Abstract:
Disclosed are a composition and a method of preparing a composition of the formula R 2  Q 4  Cu 7  O x , wherein R is a rare-earth ion or a mixture of rare earth ions, and Q is Ba, or Ba combined with either Sr or Ca or both Sr and Ca. Such compositions have a unique crystalline structure and are superconducting oxides.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to superconducting 2-4-7 oxides and their preparation. 
     Superconductors are materials which below a certain critical temperature T c  have zero resistance to the flow of electrons. Materials such as Ba x  La z-x  CuO 4  and Sr x  La z-x  CuO 4  were reported to have T c  ˜40° K., Chu, C. W. et al., 58 Phys. Rev. Lett. 405 (1987). These compounds have the K 2  NiF 4  -type structure in which copper ions are located within pseudosquare planar arrangements of the oxygen ligands. The nominal copper oxidation state for this structure is between 2+ and 3+. 
     Another superconducting material, YBa 2  Cu 3  O x , the so-called &#34;1-2-3&#34; compound, has a T c  ˜90° K.; Wu, M. K. et al., 58 Phys. Rev. Lett. 908 (1987). X-ray and neutron diffraction analyses revealed that this material essentially has a perovskite structure in which yttrium and barium ions are ordered in the c-axis direction and distinctive oxygen defects cause an orthorhombic unit cell with a space group Pmmm, see, e.g., Seigrist, S. et al., 35 Phys. Rev. B. 7137 (1987). This structure has two principal groups of Cu ions. In one group, denoted crystallographically as Cu II, the Cu ions are located within square planar or square pyramid oxygen arrangements which generate infinite [CuO 2  ] planes extending perpendicular to the c-axis on both sides of the yttrium ions. The other Cu ions (Cu I) are located in square oxygen planes, which form [CuO 3  ] chains in the b-axis direction. The superconducting properties of this compound are influenced sensitively by oxygen stoichiometry. The general consensus is that other defects, particularly twins and grain boundaries, play an important role in its electrical and mechanical properties. 
     Recently, new compounds were discovered in the Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O systems, Maeda, H. et al., 27 Jap. J. Appl. Phys. Lett. (in press) (1988), Sheng, Z. Z. et al, and Hermann, 332 Nature 138 (1988). T c  in these compounds was about 125° K. Structural analyses of these materials revealed several structures and compounds. The difference in T c  between the various compounds can be related to the number of infinite [CuO 2  ] planes perpendicular to the c-axis. The common features between the 1-2-3 phase and these new superconducting oxides indicates that infinite [CuO 2  ] planes and an intermediate copper oxidation state between 2+ and 3+ are mainly responsible for the superconducting properties. These ideas are also consistent with the result for the 1-2-3 compound presented by Xiao et al. in 322 Nature 238 (1988). 
     A derivative structure of the normal 1-2-3 structure has also been identified. This compound, named the &#34;2-4-8&#34; structure, has a T c  ˜80° K., as reported by Marshall et al., Abstracts of Mater. Res. Soc., Spring 1988 Meeting 197 (1988) in thin films having the composition Y 2  Ba 4  Cu 8  O x  and by Kogure et al., 156 Physica C 35 (1988) in oxidized metallic precursors having the composition Yb 2  Ba 4  Cu 8  O x . This structure can be described as having periodic insertions of CuO layers in the normal 1-2-3 structure, which causes double [Cu 2  O 4  ] chains in the b-axis direction. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, novel superconducting compositions and their method of preparation are provided. As embodied and described herein, the invention comprises a superconducting oxide phase having the general formula R 2  Q 4  Cu 7  O x , where R is a rare-earth ion such as Y, La, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu, and Q is at least partly Ba, the balance of which may be Sr or Ca, or both Sr and Ca. This is generally referred to as a &#34;2-4-7&#34; composition according to the general stoichiometric relationship between R, Q and Cu in the novel structure. In a preferred embodiment, R is Yb and Q is Ba, and oxygen ions are present in an amount such that x=15-β where β is in the general range of 0 to about 1.5. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1a and 1b are x-ray diffraction patterns of crystals prepared according to the preferred embodiment below; 
     FIGS. 2a and 2b are high resolution electron microscope (HREM) images of Yb 2  Ba 4  Cu 7  O x , and FIG. 2c is a HREM of a YbBa 2  Cu 3  O x  structure; 
     FIGS. 3a, 3b, and 3c are schematic structural models of 1-2-3, 2-4-8, and 2-4-7 structures, respectively; 
     FIG. 4 is an electron diffraction pattern of the 2-4-7 structure; 
     FIGS. 5a and 5b are, respectively, calculated and measured x-ray powder diffraction patterns for the 2-4-7 structure; and 
     FIG. 6 is a reflection optical micrograph of The specimen prepared according to the preferred embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The preferred embodiment of the invention is described in the following example. 
     Methods used to fabricate melt-spun metal ribbons, known as metallic precursors, are known to those skilled in the art, see, e.g., Hazen, R. M. et al., 60 Phys. Rev. Left. 1657 (1988). Accordingly, a melt-spun ribbon was formed from a starting alloy having the approximate composition YbBa 2  Cu 3  to which 33 weight percent silver was added. The melt-spun ribbons were oxidized in a furnace with 1 atmosphere of pure oxygen. The ribbons were heated at a rate of 10°-30° C. per minute to 500° C., where they were held for 10 hours. Heating was then resumed at the same rate and continued to 900° C. The ribbons were held at 900° C. for 100 hours, and were were then cooled to ambient temperature at the same rate as they were heated, again holding the temperature at 500° C. for 10 hours. 
     The oxidized ribbons, typically about 10 mm×1 mm×20 μm in size, were ground to a powder and analyzed by x-ray powder diffraction using monochromated CuK.sub.α  radiation. TEM specimens were also prepared by crushing pieces of the ribbons in an agate mortar and pestle, suspending the powder in carbon tetrachloride, and placing the suspension on a holey carbon film supported by a copper grid. The specimens were observed on the same day they were prepared using a JEOL 200 CX transmission electron microscope (200 kV, Cs=2.9 mm) with a LaB 6  filament and a side entry double-tilting sample holder. 
     X-ray diffraction patterns from the samples showed peaks characteristic of Ag, BaCuO 2 , Yb 2  BaCuO 5  (2-1-1), and CuO. The patterns also showed unidentified peaks. The unidentified peaks resembled those of the 1-2-3 phase in the higher angle range, suggesting a phase with a structure analogous to the 1-2-3 phase. 
     Analysis via electron microscopy revealed that crystals having the x-ray diffraction pattern shown in FIG. 1a and FIG. 1b were common in the specimen. These patterns were easily identified by the high density of reciprocal lattice points on one axis corresponding to a periodicity of about 25.2 Å. FIG. 1a is the pattern of the crystal from the [100] direction and is face-centered, indicating a c-axis periodicity of 2×25.2 Å, or 50.4 Å. FIG. 1b shows the pattern from the [010] direction. FIG. 1a and FIG. 1b indicate that the periods along the a- and b-axes are about 3.8 Å. 
     FIG. 2a and FIG. 2b show high resolution electron microscope (HREM) images of Yb 2  Ba 4  Cu 7  O x  present in the sample from the [100] and [010] beam directions, respectively, under the diffracting conditions of FIG. 1a and FIG. 1b. In these images, the darker dots correspond to heavy metal atoms such as Yb and Ba in the thin area. 
     FIG. 2c is a HREM of a YbBa 2  Cu 3  O x  (1-2-3) structure from the [100]/[010] direction. Compared to the 1-2-3 structure, the new structure of FIG. 2a and FIG. 2b has extra CuO layers between groups of two 1-2-3 unit cells at z=0. This is readily apparent as indicated by wider white bands and the shifting of the atoms indicated by dots by the distance b/2 in every group of two 1-2-3 unit cells. Compared to a 2-4-8 structure (not shown), an extra CuO layer is inserted in every 1-2-3 unit cell. 
     FIGS. 3a, 3b, and 3c illustrate schematically 1-2-3 and 2-4-8 structures, and The novel 2-4-7 structure, respectively. As an extra CuO layer is present in each group of two normal 1-2-3 unit cells, the chemical composition of this structure was determined to be Yb 2  Ba 4  Cu 7  O x , or &#34;2-4-7.&#34; This unit cell, shown by bold lines 6 in FIG. 3c, contains two chemical units and is orthorhombic with space group Areram. This is the same space group as a 2-4-8 structure whose c-axis has been lengthened to about 50.4 Å. The contrast at the single and double CuO layers in FIG. 2a and 2b, compared to image simulations (see, e.g., Spence, J. C. H., Experimental High--Resolution Electron Microscopy, Clarendon Press, Oxford (1981)), suggests that the CuO layers maintain the same configuration as in the normal 1-2-3 structure. In the 1-2-3 structure, oxygen mainly occupies (0,  1/2, 0) sites, FIG. 3a. X-ray diffraction measurements, discussed below, confirm this and also indicate that a- and b-axis cell lengths in The 2-4-7 and 1-2-3 phases are identical. In light of these results, the number of oxygens in the novel structure can be denoted Yb 2  Ba 4  Cu 7  O 15- β where β is in the range from 0 to about 1.5. No intergrowth of the 1-2-3 or 2-4-8 structures in 2-4-7 grains have been observed as primary transformation products. This is also consistent with the very sharp, unstreaked diffraction patterns shown in FIG. 1. 
     It is important to note that the structural illustrations of FIG. 3a, 3b, and 3c are ideal and are not maintained exactly as shown throughout actual crystals. The actual crystal lattice may be twisted or tilted, bonding angles may be variable and ions may not be centered exactly as depicted. As well, localized atomic substitutions may occur. Nevertheless, these figures are representative of the crystals&#39; structure. The present invention of the 2-4-7 compound is therefore not restricted to an embodiment with the precise, non-varying structure illustrated in FIG. 3c and claimed below. 
     FIG. 4 is an electron diffraction pattern of the prepared specimen&#39;s novel 2-4-7 structure with the beam parallel to the [001] direction. On the hk0 reciprocal plane, the extinction rule is followed and k is even. However, because of the very long c-axis, the diffraction spots on the first and second order Laue zones also appear in this view, complicating the pattern. In the normal 1-2-3 structure, the {110} mirror twins cause the splitting of high order diffraction spots on hk0 planes and the slight difference between the a and b lengths. In the novel 2-4-7 structure, FIG. 4, the absence of such splitting indicates the absence of {110} twins. 
     Using the structure model of the novel 2-4-7 phase, FIG. 3c, the x-ray diffraction pattern for the structure was calculated, FIG. 5a. The atomic coordinates and temperature factors in the calculation were derived based on the results of David et al., above, for YBa 2  Cu 3  O 7  and on the assumption that the increase in the length of the cell&#39;s c-axis caused by an excess CuO layer is 1.9 Å. The peak profile is assumed to be a Gaussian function with a half-width of 0.3 degree. The calculated pattern was compared to the measured x-ray powder diffraction pattern, FIG. 5b. The calculated peaks of FIG. 5a were easily identified in the observed diffraction pattern as indicated by arrows 14 in FIG. 5b. Of special note was the splitting of the main peak and the relatively strong peaks to either side of this main peak. Although a few 1-2-3 grains were identified during electron microscopy, the volume fraction of the 1-2-3 phase was less than about 5% of the total oxide phase based upon careful observation under the electron microscope. 
     The refined cell parameters of the novel structure using the 2-4-7 peaks observed in FIG. 5b are a=3.809 (5) Å, b=3.861 (9) Å and c=50.45 (7) Å. The a- and b-axis lengths of normal YbBa 2  Cu 3  O x  in other oxidized metallic precursors (a=3.798 (1) Å; b=3.870 (2) Å) are almost the same as the a- and b-axis lengths in the newly discovered phase. The increase (c=11.660 (3) Å) in the c-axis direction attributed to the additional CuO layer was an estimated 1.905 Å. 
     In the prepared sample, the volume fractions of BaCuO 2 , Yb 2  BaCuO 5  (2-1-1), CuO, and 2-4-7, excluding silver, were estimated as 45%, 20%, 6%, and 29%, respectively. Other samples have been prepared in which the 2-4-7 phase comprised more then 70% of the sample&#39;s volume. 
     One of the key characteristics of a superconducting material is its superconducting critical temperature T c . Analysis showed that the novel 2-4-7 phase exhibited an onset of superconductivity at T c  ˜86° K., with zero resistance at T c  ˜82° K. These results differ from those of other oxidized metal precursors containing high volume fractions of the normal 1-2-3 phase. These other materials show T c  (onset) at about 90°-93° K., with T c  (R=0) at 87°-90° K. As the volume fractions of other superconducting phases was too small to contribute to the sample&#39;s measured T c , the data therefore show that the 2-4-7 phase is responsible for the superconductivity in the specimen prepared in the preferred embodiment. 
     FIG. 6 is a reflection optical micrograph of a cross-section of the specimen. The elongated grains are 2-4-7 crystals whose plate-like shapes are about 10-20 μm long and 1-3 μm thick (the c-axis as the &#34;thickness&#34; dimension). 
     FIG. 6 shows random orientation and intersection of the grains of the 2-4-7 crystal. The intersecting grains create superconducting paths in the ribbons. As a result, the current-carrying capacity of the ribbon is very high. In the samples produced, J c  was greater than 1100 A/cm 2  at 4.4° K. This current carrying mechanism is advantageous as it may eliminate the need to texture the orientation of the oxide grains in superconducting products so that the grains are oriented in the direction of preferred current flow. 
     A superconducting article might include the oxide phase in a matrix together with another metal or metals. Such metals could include the noble metals such as gold, silver, platinum, palladium, or iridium, or copper or aluminum or their alloys, or stainless steel. The selection of the matrix metal would depend upon the particular use for the conductor. 
     Additional features of the specimen prepared in the preferred embodiment include the presence of line defects along beam direction [010]. These defects, indicated by arrows in FIG. 2b, are present at almost every CuO double layer in the relatively thin crystal areas. The faint contrast of some of these features indicated the probable short length of the defects. 
     In the above embodiment, the starting alloy from which the melt-spun ribbon was formed, before oxidation, had the approximate composition YbBa 2  Cu 3  with additional silver. Of the oxide phases produced, this yielded about 29% of the novel 2-4-7 structure. By using a Yb 2  Ba 4  Cu 7  O 15-x  precursor having more Cu, such as the metallic precursor Yb 2  Ba 4  Cu 7 , it is expected that a more Cu rich structure will result upon oxidation, yielding a higher percentage of the 2-4-7 superconductor. 
     It is expected that other elements may be substituted for Yb and Ba in the novel 2-4-7 structure to yield other superconducting compositions. For example, other rare earths such as Y, La, Eu, Gd, Tb, Dy, Ho, Er, Tm and Lu could appear either individually or in combination in the lattice in the sites where Yb was indicated as present in FIG. 3c. Similarly, Ba sites would be partially replaceable or substitutable with Sr and/or Ca; some Ba would be expected to remain in either case. These substitutions would be easily achieved by beginning with a suitable precursor composition that upon oxidation would yield the novel 2-4-7 superconducting composition. 
     The invention is not limited to the above embodiment, as other embodiments are included within the scope of the claims and their equivalents.