SCIENCE
New Class of Superconductors Pushing Temperatures Higher Rare-earth-free thallium- or bismuth-containing materials are providing a rich garden of superconductors that work at temperatures up to 125 K Ron Dagani, C&EN Washington
Until about four months ago, the term "high-temperature superconductor" meant only one thing: "rareearth-containing copper oxide." But that, like so many things in the superconductivity sweepstakes, is no longer true. In recent months, researchers, working feverishly, have greatly expanded the possibilities by making new superconducting materials that contain no rare-earth elements, and some that don't even have copper. Before this renewed burst of activity, most researchers were concentrating their efforts on the socalled 1-2-3 compounds: yttrium-barium-copper oxide (YBa2Cu3C>7-x) and analogs in which bona fide rare earths such as europium and gadolinium take the place of yttrium. But now the excitement has shifted to bismuth- or thallium-containing copper oxides and copper-free bismuth oxides because they offer interesting new structures and properties. One of the properties of greatest interest is the temperature at which a material becomes superconducting, known as the transition temperature or Tc. Experimentally, Tc is reached when the material loses all resistance to electricity and excludes an externally generated magnetic field. Until this year, the highest confirmed Tcs were in the vicinity of 95 K (for the 1-2-3 compounds). But the discovery of the thallium 24
May 16,1988 C&EN
materials has pushed Tc to a new record—125 K. Further increases are expected. The first sign that a new class of rare-earth-free superconductors was about to emerge came from Bernard Raveau and coworkers at the University of Caen, France. It was that group's published studies on lanthanum-barium-copper oxides that two years ago led IBM's J. Georg Bednorz and K. Alex Muller to their Nobel Prize-winning observation of superconductivity in this system at 30 K. More recently, Raveau's group, like many others, had been scouring the periodic table, looking for other potentially superconductive compositions. One idea they had was that bismuth might be able to substitute for the rare earth lanthanum in these copper oxides because La3+ and Bi 3+ have similar ionic radii. Sure enough, they reported last September that a bismuthstrontium-copper oxide was superconductive at 22 K. At the National Research Institute for Metals in Tsukuba, Japan, Hiroshi Maeda's group added calcium to the recipe and observed superconductivity above liquid nitrogen temperature (77 K). A similar observation was made independently at the University of Houston by superconductivity superstar ChingWu (Paul) Chu and coworkers. But Maeda took the media plunge first, announcing his findings in Japan on Jan. 22. Within days, researchers at other laboratories confirmed Maeda's results and even improved upon them (C&EN, Feb. 1, page 5). On the day of Maeda's announcement, a press conference was held at the University of Arkansas in Fayetteville. There, Allen M. Hermann, chairman of the physics department, and chemist Zhengzhi Sheng unveiled their own coup: a
thallium-barium-copper oxide that was fully superconducting at 81 K. Neither Maeda nor the Arkansas workers were aware of the other group's discovery. "We thought we had the world's first non-rare-earth high-temperature superconductor," Hermann later told C&EN. "It turns out we're tied for the world's first." The Arkansas announcement, unlike the Japanese one, didn't generate international ripples because major news organizations showed no interest in picking it up. But even Hermann and Sheng soon lost interest in their Tl-Ba-Cu-O compound when they discovered another composition that was much easier to make and had a much higher Tc. The key ingredient—again—was calcium. On Feb. 15, Hermann and Sheng were able to announce that a multiphase Tl-Ca-Ba-Cu-O material became fully superconducting at about 107 K. They got plenty of regional press attention, but "virtually zip from anywhere else," Hermann notes. One week later, they disclosed details at a poster session at the 1st World Congress on Superconductivity in Houston. Within a day, the new results were reproduced at other laboratories, Hermann says. "That's when we got the credibility we deserve and that's when people made a big to-do over it." Then on March 3, Stuart S. P. Parkin and coworkers at the IBM Almaden Research Center in San Jose, Calif., disclosed that they had prepared a single-phase Tl-Ca-BaCu-O sample that becomes totally superconducting (zero resistance) at 125 K, setting a new record (C&EN, March 14, page 4). Euphoria over these discoveries has fueled round-the-clock explorations into the chemistry and physics of the new bismuth and thalli-
um compounds. At last month's Materials Research Society meeting in Reno, Nev., Arthur W. Sleight, who leads a high-power research team at Du Pont, joked that his role has been to watch over the shoulders of his coworkers and "make sure nobody'goes home on weekends." Such efforts have paid off handsomely. Researchers say they have uncovered "a very rich garden" of new superconductors, most of them in the thallium system. At least six distinct phases, or chemical compositions, of thallium superconductors have been isolated and identified. The bismuth system so far has
yielded at least two. But researchers have evidence for a number of additional phases as well. Bismuth and thallium phases discovered to date have corresponding stoichiometries. This account will focus on the thallium system. The first thallium compound synthesized by Sheng and Hermann later was identified for them as Tl 2 Ba 2 Cu0 6 by crystallographer Robert M. Hazen and coworkers at the Geophysical Laboratory of the Carnegie Institution of Washington (D.C.) The shorthand name for this phase, 2-0-2-1, can be derived by writing the formula as Tl2CaoBa2-
CuiCV Different groups have determined its zero-resistance Tc to be in the range 80 to 85 K. When the Arkansas researchers added calcium to the recipe, their first samples of Tl-Ca-Ba-Cu-O material were found to contain two superconducting phases. Hazen's group identified these as Tl2CaBa2Cu 2 0 8 (the 2-1-2-2 phase) and T12Ca2Ba2Cu30io (the 2-2-2-3 phase). The former, it turned out, was responsible for the 107 K transition temperature in the original multiphase sample. The 2-2-2-3 material, once it was synthesized separately, showed itself to be fully supercon-
First series of thallium superconductors has double thallium-oxygen sheets
^NJpl^
% % %
TI 2 Ba 2 Cu0 6
Copper
# • • •
Oxygen
U
Thallium Calcium Barium
wNSWfe V %, TI 2 CaBa 2 Cu 2 0 8
Source: Du Pont
TI2Ca2Ba2Cu301o May 16, 1988C&EN
25
Science ducting at temperatures as high as 125 K. The structures of these three superconductors were determined at Du Pont by single-crystal x-ray diffraction. The three structures are similar, but distinct. All have double thallium-oxygen planes. And as the copper subscript in each case suggests, the 2-0-2-1 phase has single copper-oxygen layers, 2-1-2-2 has double Cu-O layers, and the 2-2-2-3 phase—which is unprecedented— has triple Cu-O layers. A single layer of barium ions separates the thallium and copper regions, and calcium ions are interspersed between the copper planes. Researchers couldn't help but notice an interesting trend: As the number of copper layers increases, so does Tc, apparently in a linear fashion. A simpleminded extrapolation of these three data points (one of which is now in dispute) would suggest that a room-temperature superconductor could be achieved by preparing a thallium phase with 10 consecutive copper planes. Scientists now have good reason to believe that this approach won't work. It did, however, furnish some laughs at a recent meeting. Still, it may be possible to reach new Tc milestones with more elaborate structures. According to Hazen, a number of groups have sighted four- and five-layer copper regions in the thallium materials, but no
Raveau: studies that paved the way 26
May 16, 1988 C&EN
one has reported a bulk material with such large copper stacks. In recent weeks, Parkin and associates at IBM Almaden have further enriched the field by producing a set of three new thallium phases. These all have a single Tl-O plane alternating with either one, two, or three Cu-O planes. Of the three, the phase with the most elaborate structure also has the highest Tc— about 110 K. This phase, the 1-2-2-3, also was announced independently by Raveau's group in Caen. Having these six related thallium structures available for study will allow scientists to clarify the role played by the thallium and copper planes, Parkin says. "This is a very important database from which you can develop theories about the origin of high-temperature superconductivity," he tells C&EN. The study of these new phases has already changed scientists' ideas about what structural features are necessary for superconductivity. For example, many theorists had thought that the linear Cu-O chains in the 1-2-3 compounds were more important for superconductivity than the Cu-O planes. Since the bismuth and thallium materials don't have Cu-O chains, this feature obviously cannot be essential for high-temperature superconductivity. Theoretical attention then shifted to the Cu-O planes. Because these planes are buckled or rippled in the 1-2-3 materials and in the bismuth copper oxides, many theorists assumed that this feature was important. But scientists have now found that the copper sheets in at least some of the thallium materials are planar. So much for the theoretical significance of buckled planes! Of course, the even more recent discovery of copperless superconductors suggests that copper sheets aren't necessary for high-temperature superconductivity either. Researchers are finding that the structure and properties of these new superconductors depend very much on the preparative conditions. Hence, they have spent a lot of time trying to find the optimum conditions to make the best samples. Like the conventional preparation of the 1-2-3 materials, the Tl-Ca-BaCu-O compounds can be made by
Tc rises as number of copper layers increases Ti-Ca-Ba-Cu-0
Thallium layers
1-0-2 - 1 b 1-1-2-2 b 1-2-2-3 b
1 1 1
1 2 3
~80, 103 -110
2-0-2 - 1 6 2-1-2-2 8 2 - 2 - 2 - 3 10
2 2 2
1 2 3
~80-85 d 108 125
Bismuth layers
Copper layers
Bi-Ca-Sr-Cu-O
2-0-2 - 1 6 2-1-2-2 8 2 - 2 - 2 - 3 10 e
Copper layers
V(K) C
T c a (K)
-12 -85-95
-115
a Highest value(s) reported for transition temperature at zero resi tance. b Not determined, c Not superconducting, d Measurement dispute; may not be superconducting, e Phase identified but n isolated. Sources: IBM Almaden, Du Pont, other labs
heating a mixture of simple metal oxides: TI2O3 (thallic oxide), CaO or Ca0 2 , BaO or Ba0 2 , and CuO. But complications arise because thallic oxide is unstable at the high temperatures (800 to 900 °C) required to make the superconducting phases, explains materials scientist David S. Ginley of Sandia National Laboratories in Albuquerque. Thallic oxide decomposes to thallous oxide (T120), which melts between 300 and 400 °C. That's both "a blessing and a curse," says Ginley. The blessing is that the melting forms a liquid phase that promotes reaction of
Hermann: tied for world's first
the oxides. The curse is that thallous oxide is volatile and can decompose to volatile — and toxic — thallium metal, which is easily lost. The loss of thallium leads to variations in the stoichiometry of the final products. Researchers have found ways to minimize the loss of thallium. At IBM Almaden, for instance, chemists Victor Y. Lee and Adel I. Nazzal make their samples by first pressing a mixture of the starting oxides into a pellet. The pellet is wrapped in gold foil and sealed inside a quartz tube under an oxygen atmosphere. It is then heated at 880 °C for about three hours. The 1-2-3 materials require a lengthy period of annealing in flowing oxygen to produce the superconducting structure. The thallium materials, on the other hand, don't require oxygen annealing, although the Sandia group has found this step to be beneficial. By optimizing the processing conditions, the IBM researchers have found that they can make a material containing only one superconducting phase, even though it also contains insulating phases such as barium copper oxide, Parkin notes. Another quirk of this high-temperature thallium chemistry, Parkin says, is that the starting composition of the unreacted oxide mixture (the ratio of metal atoms) "doesn't necessarily bear much relationship" to the composition of the resultant superconducting phase. For instance, the IBM workers found that when the starting composition is 2:2:2:3, they get the 2-1-2-2 phase. To make the 125-K 2-2-2-3 phase, they had to start with the 1:3:1:3 composition. A 1:2:1:3 starting composition also yields the 2-2-2-3 phase, Parkin says, although the product's Tc is only about 118 K. After examining the microscopic structure of these samples under high resolution, the IBM researchers concluded that the 125 K superconductor represents a "perfect" 2-2-2-3 structure. The structure of the 118 K sample, on the other hand, contains defects: Here and there a double copper layer is seen instead of the expected triple copper layer. These defects, which Parkin calls "intergrowths," account for the ma-
Bismuth and thallium superconductors have platy makeup Scanning electron micrographs of TI 2 CaBa2Cu 2 0 8 (top) and the corresponding bismuth-strontium analog, Bi 2 CaSr 2 Cu20 8 , (bottom) show the platy morphologies of these two classes of superconductors. The thallium materials are more chunky. The bismuth materials are more flaky, resembling mica, a mineral that is readily separated into thin, transparent leaves. These bulk characteristics are a direct consequence of the layered structure of these materials. Du Pont researchers determined that the distance between adjacent thallium-oxygen sheets is about 2.2 A, whereas the distance between adjacent bismuth-oxygen sheets is about 3.2 A. The greater separation in the bismuth compounds leads to better cleavage, and hence their micalike character.
terial's depressed transition temperature, he explains. Using a range of slightly different starting compositions, Parkin's group prepared a range of 2-2-2-3 samples in which Tc varied continuously from 118 K to 125 K. The more intergrowths a sample had, the lower was its Tc. "That's rather an intriguing result," he says. Other groups that have made samples of the 2-2-2-3 compound report values for the zero-resistance Tc as low as 114 K. The implication is that these samples, too, have intergrowths that lower the Tc from its "optimum" value. The range of transition temperatures that have been reported for other thallium and bismuth phases probably also are due to intergrowths, Parkin believes. At Du Pont, electron microscopy of some samples of the 2-2-2-3 phase has revealed intergrowths of five consecutive Cu-O layers. These sam-
ples have an odd bend in the resistivity curve at 140 K, according to crystallographer Charles C. Torardi. That suggests that five-layer defect regions may have significantly higher Tc than the surrounding material. Furthermore, intergrowths aren't confined to the copper units. According to Parkin, samples of the single-thallium-layer 1-2-2-3 phase, for example, show occasional double-layer Tl-O intergrowths that also cause small variations in the material's Tc. Even single crystals aren't immune from intergrowths*. According to Ginley, Sandia researchers have grown large crystals that consist of layers of different thallium phases stacked on top of one another "in perfect register." The thallium structures, with all their marvelous diversity/ appear to be less complex than the bismuth structures, according to Du Pont's May 16, 1988 C&EN
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Science Torardi (left): studies on defect regions. Sleight: thallium structures less complex than bismuth structures
Sleight. "We see an obvious superstructure [a longer-range ordering] in the bismuth materials/' but not in the t h a l l i u m materials, says Sleight's coworker Torardi. In essence, the Bi-O units in the crystal lattice don't repeat as often as the copper, calcium, and strontium units, he explains. Whether thallium or bismuth superconductors eventually find their way into practical applications will depend in large part on their physical, chemical, and electrical properties. Scientists have only begun to study these properties, but they appear quite promising. For example, the thallium and bismuth materials are more stable in air than the 1-2-3 materials, Sleight says. And single crystals of a Bi-Ca-Sr-Cu-O compound grown at Bell Communications Research in Red Banks, N.J., have shown excellent superconducting properties. To be useful in microelectronics applications, the new materials must be able to carry current densities of 105 to 106 amp per sq cm without developing resistance and hence losing their superconductivity. Such critical current densities, or Jcs, have already been achieved in singlecrystal epitaxial (substrate-oriented) thin films of YBa2Cu3C>7. But because such films can only be deposited on certain substrates, their applications are very restricted, says Paul S. Peercy, who manages semiconductor research at Sandia. Un28
May 16, 1988 C&EN
fortunately, unoriented polycrystalline films of YBa2Cu3C>7 carry virtually no current because the grains in these films are only weakly linked, explains Sandia's Ginley. Ginley and his coworkers thus were understandably excited when they recently made polycrystalline thin films of the thallium 2-1-2-2 phase that carry 110,000 amp per sq cm at 77 K. This critical current density is by far the highest ever measured in an unoriented polycrystalline superconducting oxide film at
Parkin: quirks of thallium chemistry
any temperature. And it's "nearly good enough" for all the microelectronics applications they're interested in, Ginley says. Moreover, preliminary tests indicate that the critical current in these films, as well as in the bulk material, is much less sensitive to magnetic fields than polycrystalline YBa2CU3O7. In the 1-2-3 superconductors, gradual increases in the magnetic field strength lead to a rapid falloff in Jc. But at 77 K, Ginley's group sees only a 20 to 30% fall-off in the Jc of the thallium film at 1 tesla. This indicates that the grains are strongly linked to one another, according to Peercy. Thus, the thallium family of compounds appears to be the best candidate for all highcurrent applications, including those involving high magnetic fields, Ginley adds. Researchers are seeking high transition temperatures in thin films because the h i g h e r the T c , the more current the film can carry at liquid nitrogen temperature. Sandia's 2-1-2-2 films show zero resistance at 97 K. C&EN learned last week that this figure has now been surpassed by IBM Almaden researchers, who have achieved a Tc of 120 K in a thin film of the 2-2-2-3 compound. No further details were available at press time. It's ironic that thallium, which has made these superconductivity gains possible, is also a highly toxic metal. But Ginley, who supervises Sandia's semiconductor materials division, points out that the microelectronics industry is used to handling highly toxic starting materials. So producing thallium-containing thin films on a large scale would not be a big problem, he thinks. Furthermore, thallium components could be made acceptably safe by suitable packaging, he adds. In the research lab, though, chemists who prepare thallium compounds should guard against thallium exposure, Hermann warns. The precautions he urges include working in a hood and using a mask, goggles, rubber gloves, and apron. Some research groups, though, have decided not to investigate the thallium compounds because of the health risk. They are hoping that even better superconductors not
Ginley: polycrystalline thin films containing thallium will come along shortly. Indeed, as Hermann told attendees of last month's International Conference on the First Two Years of High-Temperature Superconductivity in Tuscaloosa, Ala., "I hope we can learn to replace thallium with something that's not so nasty to deal with." But finding new higher-T c superconducting compositions remains a great challenge. "All the obvious compositions have already been tried" many times over by many groups, Hermann tells C&EN. "We're really scratching our heads now." The solution might lie in a totally different chemical system, perhaps a noncopper one. Such an approach gained some credibility about two weeks ago when Robert J. Cava, Bertram Batlogg, and coworkers at AT&T Bell Laboratories announced the discovery of new superconducting oxides not based on copper. These new materials are bariumbismuth oxides containing either potassium or rubidium (C&EN, May 2, page 7). They are based on the parent compound BaBiC>3, with potassium or rubidium replacing some of the bariums. The new bismuth oxides have Tcs near 30 K, the operating temperature of the lanthanum copper oxides that triggered the superconductor revolution two years ago. The Bell Labs group magnetically measured the onset of superconductivi-
ty at 29.8 K in Bao.6Ko.4Bi03. This is the highest Tc value they have reported, and is significantly higher than the 13 K transition temperature achieved more than a decade ago in a closely related barium-leadbismuth oxide (BaPbo.75Bio.25O3). The hope is that scientists can further raise Tc in bismuth oxides by making other elemental substitutions. The synthesis of the bismuth oxides involved first grinding together oxides of the constituent metals (BaO, Bi2C>3, and an excess of KO2 or Rb20). The mixture was heated at 675 °C for three days inside a sealed silver tube. The resulting powder was annealed in flowing oxygen at 475 °C for 45 minutes, quickly cooled, and then pressed into pellets. Annealing for longer periods of time or at higher temperatures caused the loss of potassium from Ba-K-Bi-O compositions, the researchers say. The new bismuth oxides belong to the same mineral class (perovskites) as the copper oxide superconductors. But the bismuth oxides have a three-dimensional arrangement of bismuth and oxygen atoms rather than the layered arrangement found in the copper oxides. This means that properties such as critical current density are likely to be isotropic, or similar in all directions. In the copper oxides, by contrast, current flows much more efficiently along the copper planes than perpendicular to them. Thus, it may be easier to fashion high-performing superconducting wires and films from the bismuth oxides. Theoretical understanding of superconductivity also may benefit from the Bell Labs discovery. The missing copper, Batlogg predicts, "will stimulate some theorists to rethink their models for the underlying mechanism of high-T c superconductivity." Although the spotlight for the past few months has been trained on the new thallium and bismuth materials, investigation of the lanthanum and yttrium superconductors continues. The more researchers learn about these fascinating materials, the more questions they have. As Caen's Raveau recently noted with a great deal of understatement, "There is much to do in this field."D
Chemist wins NSF's Waterman award Peter G. Schultz, associate professor of chemistry at the University of California, Berkeley, recently was selected to receive the National Science Foundation's prestigious Alan T. Waterman Award. The award has been given annually since 1975 to an outstanding young researcher in any field of science, mathematics, or engineering. Schultz, who was selected from 128 nominees, is only the third chemist to win the Waterman award, which includes a medal and NSF grants of up to $500,000 over three years to fund research and advanced studies. Schultz was cited for "innovative research at the interface of chemistry and biology, both in the development of new approaches for the study of molecular recognition and catalysis and in the application of these studies to the design of selective biological catalysts." NSF Director Erich Bloch said of Schultz that "his achievements, in just two short years, have changed the way scientists think about bioorganic and biotechnologic areas and undoubtedly will continue to define the leading edge in this area of science." Of the award, Schultz says that "I am delighted to know that our work is really appreciated by the scientific community. It is also a great recognition for my s t u d e n t s , who started out not long ago setting up this lab from scratch." Because his research group has grown rapidly to more than 20 people, he also notes that the grants that accompany the award will be very useful. "We need to buy a lot more equipment, so the timing of the gift is perfect," he says. One area of Schultz's research singled out by NSF is his continuing investigation of catalytic antibodies. Independently, in 1986, Schultz and a group at the Research Institute of Scripps Clinic reported production of antibodies that catalyze, with a high degree of specificity, hydrolysis of esters and carbonates (C&EN, April 6, 1987, page 30). The antigenic molecules used to elicit these antibodies were phosphonates May 16, 1988 C&EN
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