 1b 2c 2c
0 0 0.5 0.5 0.5 0.5 0.5
0 0 0.5 0.5 0.5 0 0
0.463 0.029 0.683 0.228 0.0328 0.279 0.650
a
(2) (1) (1) (1) (5) (2) (1)
Fig. 7. Temperature dependence of the homogeneous spin susceptibility of YBaCuFeO5 measured with the field of 1 T either parallel or perpendicular to the (0 0 1) direction.
Table 2 Summary of Curie-Weiss law fitting of M/H in the temperature range of 600 to 800 K for YBaCuFeO5. Field orientation
C (cm3K/mole)
θ (K)
χ0 (cm3/mole)
H//ab-plane H//c-axis Powder
3.80963 11.5678 4.97528
675.43 1800.15 893.12
0.0002575 0.00155 0.0007943
Uiso / Å2
0.0123 0.0211 0.001 0.019 0.016 0.016 0.016
(8) (6) (3) (3) (1) (1) (1)
a¼ b ¼3.87166(5) Å, c ¼7.65703(9) Å, Rwp ¼ 5.24%, Rp ¼ 2.81%, χ2 ¼7.81. a The isotropic thermal parameter Uiso of oxygen was constrained to be identical by atom type.
The temperature dependence of the susceptibility above TN1 was fitted using the Curie-Weiss law, χ ðTÞ ¼ χ 0 þC=ðT θÞ, where χ0, C, and θ are the temperature independent susceptibility, the Curie constant, and the Weiss temperature, respectively. The fitting results between T¼ 600–800 K are summarized in Table 2. The Curie constants for H//c-axis and H//ab-plane were found to be different, which may be ascribed to the anisotropic g-factor. The powder-averaged Curie constant is Cexp ¼ 4.975 cm3K/mole, which corresponds to an effective magnetic moment of μexp ¼ 6.309 μB. Because the Mossbauer spectra analysis indicates that the spin of Fe3 þ is at a high-spin of 5/2 and the spin of Cu2 þ is at 1/2, [11] the expected Curie constant is C ¼ ðN Fe3 þ ðμFe3 þ Þ2 þ N Cu2 þ ðμCu2 þ Þ2 þ Þ=
3kB ¼ 4:754 cm3 K=mole, i.e., μcal ¼6.144μB. The satisfactory agreement between the theoretical and experimental results confirms that Fe3 þ is in the HS state of S ¼5/2 and that Cu2 þ is in the state of S ¼1/2.
4. Conclusion Large size and high quality single-crystal YBaCuFeO5 was grown using a modified traveling solvent floating zone (TSFZ) method without prior knowledge of the complex phase diagram, which involves four cation elements. A partial pseudo-binary phase diagram of the YBaCuFeO5-CuO system was determined using combined DTA thermal analysis and the self-adjusted flux. We believe that the proposed TSFZ technique is particularly useful for the crystal growth of cuprate compounds involving more than three cation elements because it does not require prior phase diagram construction.
Acknowledgments We are grateful to S.Y. Tsai for the EPMA measurements performed at National Tsing Hua University, Hsinchu. This work
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was supported by the Ministry of Science and Technology of Taiwan under project number MOST-102-2119-M-002-004. C.H. Du is grateful to MOST for the financial support through MOST-992112-M-032-005-MY3 and MOST-102-2112-M-032-004-MY3. References [1] L. Er-Rakho, C. Michel, P. Lacorre, B. Raveau, J. Solid State Chem 73 (1988) 531–535. [2] W.I.F. David, W.T.A. Harrison, J.M.F. Gunn, O. Moze, A.K. Soper, P. Day J.D. Jorgensen, D.G. Hinks, M.A. Beno, L. Soderholm, D.W. Capone, Ii, I.K. Schuller, C.U. Segre, K. Zhang, J.D. Grace, Nature 327 (1987) 310–312. [3] Y. Maeno, T. Tomita, M. Kyogoku, S. Awaji, Y. Aoki, K. Hoshino, A. Minami, T. Fujita, Nature 328 (1987) 512–514. [4] B. Kundys, A. Maignan, C. Simon, Appl. Phys. Lett 94 (2009) 072506. [5] T. Kimura, Y. Sekio, H. Nakamura, T. Siegrist, A.P. Ramirez, Nat. Mater. 7 (2008) 291–294.
[6] A.W. Mombru, C. Christides, A. Lappas, K. Prassides, M. Pissas, C. Mitros, D. Niarchos, Inorg. Chem 33 (1994) 1255–1258. [7] M.J. Ruiz-Aragon, U. Amador, E. Morán, N.H. Andersen, Phys. C: Superconductivity 235–240 (Part 3) (1994) 1609–1610. [8] V. Caignaert, I. Mirebeau, F. Bourée, N. Nguyen, A. Ducouret, J.M. Greneche, B. Raveau, J. Solid State Chem 114 (1995) 24–35. [9] M.J. Ruiz-Aragón, E. Morán, U. Amador, J.L. Martínez, N.H. Andersen, H. Ehrenberg, Phys. Rev. B 58 (1998) 6291–6297. [10] A.W. Mombrú, K. Prassides, C. Christides, R. Erwin, M. Pissas, C. Mitros, D. Niarchos, J. Phys.: Condensed Matter 10 (1998) 1247. [11] C. Meyer, F. Hartmann-Bourton, Y. Gros, P. Strobel, Solid State Commun 76 (1990) 163–168. [12] M. Pissas, C. Mitros, G. Kallias, V. Psycharis, A. Simopoulos, A. Kostikas, D. Niarchos, Phys. C: Superconductivity 192 (1992) 35–40. [13] A. Maljuk, S. Watauchi, I. Tanaka, H. Kojima, J. Cryst. Growth 212 (2000) 138–141. [14] D. Elwell, H.J. Scheel, Crystal Growth from High-Temperature Solutions, Academic Press, London, 1975.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Self-adjusted flux for the traveling solvent floating zone growth of YBaCuFeO5 crystal Yen-Chung Lai a,b, Guo-Jiun Shu a, Wei-Tin Chen a, Chao-Hung Du b, Fang-Cheng Chou a,c,d,n a
Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan Department of Physics, Tamkang University, Tamsui 25137, Taiwan c National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan d Taiwan Consortium of Emergent Crystalline Materials, Ministry of Science and Technology, Taipei 10622, Taiwan b
art ic l e i nf o
a b s t r a c t
Article history: Received 19 September 2014 Received in revised form 3 November 2014 Accepted 21 December 2014 Communicated by: R.S. Feigelson Available online 29 December 2014
A modified traveling solvent floating zone (TSFZ) technique was used to successfully grow a large size and high quality single crystal of multiferroic material YBaCuFeO5. This modified TSFZ growth uses a stoichiometric feed rod and pure copper oxide as the initial flux without prior knowledge of the complex phase diagram involving four elements, and the optimal flux for the growth of incongruently melt crystal is self-adjusted after a prolonged stable pulling. The wetting of the feed rod edge that often perturbs the molten zone stability was avoided by adding 2 wt% B2O3. The optimal flux concentration for the YBaCuFeO5 growth can be extracted to be near YBaCuFeO5:CuO¼ 13:87 in molar ratio. The crystal quality was confirmed by the satisfactory refinement of crystal structure of space group P4mm and the two consecutive anisotropic antiferromagnetic phase transitions near 455 K and 170 K. & 2014 Elsevier B.V. All rights reserved.
Keywords: Single crystal growth Floating zone technique Phase diagram X-ray diffraction Magnetic materials
1. Introduction The oxygen-deficient perovskite material YBaCuFeO5 was first identified by Er-Rakho et al. [1] before it was treated as an impurity phase in the Fe-doped superconductor YBa2Cu3O7 x and the related Y2BaCuO5 [2,3]. YBaCuFeO5 has been reported to be multiferroic, with intriguing couplings that enable the material to exhibit ferroelectricity and incommensurate magnetic ordering below 230 K [4]. Note that after the discovery of multiferroicity of CuO at 220 K, [5] YBaCuFeO5 became the first example of a rare oxide material demonstrating a multiferroic transition temperature above 200 K. More attention should be placed on YBaCuFeO5 due to the fascinating phenomena in fundamental physics and the possible technical application of the material. The crystal structure of YBaCuFeO5 can be derived from YBa2Cu3O7 by removing a layer of BaO and the Cu(1)O chain layer and then replacing one-half of the Cu in the Cu(2)O2 layer by Fe, as illustrated in Fig. 1. The tetragonal structure was confirmed by X-ray powder diffraction, with lattice parameters of a ¼b¼ 3.867 Å and c¼ 7.656 Å E2a. It is still under debate whether the Cu and Fe
n Corresponding author at: Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan. Tel.: þ 886 2 33665254; fax: þ 886 2 33663843. E-mail address: [email protected] (F.-C. Chou).
http://dx.doi.org/10.1016/j.jcrysgro.2014.12.020 0022-0248/& 2014 Elsevier B.V. All rights reserved.
cations order or not. Two models have been considered in the earlier crystal structural works, [1,6–10] including (a) a complete disordering between Fe/Cu in the (Fe/Cu)O2 layer (P4/mmm space group) and (b) a fully ordered structure between Fe and Cu ions occupying distinct crystallographic sites (P4mm space group). Mossbauer spectroscopy studies suggested that iron and copper ions are fully ordered and are located in separate layers of the space group P4mm [11,12]. Conflicting reports from neutron powder diffraction were found, where either space group P4mm or P4/mmm was satisfactorily assigned to the crystal and magnetic structures. Although disagreement exists regarding the symmetry space group assignment in Rietveld refinements, no significant difference between the two space groups is shown on the structural parameters. The nature of two magnetic transitions have been reported with TN1 ¼450 K and TN2 ¼190 K based on neutron powder diffraction [10]. Below TN1, the diffraction patterns reveal additional magnetic reflections at (h/2 k/2 l/2) to indicate a magnetic unit cell of amag ¼ 2a and cmag ¼ 2c. Below TN2, two sets of satellite peaks exist surround the (1/2 1/2 1/2) magnetic peak. A tentative magnetic structure model was proposed, based on neutron powder diffraction results so far. A large single crystal sample is desirable to resolve the contradicting results on the crystal and magnetic structures conclusively. Here, using an improved optical floating zone method, we report the successful growth of a high quality YBaCuFeO5 single crystal with a size as large as 5 cm long
Y.-C. Lai et al. / Journal of Crystal Growth 413 (2015) 100–104
101
3. Results and discussion
Fig. 1. Crystal structures of (a) YBaCuFeO5 (P4mm or P4/mmm) and (b) YBa2Cu3O7 (Pmmm). The space group P4/mmm for YBaCuFeO5 is defined when Fe and Cu atoms in the (Fe/Cu)O2 layer are disordered at Fe/Cu site of half-occupancy.
and 0.5 cm in diameter. In particular, a tentative sectional view of the YBaCuFeO5-CuO pseudo-binary phase diagram is proposed based on the corresponding DTA studies.
2. Experiment Powder sample of YBaCuFeO5 was prepared in the air through solid state reaction. Stoichiometric amounts of Y2O3, BaCO3, Fe2O3, and CuO were mixed and heated at 900 1C in the air. The mixture was ground and fired repeatedly at 1000 1C for 48 h. The obtained powder was packed and subsequently sealed into a thin-rubber tubing to form a cylindrical rod as the feed rod. The solvent rods consisted of CuO with 2 wt% B2O3 added to increase the molten zone viscosity. The typical dimensions of feed and solvent rods were 5 mm in diameter and 50 mm in length after being shaped with a hydraulic press under a pressure of 60 MPa. The feed and solvent rods were sintered in the air at 1150 1C and 900 1C, respectively, for 24 h. Single crystal growth was performed through the traveling solvent floating zone (TSFZ) method using a four-mirror optical furnace in which four 150 Watt halogen lamps were used as the heating source (Crystal System Corporation, Japan). Thermal analysis for YBaCuFeO5 and CuO powder mixtures with various molar ratios and the solvent flux after the crystal growth were performed using a TG-DTA thermal analysis system (STA 449 F3, NETZSCH). The measurements were performed with the heating and cooling rate of 10 1C/min in air flow of 50 ml/min. The as-grown single crystals were annealed at 1000 1C for 12 h in the air. Phase characterization and crystal structure analysis were performed using synchrotron X-ray powder diffraction (SXRD). The SXRD pattern for a pulverized crystal was collected at room temperature using synchrotron X-ray (20 keV energy, wavelength¼ 0.619925 Å) at beam line Bl01C2 of the National Synchrotron Radiation Research Center (NSRRC) in Taiwan. The obtained diffraction patterns were analyzed with the Rietveld method using the General Structure Analysis System (GSAS) software package. The composition of the solvent and single crystal was analyzed using an electron probe micro-analyzer (EPMA). The magnetic susceptibility was measured using a superconducting quantum interference device magnetometer (Quantum Design, VSM) with a 1-T magnetic field perpendicular and parallel to the ab-plane.
The traveling solvent floating zone (TSFZ) method is a widely used technique to grow large size oxide single crystals. Compared to flux growth, the TSFZ method has the advantage that large size single crystal can be grown without contamination from the crucible material. However, there are some challenges when applying the TSFZ method to the growth of YBaCuFeO5. First, YBaCuFeO5 melts incongruently near 1220 1C, and a proper solvent rod is required, with CuO preferably used as one of the constituents. However, there is no reported phase diagram that involves all four elements simultaneously, and constructing such a complex phase diagram would not be practical. A pseudo-binary phase diagram of YBaCuFeO5-CuO is an alternative choice, but the complexity of the diagram remains high. In this work, we used pure CuO as the initial solvent in the molten zone and slowly modified the relative YBaCuFeO5-CuO ratio during the solidification process; single crystal YBaCuFeO5 can be grown after the supersaturation condition is reached naturally without the need of prior knowledge of the phase diagram, i.e., the optimal flux is obtained through a self-adjusted method after more YBaCuFeO5 is dissolved into the initial CuO solvent. The penetration effect, or wetting phenomenon at the edge of the feed rod, is known to be the greatest problem in the TSFZ growth of similar oxide systems [13]. The growth of many high-temperature superconductor materials, such as La2 xSrxCuO4 and YBa2Cu3O7 x, [14] encountered difficulties due to strong penetration effects in the growth process. The solvent in the molten zone tends to penetrate into the feed rod edge during the growth. As a result, the molten zone cannot remain stabilized for the required long time crystal pulling. Furthermore, the CuO flux becomes unstable at high temperature after melting because of not only the copper oxide vapor loss at high temperature but also the oxygen loss, which often creates bubbles in the molten zone from the reaction of CuO(liq.)-Cu2O(liq.)þ O2(gas). The Cu2O raises the melting point of the molten zone, and O2 bubbles collapse the molten zone eventually. We can solve this problem by adding 2 wt% B2O3 into the solvent rod to increase the viscosity of the molten zone and decrease the melting temperature significantly [13]. With the decrease in the molten zone temperature, the bubbles in the molten zone are reduced substantially. Moreover, the higher viscosity of the solvent provides stability to the molten zone, thereby preventing the penetration effect. Considering the small radius of the boron ion, no boron contamination due to substitution is expected. In the TSFZ method of the growth of single crystal YBaCuFeO5, the prepared stoichiometric feed rod was hung at the upper shaft and also used as the seed rod at the lower shaft in the beginning. The feed and seed rods rotate at 25 rpm in opposite directions to form a molten zone, which is composed of pure CuO in the beginning, as shown in Fig. 2. A slow pulling rate of 0.25 mm/h is beneficial for the gradual adjustment of the molten zone composition, and the lamp power is adjusted accordingly during the initial pulling before the single phase sample can be extracted due to supersaturation. A constant power was maintained to keep the molten zone in the liquid state balanced among viscosity, surface tension, and gravity during the entire pulling process for more than a week; as a result, more YBaCuFeO5 solute is dissolved gradually into the molten zone until supersaturation is reached in one day, which allows for only YBaCuFeO5 precipitates at the lower shaft. The evolution of the solidified part has been cut and polished to obtain a backscattered electron image (BEI) and to perform EPMA chemical analysis, as shown in Fig. 2. In the initial stage of growth, Cu2O dominates with a small amount of needle-like YBaCuFeO5 crystals grown. The grain size of YBaCuFeO5 grows until single grain single phase is achieved, which indicates that the YBaCuFeO5 is grown out of the Cu2O in the air atmosphere. Because it takes a long time to ensure that the solute YBaCuFeO5 is dissolved completely in the Cu2O flux, the stability of the molten zone becomes extremely
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Fig. 2. Schematic plot of the modified traveling solvent floating zone technique, and the backscattered electron image (BEI) of the indicated area is shown on the right. EPMA results indicate that the atomic ratio of the Cu2O area (deep gray) is Cu:O¼65.33:34.67, and the YBaCuFeO5 area (light gray) is Y:Ba:Cu:Fe:O¼ 11.03:11.63:11.04:10.52:55.68.
Fig. 3. DTA curves of the YBaCuFeO5/CuO mixtures with molar ratios of (a) 20:80, (b) 15:85, (c) 13:87, (d) 10:90, and (e) 5:95; (f) the solidified flux after the crystal growth. The heating and cooling rate is 10 1C/min in the air atmosphere. The heating curve is shown for (a) only, and only the cooling curves are shown for the rest.
important, and the pulling rate must be as slow as possible to reach single phase and single grain. To determine the crystal growth temperature and flux concentration in the process, DTA analysis was performed on the solidified flux after the growth and compared with the specimens of various YBaCuFeO5-CuO ratios, including YBaCuFeO5:CuO molar ratios of (a) 20:80, (b) 15:85, (c) 13:87, (d) 10:90, and (e) 5:95. The DTA results between 750 1C and 1200 1C are summarized in Fig. 3. All of the samples exhibit endothermic reactions during heating between 1010 and 1100 1C and exothermic reactions during cooling above 805 1C. The reactions at approximately 1020 1C are possibly due to pure copper oxide, and the lowest 805 1C exothermic reaction indicates the eutectic point of the YBaCuFeO5-CuO system. The endothermic reaction at 1100 1C is related to the dissolution of YBaCuFeO5 compound into the CuO melt. The liquidus for various YBaCuFeO5/CuO ratios can be determined from the exothermic reactions between 1126 1C and 1020 1C. The DTA scan for the solidified flux after crystal growth is analyzed for comparison, as shown in the (f) line shown in Fig. 3, which indicates that the first solidification temperature is near 1088 1C and close to that of sample (c); in addition, three more endothermic reactions near 990 1C, 970 1C, and 900 1C are found before reaching the eutectic point near 805 1C. The optimal flux concentration for YBaCuFeO5 growth can thus be extracted to be approximately YBaCuFeO5:CuO¼13:87.
Fig. 4. The proposed pseudo-binary phase diagram of YBaCuFeO5-CuO, which is based on the combined DTA and the modified TSFZ technique shown in Fig. 3.
A partial phase diagram of the YBaCuFeO5-CuO pseudo-binary system is proposed tentatively based on the DTA analysis, as shown in Fig. 4. The peritectic and eutectic lines are proposed tentatively based on the initial and final solidification temperatures for various YBaCuFeO5-CuO ratios, and the analysis of the solidified flux from TSFZ method is used to identify the three possible tie lines. The as-grown YBaCuFeO5 single crystal exhibits a shiny and clean surface in black color. The (0 0 1) plane is easy to cleave, and the (1 0 0) direction is perpendicular to the growth direction, as shown in Fig. 5. The back reflection Laue X-ray diffraction pattern with a 451 tilt is shown in the inset of Fig. 5, which confirms the good crystallinity from the diffraction spots with (0 0 1) orientation. The SXRD pattern of pulverized crystal is shown in Fig. 6. The SXRD patterns for samples taken from the beginning and the end sections of the growth are identical, which confirms that the sample is homogeneous single phase. Due to the similarity of the X-ray scattering cross-sections between Fe and Cu, it is difficult to distinguish the contributions from Fe and Cu, making it difficult to differentiate the order/disorder structure. The obtained SXRD patterns were analyzed using the Rietveld method, and both the reported P4/mmm and P4mm space groups from the literature were examined, [9,10] The resultant weighted profile residual and goodness-of-fit were Rwp ¼5.32% & χ2 ¼8.04 and Rwp ¼5.24% & χ2 ¼ 7.81 for the P4/mmm and P4mm space groups, respectively. Although the difference is not significant between the two models, the Rietveld refinement with the latter model achieves better values; thus, the P4mm space group is considered in the following discussion. A satisfactory refinement for the room temperature SXRD pattern is shown in Fig. 6, and the structural parameters are listed in Table 1. The sample crystallizes into a tetragonal lattice, with a¼ b¼ 3.87166(5) Å and c¼7.65703(9) Å. Because neither impurity phases nor obvious indications of oxygen off-stoichiometry were observed in the refinement, the sample contribution is set to be pure YBaCuFeO5. The temperature dependence of the homogeneous spin susceptibility for single crystal YBaCuFeO5 was measured in a field of 1 T oriented either along or perpendicular to the (0 0 1) direction at temperatures ranging from 2 to 1000 K, as shown in Fig. 7. A near paramagnetic behavior in the entire measurement temperature range is found for the field applied parallel to the c-axis. Two transitions of antiferromagnetic character can be identified with field applied parallel to the ab-plane, namely the antiferromagnetic transition at TN1 ¼455 K and the TN2 ¼170 K, which has been suggested to be a commensurate-incommensurate transition [4,10]. Based on the spin anisotropy implied from the measurement results of both orientations, the copper and iron spins lie in the abplane; a neutron scattering experiment based on a large single crystal grown in this study is proposed for future work.
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Fig. 5. YBaCuFeO5 single crystal grown through the TSFZ method, with the direction identified in the scheme. The Laue X-ray backscattering pattern of the cleavage face (0 0 1) at a 451 tilt angle is shown on the right.
Fig. 6. SXRD pattern at ambient conditions and the result of Rietveld refinement with space groups P4mm. The observed pattern, calculated profile, and the Bragg peak positions are indicated by cross marks, a red curve, and tick marks, respectively. The bottom curve shows the difference between the observed and calculated intensities. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Table 1 Structure parameters derived from the Rietveld refinements of YBaCuFeO5 with the space group of P4mm at room temperature. Atom
Site
x
y
z
Y Ba Fe Cu O1 O2 O3
1a 1a 1b 1b 1b 2c 2c
0 0 0.5 0.5 0.5 0.5 0.5
0 0 0.5 0.5 0.5 0 0
0.463 0.029 0.683 0.228 0.0328 0.279 0.650
a
(2) (1) (1) (1) (5) (2) (1)
Fig. 7. Temperature dependence of the homogeneous spin susceptibility of YBaCuFeO5 measured with the field of 1 T either parallel or perpendicular to the (0 0 1) direction.
Table 2 Summary of Curie-Weiss law fitting of M/H in the temperature range of 600 to 800 K for YBaCuFeO5. Field orientation
C (cm3K/mole)
θ (K)
χ0 (cm3/mole)
H//ab-plane H//c-axis Powder
3.80963 11.5678 4.97528
675.43 1800.15 893.12
0.0002575 0.00155 0.0007943
Uiso / Å2
0.0123 0.0211 0.001 0.019 0.016 0.016 0.016
(8) (6) (3) (3) (1) (1) (1)
a¼ b ¼3.87166(5) Å, c ¼7.65703(9) Å, Rwp ¼ 5.24%, Rp ¼ 2.81%, χ2 ¼7.81. a The isotropic thermal parameter Uiso of oxygen was constrained to be identical by atom type.
The temperature dependence of the susceptibility above TN1 was fitted using the Curie-Weiss law, χ ðTÞ ¼ χ 0 þC=ðT θÞ, where χ0, C, and θ are the temperature independent susceptibility, the Curie constant, and the Weiss temperature, respectively. The fitting results between T¼ 600–800 K are summarized in Table 2. The Curie constants for H//c-axis and H//ab-plane were found to be different, which may be ascribed to the anisotropic g-factor. The powder-averaged Curie constant is Cexp ¼ 4.975 cm3K/mole, which corresponds to an effective magnetic moment of μexp ¼ 6.309 μB. Because the Mossbauer spectra analysis indicates that the spin of Fe3 þ is at a high-spin of 5/2 and the spin of Cu2 þ is at 1/2, [11] the expected Curie constant is C ¼ ðN Fe3 þ ðμFe3 þ Þ2 þ N Cu2 þ ðμCu2 þ Þ2 þ Þ=
3kB ¼ 4:754 cm3 K=mole, i.e., μcal ¼6.144μB. The satisfactory agreement between the theoretical and experimental results confirms that Fe3 þ is in the HS state of S ¼5/2 and that Cu2 þ is in the state of S ¼1/2.
4. Conclusion Large size and high quality single-crystal YBaCuFeO5 was grown using a modified traveling solvent floating zone (TSFZ) method without prior knowledge of the complex phase diagram, which involves four cation elements. A partial pseudo-binary phase diagram of the YBaCuFeO5-CuO system was determined using combined DTA thermal analysis and the self-adjusted flux. We believe that the proposed TSFZ technique is particularly useful for the crystal growth of cuprate compounds involving more than three cation elements because it does not require prior phase diagram construction.
Acknowledgments We are grateful to S.Y. Tsai for the EPMA measurements performed at National Tsing Hua University, Hsinchu. This work
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was supported by the Ministry of Science and Technology of Taiwan under project number MOST-102-2119-M-002-004. C.H. Du is grateful to MOST for the financial support through MOST-992112-M-032-005-MY3 and MOST-102-2112-M-032-004-MY3. References [1] L. Er-Rakho, C. Michel, P. Lacorre, B. Raveau, J. Solid State Chem 73 (1988) 531–535. [2] W.I.F. David, W.T.A. Harrison, J.M.F. Gunn, O. Moze, A.K. Soper, P. Day J.D. Jorgensen, D.G. Hinks, M.A. Beno, L. Soderholm, D.W. Capone, Ii, I.K. Schuller, C.U. Segre, K. Zhang, J.D. Grace, Nature 327 (1987) 310–312. [3] Y. Maeno, T. Tomita, M. Kyogoku, S. Awaji, Y. Aoki, K. Hoshino, A. Minami, T. Fujita, Nature 328 (1987) 512–514. [4] B. Kundys, A. Maignan, C. Simon, Appl. Phys. Lett 94 (2009) 072506. [5] T. Kimura, Y. Sekio, H. Nakamura, T. Siegrist, A.P. Ramirez, Nat. Mater. 7 (2008) 291–294.
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