Growth and Structural Characterization of Needlelike Crystals in the Y-Ba-Cu-O System P. Mele,† A. Ubaldini,‡ M. M. Carnasciali, and G. A. Costa INFM-LAMIA and Dipartimento di Chimica e Chimica Industriale, UniVersita` di GenoVa, Via Dodecaneso 31, I-16146 GenoVa, Italy
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 8 1761-1765
M. Scavini* Dipartimento di Chimica Fisica ed Elettrochimica, UniVersita` di Milano, Via C. Golgi 19, I-20133 Milano, Italy ReceiVed October 5, 2005; ReVised Manuscript ReceiVed May 16, 2006
ABSTRACT: A system formed by a bar of Y2BaCuO5 (Y211) phase supported on two YBa2Cu3Ox (Y123) pellets was processed by thermal treatment at high temperature. The first effect was a change in chemical composition inside the bar, yielding new phases: mostly Y123 and BaCuO2. Their concentrations were higher near the bar ends. A second, very interesting result was the growth of two kinds of acicular crystals underneath the whole bar: green, transparent Y211 needles and needles with a green core of Y211, covered by a Y123 layer. Micro Raman and SEM analyses clearly identified the two different phases. XRD inspection has confirmed that Y211 crystals are high-quality single crystals. The preferential growth direction for Y211 needles is along the crystallographic b axis. A possible explanation, based on the VLS mechanism, for their growth is suggested. 1. Introduction Single crystals are required to study the intrinsic physical properties of materials. Also in the case of the YBa2Cu3Ox (Y123) superconductor, several methods were developed to grow single crystals. These methods can be classified into two groups. The first group consists of the free growth techniques, among the most common being the “self-flux method”. Through this process, high-quality crystals grow as a consequence of free nucleation1-3 from a high-temperature solution, where the solvent (the “flux”) is a mixture of oxides belonging to the Y-Ba-Cu-O phase diagram. The second group consists of the so-called directional growth techniques, the most important being the traveling solvent floating zone method (TSFZ)4,5 and the solute-rich liquid crystal pulling method (SRL-CP).6,7 In the case of the TSFZ, single crystals grow as a consequence of a controlled recrystallization of a melted sample slowly moved through an appropriate thermal gradient. In the SRL-CP, crystal growth is achieved by slowly pulling a crystal seed, initially brought into contact with the surface of a Ba-Cu-O liquid phase-Y2BaCuO5 system. In any case, the crystals’ external habits depend on the experimental conditions. For Y123, it is platelike in the case of the flux method,8 while the TSFZ and SRL-CP techniques allow the growth of large, long rod-shaped crystals.4,6 In this work, the results on the growth of crystals with an unusual habit are presented. During an experiment performed to test the effects of the presence of the liquid phase, derived from the peritectic decomposition of YBa2Cu3Ox, on the sintering of a Y2BaCuO5 (Y211) bar, many needle-shaped crystals, often formed by a core and an external layer of a different phase, were obtained underneath the bar. Their morphology was different from that achieved by performing * To whom correspondence should be addressed. Tel: +39-0250314221: Fax: +39-02-50314300. E-mail:
[email protected]. † Present address: Department of Material Science and Engineering, Kyoto University, Yoshida-Honmachi, Sakyo-Ku, Kyoto-Shi 606-8501, Japan. ‡ Present address: Superconducting Materials Center, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan.
typical crystal experiments, suggesting an unusual growth mechanism, at least in the Y-Ba-Cu-O system. Generally speaking, it is possible to obtain needle-shaped single crystals, or whiskers,9,10 in nearly all classes of materials.11 In the field of ceramic superconducting oxides, a great number of reports exist about growth and characterization of BiSCCO whiskers (see ref 12, for example) that present excellent mechanical properties as grown. Several papers13-17 on the preparation and characterization of YBCO and REBCO whiskerlike crystals have been published: Lepisto et al. grew13 and characterized14 whiskers of YBCO by in situ sputtering, on a MgO substrate; Pathak et al.15 synthesized whiskers from YBCO powders; Zhang et al.16 reported growth of SmBCO needle-shaped crystals on the surface of melt-textured bulk pellets; Klemenz and Scheel17 grew NdBCO from the BaO/CuO high-temperature flux in Nd2O3 crucibles. However, so far there have not been reports on the growth of whiskers in the case of the Y2BaCuO5 phase. The aim of this work is then to characterize these crystals, grown in an unusual way, investigating their structure and composition. 2. Experimental Details The Y211 powders were prepared by solid-state reactions in flowing oxygen at 1000 °C for 48 h, mixing commercial Y2O3, CuO, and BaO2 in stoichiometric amounts. These mixtures were then pressed into bar shapes, measuring 20 × 5 × 3 mm3. The Y123 powders were also prepared in a solid-state reaction by a well-established preparation method.18 With the stoichiometric mixture as a starting point, the same oxides were calcined at 930 °C for 8.5 h and then ground, remixed, and finally heated to 980 °C for 8.0 h. Such treated powders were pressed into pellets, measuring about 1 cm in diameter and height. The Y211 bar was laid on two Y123 pellets to avoid direct contact with the alumina crucible. This system was heated in a horizontal furnace in flowing oxygen with a 200 °C/h heating rate to 1250 °C, held for 0.5 h at the same temperature, rapidly cooled to 1030 °C over 2 h, slowly cooled to 1010 °C over 20 h, and finally cooled to room temperature at a rate of 60 °C/h. To investigate the effects of the contact between the solid and the liquid phases, the maximal temperature, Tmax, was lower than the Y211 decomposition temperature, TP,Y211 (1310 °C), and greater than the Y123 peritectic decomposition temperature, TP,Y123 (1030 °C), both determined by a Netzsch 408 thermal analyzer. The samples were investigated by the following techniques: SEM-
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Table 1. Crystal Data and Collection and Refinement Details formula formula wt color habit dimens space group Z µ(Mo KR) temp radiation scan type max 2θ spheres of data total no. of rflns no. of unique rflns cell param a (Å) cell param b (Å) cell param c (Å) R(F) Rw(F2) goodness of fit
Y2BaCuO5 458.67 green prismatic 65 × 70 × 90 µm3 Pnma 4 35.50 mm-1 293 K 0.710 73 (Mo KR) ω-φ 104.85° -26 e h e 26 -11 e k e 11 -15 e l e 15 30 538 2957 12.175(2) 5.659(1) 7.129(1) 0.0417 (all data) 0.0695 1.028
EDAX, using an Oxford Stereoscan 440 electronic microscope; optical microscopy, by normal and polarized light; Raman spectroscopy at room temperature, using a Renishaw System 2000 Raman imaging microscope. Spectra were collected, using as the source a 633 nm He-Ne laser, in the range 2000-100 cm-1 (20 s, 20×, 9 accumulations). X-ray diffraction analysis was performed on a crystal of prismatic habit (65 × 70 × 90 µm3). It was mounted on a Bruker SMART CCD area-detector diffractometer (graphite-monochromated Mo KR radiation, λ ) 0.710 73 Å). Table 1 summarizes the experimental details and the refinement. The SAINT program has been utilized for cell refinement and data reduction, while empirical absorption collection (µ ) 35.50 mm-1) has been performed by the SADABS program;19 the resulting range of the transmission coefficient (Tr) is 0.21 < Tr < 0.34. Structure solution (direct methods) and structure refinements were performed by means of the SHELX97 program.20 Scattering factors, including anomalous dispersion, were taken from ref 21. Rotation flat plate photographs (120 min) on needles have been collected on a four-circle Syntex P4 diffractometer, using graphitemonochromated Mo KR radiation.
3. Results and Discussion After the thermal cycle described in the Experimental Section, the external color of the bar turned black, instead of the initial green color, typical of the Y211 phase. Using an optical microscope in polarized light, many small grains, closely interconnected, of a green phase and of a black phase were observed inside the bar, as shown in Figure 1. Similarly, small amounts of the green phase were observed inside the formerly Y123 supporting pellets. These findings show that a chemical reaction occurred in the bar and that its composition changed. The sample was cut lengthwise, and its atomic average composition was determined by EDAX analysis, performed in the whole sample. The cationic composition changed from point to point, because of the polyphasic nature of the bar. Actually, by X-ray powder diffraction analysis the presence in the bar of Y123 and BaCuO2 in addition to the Y211 phase was revealed. The yttrium average concentration is about 45% in the central part of the bar, i.e., very close to the nominal stoichiometry of the Y211 phase, while it falls to about 30% toward the ends. In contrast, the average copper concentration decreases from 40% on the fringe areas to about 25% in the central part of the bar. It should be remembered that the copper percentage is 25% in the Y211 phase, while it is 50% in the BaCuO2 or Y123 phase.
Figure 1. Optical micrography of the bar section, showing the fine Y211 particle dispersion. White grains are Y211 particles.
The different relative ratios among the cations in each area depend on the different relative percentages of all the phases present in that area. Therefore, it seems that, after thermal treatment, the central part of the bar is almost completely constituted by the Y211 phase, while its concentration is low toward the bar ends, which are in contrast mostly formed by Y123 and BaCuO2. This means that the reactions in the bar are determined by diffusion from the supporting pellets toward the center. During the first part of the thermal program at around 1030 °C, a (Ba, Cu, O) liquid phase appears as the consequence of peritectic decomposition of the Y123 pellets. According to the results of several researchers,22.23 this liquid can soak the Y211 bar by a capillarity effect, wetting the Y211 particles. During the cooling stage, the (Ba, Cu, O) liquid excess in the bar can react with the Y211 phase, yielding Y123 according to the reaction
Y2BaCuO5,sol + (3BaCuO2 + 2CuO)liq f 2YBa2Cu3O6.5,sol (1) As the (Ba, Cu, O) liquid moves from the pellets toward the center of the bar following an intergranular path, the concentration of the Y123 phase is higher close to the ends of the bar than to its center. In fact, the viscosity of the liquid is high and the liquid cannot easily reach all the parts of the bar, under these experimental conditions. Moreover, as a part of the liquid phase diffuses into the bar, only a fraction of Y211 formed inside the pellets at high temperature can react during the cooling step to form Y123. After the thermal treatment many acicular needles grew into a bushlike structure (Figure 2). The needles grew underneath the whole bar, in the empty space between the sample (that lies on two Y-123 pellets) and the alumina crucible. There was a larger number near the bar ends than the center, where the Y211 phase concentration is higher. No crystals were found on top of the bar. Two kinds of crystals can be observed under an optical microscope: green, transparent needles, clean or covered by a black surface, and others with a black core covered by a spurious phase. The micro Raman spectrum of the core of the green crystals presents the characteristic vibrational modes of Y211,24 while its black surface can be identified with the Y123 phase (Figure 3). The micro Raman results were in perfect agreement with EDAX analysis performed on many randomly chosen points of several samples, which determined stoichiometries of Y1.98-
Needlelike Crystals in the Y-Ba-Cu-O System
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Figure 2. SEM photographs: (a) needlelike crystals organized in a bush structure; (b) detail.
Figure 3. Micro Raman spectra of biphasic needles: (a) black phase; (b) green phase.
Ba1.05Cu1.03Ox and Y1.02Ba1.95Cu3.01Ox for the green phase and the black phase, respectively. As shown in Figure 2b, a SEM microscope revealed that generally both kinds of needles have a width of 100 µm and a thickness of 800 µm. The green crystals have hexagonal sections and often reveal very clean surfaces and tips. Conversely, the black crystals have square sections and they usually appear partially covered by a spurious phase. SEM/EDAX analysis revealed that this latter phase is almost completely formed by copper and barium with a relative ratio of close to 1, while the concentration of yttrium is very low. The layer formed by this phase is likely to be very thin so that the electronic beam can reach the crystal situated below, therefore suggesting that it is probably the BaCuO2 phase. It seems that needle formation is a further consequence of the reactions in the bar. To get more information about the needles’ structure, some large needles were separated from the matrix in order to perform rotation flat plate photographs, an example of which is shown in Figure 4. The photograph shows spots and rings revealing that both monocrystalline and polycrystalline phases are present in the needle. The XRD analysis revealed that the green core of each needle is a single crystal of the Y211 orthorhombic phase, space group Pnma, with the cell parameters a ) 12.189(2) Å, b ) 5.661(1) Å, and c ) 7.137(1) Å, similar to the findings by Buttner and Maslen.25 The preferential growth direction of the needles occurs along the crystallographic b axis. Moreover, XRD shows that the black external surface, which covers the core of the needles, is polycrystalline. In addition to spots, which can be ascribed to the Y211 phase, the photo shows continuous concentric rings, given by the intersection of the diffraction cones with the surface of the photograph. The homogeneous brightness of the rings probes the null or at least scarce iso orientation of the polycrystalline surface.
Figure 4. Rotation flat plate photograph of Y211/Y123 needles.
This suggests that the needle core and the covering layer form according to a different process or in a different moment of the thermal treatment. A green single crystal of prismatic habit, free of Y123 powder, has been cut from the top of a biphasic needle for a more accurate XRD analysis. For the last refinement cycles 50 parameters have been varied (scale factor, isotropic extinction, 16 atomic positions, and 32 thermal parameters). The low values of R factors (see Table 1) and difference in density ∆F values (∆Fmax ) 3.59 e Å-3; ∆Fmin ) -2.27 e Å-3), if compared with the work of Buttner and Masden,25 highlight the good quality of the crystal. With regard to the crystal structure in the Y2BaCuO5 phase, all the cations plus O3 lie on the y ) 1/4, 3/4 planes (on the mirrors), while O1 and O2 are close to the y ) 0, 1/2 level. Table 2 reports the closest interatomic distances. Each barium is surrounded by 11 oxygen ions. The Ba-O distances are widespread, centered on the ideal Ba-O ionic radius (∼2.95 Å) calculated on the basis of Shannon and Prewitt’s work.26 The unusually short Ba1-O3 distance ()2.611 Å) should be noted. The elongation of Ba1-O3 would increase the tilt angle of the Cu1-O3 bond (see Figure 5, top) and decrease the O3-Y2 distance, which already is the shortest interatomic distance for this ion.
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Figure 5. Connectivity of Cu-O polyhedra along the a (top), b (middle), and c (bottom) directions. Table 2. Selected Interatomic Distances bond
dist (Å)
multiplicity
bond
dist (Å)
multiplicity
Ba1-O3 Ba1-O3 Ba1-O2 Ba -O2 Ba1-O1 Ba1-O1 Y1-O3 Y1-O1 Y1-O1
2.611(2) 2.831(1) 2.950(2) 3.007(2) 3.071(2) 3.247(2) 2.278(2) 2.293(2) 2.365(2)
×1 ×2 ×2 ×2 ×2 ×2 ×1 ×2 ×2
Y1-O2 Y2-O3 Y2-O1 Y2-O2 Y2-O2 Cu1-O1 Cu1-O2 Cu1-O3
2.382(2) 2.299(2) 2.309(2) 2.325(2) 2.353(2) 1.976(2) 2.013(2) 2.203(2)
×2 ×1 ×2 ×2 ×2 ×2 ×2 ×1
The two symmetrically nonequivalent yttrium ions show a similar environment. They are both seven-coordinated, with Y-O distances varying from 2.278 to 2.382 Å for Y1 and from 2.299 to 2.353 Å for Y2. Cu1 is fivefold coordinated, like the Cu2 ion in Y123. The square pyramid formed by oxygen in Y123 is distorted, as O1 and O2 are not symmetrically equivalent. Cu1-O1 and Cu2O2 distances are shorter than the ideal ionic Cu-O distance (∼2.05 Å)22 but longer than Cu2-O2 and Cu2-O3 distances in YBCO, which are 1.942 and 1.955 Å, respectively,27 indicating that, in Y211, Cu-O bonds have a certain degree of covalence that is, however, less than that of the YBCO superconductor. For a graphical representation of the cation environment we refer to ref 25. Unlike Y123, in Y211 Cu-O polyhedra are not directly connected to each other; in Figure 5 their connectivity along the three crystallographic directions is shown through an ORTEP plot.28 Along the a direction (Figure 5, top) the Cu-O polyhedra are connected to each other by Ba1; the polyhedra are connected along the b direction (Figure 5, middle) by Y2 and along the c direction (Figure 5, bottom) by Y1. The growth of these needlelike single crystals is a surprising result. The following discussion is dedicated to a possible explanation of their formation. As already noted, the maximal processing temperature (Tmax ) 1250 °C) is lower than the
peritectic temperature of the pure Y211 phase (TP,Y211), determined by DTA measurements to be about 1310 °C. Hence, needle growth is not due to the recrystallization of the Y211 phase. Moreover, it is important to note that needle growth was observed only on the lower side of the bar, where the liquid can accumulate due to the effect of gravity. For this reason, the liquid is a very important factor for the crystal growth. Actually, the presence of the liquid phase is a favorable factor for cation diffusion29,30 and for the coarsening of Y211 particles.31,32 Even if the yttrium concentration in the Ba-Cu-O melt is low,33 it can be considered constant, as the Y123 pellets can act as a source. Thus, a chemical equilibrium can be established inside the bar and a diffusion from the small Y211 particles to the larger ones occurs. This effect is stronger in areas close to the pellets than in the center of the bar and stronger at the bottom of the bar than at the top, as the concentration of liquid is higher in these areas. In agreement with this, it can be seen that there is a larger number of needles near the ends than in the center of the bar: i.e., where the liquid is more abundant. The formation of these crystals is probably due not only to a diffusion process inside the liquid. The growth of whiskers or of other needlelike crystals is often a complex process, and many factors can play an important role. The whiskers are very commonly observed in many kinds of materials,9-17 and among the theories proposed to explain their formation, the vaporliquid-solid (VLS) growth model11 is the most popular. According to this theory, which was initially proposed for the growth of silicon whiskers, the formation requires the contemporary presence of a solid phase, of a liquid that is in contact with the solid surface, and of a vapor, which is necessary to allow the matter transport. In the silicon case,9 the crystals form if over the sample surface small liquid drops (for instance of a melted gold-silicon alloy) are present and if in the reacting atmosphere there are small quantities of H2 and SiCl4. The latter is required for the matter transport. The SiCl4 is reduced by hydrogen at the liquid droplet surface, as the liquid phase can absorb the gas much more quickly than can the free solid surface. The droplet becomes quickly supersaturated with Si and growth at the solidliquid interface occurs. As a consequence, the droplet rises from the initial level on the solid surface, leading to the formation of very long and thin crystals. This process could proceed indefinitely if the external conditions do not change and the matter transport continues. Of course, this does not happen, because the drop can solidify (for example, if the temperature becomes too low), or it can evaporate or be irreversibly polluted by impurities. The VLS process could occur if the cations can be effectively transported in the vapor phase. For instance, it has been reported that even a small amount of water in the reacting atmosphere is effective for the vapor transport of Ba.34 It seems reasonable that something similar might occur also in this case. It is possible that in the reacting atmosphere small quantities of impurities were present, such as H2O, CO2, and CO, and that they could react with the cations, forming some volatile compounds and allowing in this way the matter transport in the vapor phase. These compounds can react with the liquid phase present in the system. The liquid phase quickly would become saturated and the gas vector would return to the atmosphere. This could offer an explanation for the fact that crystals form under the bar and at its ends, where the liquid is more abundant. Thus, it is possible to imagine that a Y211 crystal can grow unidirectionally if over its tip there is a droplet of the BaCu-O liquid and if there is a continuous transport of matter
Needlelike Crystals in the Y-Ba-Cu-O System
from the vapor. In this case, the crystal growth rate depends on the slowest transport rate of the cations. Nevertheless, it must be pointed out that the VLS mechanism itself cannot explain all the characteristics of these needlelike crystals. Especially, by this theory it is not possible to explain why many crystals have an external layer different from their core. Actually, the external Y123 layer forms only after the Y211 core, during the cooling in the last part of the thermal treatment, as a consequence of reaction 1. If the needle is small enough, this reaction could lead to conversion of the initial Y211 single crystal into a polycrystalline Y123 needle; otherwise, an inner green core can survive. This means that the needle surface must be in contact with the liquid, at least during the last part of the process. It is probable that the liquid can wet the needle falling down the bar. Alternative models could be invoked, particularly because the VLS growth model implicitly assumes that the “last” atom added to the growing crystal goes on the crystal top. If, on the contrary, the last atom reaches the bottom of the crystal (i.e., in this case the part closer to the bar), the growth would follow a completely different mechanism. For instance, recently the growth of the EuBa2Cu3Ox whiskers was observed by Islam et al.35 during the synthesis of EuBa2Cu3CaTe0.5Oy samples. They argued that in this case the whisker growth does not follow the VLS mechanism but, rather, a mechanism similar to what was proposed by Barsoum et al.36 in the case of some intermetallic whiskers. In these cases, the mechanic strain,36 due to reactions leading to volume changes, could be the driving force for whisker growth, so that they are extruded from the bulk materials. In this case, the presence of the liquid phase inside the sintered Y211 bar might be the source for the mechanical stress. In any event, at this stage any growth model might be considered as an hypothesis, because more experimental data are required in order to confirm it or reject it. 4. Conclusion A system formed by a bar of Y211 green phase and by two supporting Y123 pellets was processed by thermal treatment whose maximal temperature was greater than the Y123 peritectic decomposition temperature and lower than the Y211 peritectic decomposition temperature. The effects of the thermal treatment during this work are quite surprising. The modifications occurring in the bar can be ascribed to the diffusion of a liquid phase, formed during the peritectic decomposition of the Y123 pellets. This phase moves from the ends of the bar toward its center and, due to the effect of gravity, it accumulates in the lower part of the bar. During the cooling stage, a reaction between this liquid and the Y211 phase occurs, leading to the formation of different phases, above all Y123 and BaCuO2. The concentration of these phases is higher toward the ends and lower in the center. A second, very interesting, result is the growth of many needlelike crystals at the lower side of the bar. There are two kinds of crystals: green, transparent Y211 needles with a hexagonal section and square needles with a green core of Y211 covered by a black layer of Y123. It may be assumed that the former needles form first and then react with the excess of liquid phase present in the bar, forming the Y123 layer. XRD analysis revealed that the Y211 core of each needle is a single crystal, while the external Y123 layer is polycrystalline. The Y211 crystals are orthorhombic, in space group Pnma. The needles grow preferentially along the b crystallographic axis. Structure refinement confirmed the high quality of the Y211 crystals.
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The growth of these crystals might be explained by a modified VLS mechanism, if matter transport in the gas phase is effective, but different mechanisms might be possible. Acknowledgment. We gratefully thank Prof. M. Ferretti for useful discussions and the CNR-ISTM, Milano, Italy, especially Dr. Tullio Pilati, for support during XRD data collection and analysis. Supporting Information Available: A CIF file giving crystal data for the Y211 crystals. This material is available free of charge via the Internet at http://pubs.acs.org.
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CG050519C
