Growth and Structural Characterization of Needlelike Metastable

Dipartimento di Chimica e Chimica Industriale, Universita` di Genova, and INFM-LAMIA,. Via Dodecaneso 31, I-16146 Genova, Italy. Received July 10, 200...
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Growth and Structural Characterization of Needlelike Metastable Crystals in the Nd-Ba-Cu-(Al)-O System M. Scavini* Dipartimento di Chimica Fisica ed Elettrochimica, Universita` di Milano, Via C. Golgi 19, I-20133 Milano, Italy

CRYSTAL GROWTH & DESIGN 2004 VOL. 4, NO. 6 1259-1263

P. Mele,† G. A. Costa, and M. Ferretti Dipartimento di Chimica e Chimica Industriale, Universita` di Genova, and INFM-LAMIA, Via Dodecaneso 31, I-16146 Genova, Italy Received July 10, 2004

ABSTRACT: An unconventional technique was used to grow crystals in the Nd-Ba-Cu-O system. A Nd4Ba2Cu2O10 (Nd-422) bar was supported on two NdBa2Cu3Ox (Nd-123) pellets in an alumina crucible and was heated in a horizontal furnace at a higher temperature than the peritectic decomposition point of Nd-422. Brown Al-doped Nd-Ba-Cu-O needlelike crystals belonging to a new phase were grown directly on the crucible. The structure of the new phase, whose composition is Nd6Ba9.78Sr0.22Cu2Al6O30, has been determined by single-crystal X-ray diffraction. The space group is P63mc; the unit cell parameters are a ) 11.526(2) Å, c ) 6.963(1) Å, and Z ) 1. A full matrix least-squares refinement yielded R(F) ) 0.0405 for 3131 independent reflections. This new phase is a metastable polymorphic modification of Nd-422 solid solution. The preferential directional growth of the needles lies along the c-axis. Many needles are affected by twinning by merohedry, with the twinning plane parallel to the (001) crystallographic plane. Introduction Single crystals are useful to study the intrinsic physical properties of materials. In the case of the REBa2Cu3Ox (RE-123, RE ) Y or rare earth) superconductors, several methods were developed to grow single crystals. These methods can be classified into two groups. The first group consists of free growth techniques, the most common of which is the “self-flux method”. As a consequence of free nucleation,1-3 high quality crystals grow from a high-temperature solution, where the solvent (the “flux”) is a mixture of oxides belonging to the REBa-Cu-O phase diagram. The second group consists of the so-called directional growth techniques, the most important of which are the travelling solvent floating zone (TSFZ) method4 and the solute-rich liquid crystal pulling (SRL-CP) method.5 In the TSFZ method, 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 method,5 a crystal seed is brought into contact with the melt surface, and the crystal can grow by slowly pulling. The crystals’ external habits depend on the experimental conditions. It is platelike in the case of the flux method1,2 while the TSFZ and the SRL-CP techniques allow the growth of large, long, rod-shaped crystals.4,5 In a previous work, the results of the growth of Y-123 crystals with unusual habits were presented.6 During an experiment performed to test the effects of the * 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.

presence of the liquid phase, derived from the perictetic decomposition of YBa2Cu3O7-δ, on the sintering of a Y2BaCuO5 (Y-211) bar, many needle-shaped crystals underneath the bar were obtained. The needles were formed by a core and an external layer of a different phase. Then, this growth technique was extended to other systems with different RE (RE ) Pr, Nd, Sm, Eu) as described herein.7 Concerning the growth of needles in the Nd-BaCu-O system, to our knowledge, only a report by Klemenz and Scheel8 on Nd-123 needles grown from the BaO/CuO high-temperature flux in Nd2O3 crucibles has been published so far. In this work, we focus our attention on the structural characterization of the needlelike crystals obtained in this system. Experimental Section Ceramic materials of nominal composition NdBa2Cu3Ox (Nd123), Nd4Ba2Cu2O10 (Nd-422), and Nd6Ba9.78Sr0.22Cu2Al6O30 (see below) were obtained by the solid state reaction, with a preparation method well-standardized in our laboratory,9 mixing Nd2O3 (purity 99.99%), CuO (purity 99+), Al2O3 (purity 99.9%) all, and BaO2 (purity 95%) in stoichiometric amounts and processing in flowing oxygen at 1000 °C for 48 h. Crystal growth was performed in the Nd-Ba-Cu-O system on unusually arranged samples. A Nd-422 bar was supported on two Nd-123 pellets and put in an alumina crucible. Then, a thermal program was applied, which involves partially melting the system by peritectic decomposition and slowly cooling it to room temperature. The Nd-422 bar, placed on two Nd-123 pellets, was heated at 200 °C/h to a temperature of TMAX ) 1250 °C in flowing oxygen. This temperature was held for 1/2 h, and then, the sample was rapidly cooled to 1030 °C, cooled to 1010 °C in 20 h, and finally to room temperature at 60 °C/h. For a detailed description of the crystals’ growth process, see ref 7. The crystals were subsequently investigated by optical microscopy (Reichert microscope equipped with Zeiss lens),

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Table 1. Crystal Data Collection and Refinement formula formula weight color habit dimensions space group Z value µ (Mo KR) temperature radiation scan type max 2θ sphere of data total no. of reflections no. of unique reflections Rint cell parameter a (Å) cell parameter c (Å) R(F) wR(F2) goodness of fit

Nd6Ba9.78Sr0.22Cu2Al6O30 2997.72 brown prismatic 20 × 20 × 160 µm3 P63mc 1 23.31 mm-1 293 K 0.71073 (Mo KR) ω-φ 105.14° -25 e h e 25 -25 e k e 25 -14 e l e 13 53805 3131 0.0557 11.526(2) 6.963(1) 0.0405 all data 0.0882 1.269

scanning electron microscopy (SEM; Oxford Stereoscan 440 microscope), and single-crystal X-ray diffraction. For single-crystal X-ray diffraction, a single brown crystal of prismatic habit (20 × 20 × 160 µm3) was mounted on a Bruker SMART CCD area detector diffractometer (graphitemonochromatized Mo KR radiation, λ ) 0.71073 Å). A data collecting strategy was adopted as follows: (i) three sets (606 frames each), 30 s/frame, detector arm at 2θ ) 32° (φ ) 0, 120, and 240°); (ii) six sets (606 frames each), 30 s/frame, detector arm at 2θ ) 75° (φ ) 0, 60, 120, 180, 240, and 300°). Table 1 summarizes the experimental details and the refinement. For cell refinement, data reduction, and multiscan absorption correction, the programs SAINT and SADABS by Bruker have been utilized.10 Structure solution (direct methods) and structure refinements were performed by means of the SHELX97 program.11 Scattering factors, including anomalous dispersion, were taken from the International Tables for X-ray Crystallography.12 Further details on the crystal structure investigation can be obtained from the Fachzentrum Karlsruhe (EggensteinLeopoldshafen, Germany) on quoting the depository number CSD 413246. Room temperature X-ray powder diffraction (XRPD) patterns were collected on the ceramic materials between 10 and 100° (2θ range, ∆2θ ) 0.025°) and a counting rate of 10 s/step, with a Philips 1729 diffractometer operating with Ni-filtered Cu KR radiation. Rietveld refinement has been performed using the GSAS software suite13 and its graphical interface EXPGUI.14 The backgrounds have been subtracted using a shifted Chebyshev polynomial. The diffraction peak’s profile has been fitted with a pseudo-Voight profile function. Site occupancies have been taken fixed during the refinement while all isotropic thermal factors have been varied. Micro-Raman spectroscopy (Renishaw System 2000 Raman Imaging microscope) energy dispersion spectroscopy (EDS) has been performed on both needles and ceramic phases.

Results and Discussion After the thermal program described in the Experimental Section, clean needlelike brown crystals are obtained (see Figure 1). They grow around the supporting pellets, directly on the alumina boat, and form a hexagonal section whose average length and width are ∼150 and ∼20-30 µm, respectively. Table 2 shows the cationic composition determined by EDS together with the one derived by single-crystal XRD analysis (see below for details).

Figure 1. SEM images of crystals grown in the Nd-BaCu-O system. The marker is 10 µm. Table 2. Cationic Stoichiometry from EMPA and XRD Experimentsa ion

Nd-422 cationic stoichiometry

EMPA cationic stoichiometry

XRD cationic stoichiometry

Ba Sr Nd Cu Al

2 0 4 2 0

8.8(5) 0.2(1) 6.2(4) 2.0(2) 6.7(11)

9.78 0.22(1) 6 2 6

a The units are atoms per cell. For EMPA results, the data have been normalized fixing the Cu concentration to two ions per cell. The cationic stoichiometry of the ideal Nd-422 phase is also shown.

EDS reveals that the cationic composition of the needles is quite different from the ideal Nd-422 one (see Table 2). The high Al content is due to the direct contact with the alumina crucible. Conversely, strontium is present because it is the main impurity in the BaO2 precursor. Also, the Nd/Ba ratio and the Ba/Cu one are different from the Nd-422 composition. Several single crystals of the new phase have been used for single-crystal X-ray diffraction analysis. The space group is P63mc. The c-axis is parallel to the preferential growth direction of the needles. Most of the analyzed crystals showed a large degree of twinning (about 40-50%), as determined through the method of Jameson, Schnaider, Dubler, and Oswald,15 implemented in the SHELX97 program.11 The twinning plane was always parallel to the (001) plane. This is a typical example of twinning by merohedry, which may occur when the crystal point group has a lower symmetry than the lattice point group. In fact, space group P63mc belongs to the P6mm point group, while the corresponding lattice point group is P6/mmm. Table 1 shows the results of the refinement performed on the untwinned crystal described in the Experimental Section. Further details about atomic coordinates, site occupations, and anisotropic thermal parameters can be found in the Supporting Information. The stoichiometric formula determined by X-ray diffraction, Nd6Ba9.78Sr0.22Cu2Al6O30, is consistent with EMPA results (see Table 2). The greatest discrepancy concerns the barium content (see Table 2); however, the two values agree within 2σ. Table 3 shows some selected interatomic distances. Figure 2 shows an ORTEP plot16 of the local cationic environment together with the cationic connectivity along the c-axis, which is the preferential directional growth. As a general comment, Ba ions are on three nonequivalent sites, while each Nd, Cu, and Al ion occupies

Metastable Crystals in the Nd-Ba-Cu-(Al)-O System Table 3. Selected Interatomic Distances bound

distance (Å)

multiplicity

Ba1-O2 Ba1-O4 Ba1-Cu1 Ba1-O3 Ba2-O3 Ba2-O2 Ba2-O4 Ba2-O4 Ba2-O1 Ba2-O1 Ba3/Sr1-O1 Ba3/Sr1-O1 Nd1-O4 Nd1-O2 Nd1-O3 Nd1-O2 Nd1-O4 Cu1-O2 Cu1-O3 Al1-O1 Al1-O4 Al1-O3

2.734(6) 3.098(5) 3.123(2) 3.124(6) 2.696(7) 2.7868(6) 2.865(5) 2.961(5) 3.034(2) 3.313(9) 2.530(6) 2.691(8) 2.361(4) 2.375(6) 2.511(1) 2.513(2) 2.741(5) 2.031(6) 2.199(7) 1.729(6) 1.763(4) 1.796(8)

×3 ×6 ×1 ×3 ×1 ×2 ×2 ×2 ×2 ×1 ×3 ×3 ×2 ×1 ×2 ×1 ×2 ×3 ×3 ×1 ×2 ×1

one site. Ba1 (Figure 2, left) is surrounded by 12 oxygen ions, forming a local distorted cubic-closed-packed environment. The site symmetry is C3v and the Ba-O bond distances vary from 2.734 to 3.124 Å. Ba2 (Figure 2, middle) is on an m site and is surrounded by 10 oxygen ions; the Ba2-O bond distances are in the range of 2.696-3.313 Å. Ba3 (Figure 2, right) is in a distorted octahedral environment, of C3v symmetry. Six O1 oxygen ions surround Ba3 with two different values of Ba3-O1 bond distances: 2.530 and 2.691 Å. Nd1 (not shown in the figure) is on an m site; it is connected to eight oxygen ions with Nd-O distances varying from 2.361 to 2.741 Å. The Cu1 site (Figure 2, left) has C3v symmetry with three Cu1-O2 bonds (dCu1-O2 ) 2.0319 Å) and three Cu1-O3 bonds (dCu1-O2 ) 2.199 Å). The copper is in a distorted octahedral environment, and the O-Cu-O angles are between 87 and 96.8°. Aluminum (Figure 2, middle) is in a distorted tetrahedral environment with Al-O distances varying from 1.729 to 1.796 Å. The O-Al-O angles are in the range of 101-116°. For what concerns the cationic connectivity along the c-axis, Ba1 and Cu1 form infinite chains and the Ba1Ba1 distance along the c-axis corresponds to one reticular distance. The same behavior is found for Ba2 and Al1. Even Ba3 forms infinite chains along the same axis; in this case, the Ba3-Ba3 distance corresponds to half the c-axis (see Figure 2). It is worth noting that except for Ba3-O, all of the cation oxygen distances are consistent with the ones calculated by Shannon and Prewitt.17 With regard to Ba3, the sum of Ba2+(VI) and O2-(VI) ionic radii gives an ideal Ba-O distance of 2.76 Å while the mean Ba-O distance is 2.61 Å. Moreover, the thermal ellipsoids of O1 ions, which are the nearest neighbors of Ba3 ions, are asymmetrically elongated in the c direction (see Figure 2, right). These structural anomalies can be easily related to strontium substitution on the Ba3 site. In fact, the structural refinement showed that all of the sites were fully occupied, except for the Ba3 site, where a lack of electrons was found. As the main impurity, deter-

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mined by EMPA analysis, is the element strontium, in the last refinement, both Ba and Sr ions have been positioned on the Ba3 site and the fractional occupational factor has been allowed to vary. The refinement revealed that 11% of Ba3 sites are occupied by Sr ions. Strontium doping should introduce static disorder, as the barium-oxygen and the strontium-oxygen equilibrium distances should be different (the ionic radius of Sr2+ is ∼0.2 Å shorter than Ba2+ 17). This static disorder is mainly directed along the c-axis and is simulated during the refinement as an anomalous elongation of O1 thermal ellipsoids along the same direction. The crystal structure of the needles is quite different from the one determined by Stalik18 for Nd-422: a new phase has been produced. In fact, in the Nd-Ba-Cu-O ternary phase diagram, only two ternary oxides, Nd123 and Nd-422, are described.19,20 Both phases are the ideal end of wide solid solutions. In particular, Nd-422 forms solid solutions composed of Nd4-2xBa2+2xCu2-xO10-2x whose space group is P4/mbm.18 The only cited phase of the Nd-Ba-Cu-Al-O quaternary phase diagram is NdBa2Cu2.7Al0.3O6.5,21 which is derived from Nd-123 by substituting copper with aluminum. This new structure could be either a new phase of the Nd-Ba-Cu-Al-O quaternary phase diagram or a metastable phase obtained through the unconventional method adopted for the crystal growth. Thus, ceramic materials of Nd4Ba2Cu2O10 and Nd6Ba9.78Sr0.22Cu2Al6O30 (hereafter, Al-doped Nd-422) composition have been prepared and analyzed through XRPD analysis. In Figure 3, the detail of the experimental XRPD patterns relative to Nd-422 (top) and to Al-doped Nd-422 (middle) materials together with the pattern calculated from the needle structure are shown. This last pattern has been obtained through the program PowderCell.22 The peak shape and the scale factor have been arbitrarily chosen. Numbers into brackets upon peaks are the Miller indices relative to the Nd-422 phase.18 For what concern the material of Nd-422 sample only, the Nd-422 tetragonal phase is apparent.18 In the other case also, some peaks relative to minority impurity phases are present, indicating that the nominal composition is slightly outside the solid solution limits of the Nd-422 phase. However, the two patterns are quite different from the one calculated from the structural model obtained for the needle (bottom). The diffraction pattern relative to the Nd4Ba2Cu2O10 sample is reported in Figure 4 as an example together with the calculated pattern obtained through Rietveld analysis. The experimental (crosses) and calculated (continuous line) X-ray patterns are shown in the figure together with the difference profile (bottom). The structural model proposed by Stalik for Nd-42217 wellinterprets our experimental data (wRp ) 0.0864, Rp ) 0.0667, and GoF ) 1.191). The same stands even for the Al-doped sample; however, in this case, the worst statistical parameters have been obtained (wRp ) 0.1072, Rp ) 0.0770, and GoF ) 0.574) due to the polyphasic nature of the sample.

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Figure 2. Cationic connectivity along the c direction shown with 80% probability displacement ellipsoids. Left, Ba1-Cu1 chains; middle, Ba2-Al1 chains; right, Ba3-Ba3 chains. The width of the c-axis is also shown.

Figure 4. Observed (crosses) and calculated (continuous line) profile of the Nd-422 sample. In the same figure, the residuals are also shown (bottom). Figure 3. Room temperature XRPD patterns. Experimental result for Nd4Ba2Cu2O10 (top), experimental result for Nd6Ba9.78Sr0.22Cu2Al6O30 (middle), and calculated pattern obtained from needle structure (bottom) through the program PowderCell.22

The XRPD results are confirmed by micro-Raman analysis, which is shown in Figure 5. The Raman spectrum of the Al-doped Nd-422 polycrystalline sample shows the vibrational modes of Nd-422 and Al2O3. The modes relative to this latter phase are not shown in Figure 5 as they are apparent at higher frequencies. Conversely, the modes determined by micro-Raman analysis for the needles belong neither to the Nd-422 phase (see Figure 5) nor to Nd-123.23 However, it must be recalled that not all of the samples have been prepared in the same thermodynamic conditions [T and P(O2)] and this could slightly affect the Raman modes. In summary, an unconventional technique was used to grow needlelike brown crystals in the Nd-Ba-Cu(Al)-O system. A new phase has been obtained (hexagonal system, space group P63mc). When ceramic samples with the same composition of the needles are produced, the Nd-422 phase is obtained, together with some minority impurity phases, indicating that the

Figure 5. Raman spectra of the Nd needles (top), Nd-422 (middle), and Nd6Ba10Cu2Al6Ox powder (bottom).

composition of the needles is slightly outside the solid solution limits of the Nd-422 solid solution. This means that the new phase produced is not a stable phase in the Nd-Ba-Cu-Al-O quaternary system but a metastable polymorphic modification of the Nd-422 solid solution.

Metastable Crystals in the Nd-Ba-Cu-(Al)-O System

Acknowledgment. We thank Dr. A. Ubaldini, Dr. M. M. Carnasciali, and Dr. R. Bianchi for useful discussions and CNR-ISTM, Milano, namely, Dr. Tullio Pilati, for the support during XRD data collection and analysis. Supporting Information Available: Further details about atomic coordinates, site occupations, and anisotropic thermal parameters in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.

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