J. Am. Ceram. Soc., 83 [1] 5–28 (2000) Oxide Superconductors Robert J. Cava* Department of Chemistry and Materials Institute, Princeton University, Princeton, New Jersey 08544 This article briefly reviews ceramic superconductors from differences in electronegativity result in semiconducting or insu- historical and materials perspectives. It describes the factors lating materials, with forbidden energy gaps between the highest that distinguish high-temperature cuprate superconductors occupied orbitals in the solid derived from oxygen electronic states from most electronic ceramics and places them in the context and the lowest unoccupied orbitals derived from the metal elec- of other families of superconducting materials. Finally, it tronic states. The technical applications of electronic ceramics describes some of the scientific issues presently being actively depend very highly on the materials scientist’s manipulations of pursued in the search for the mechanism for high-temperature these energies, the introduction or elimination of defects that alter superconductivity and the directions of research into new the distribution of energy states and/or their occupancy by charge superconducting ceramics in recent years. carriers, and other factors such as the polarizabilities of the component atoms. Not all metal oxides are transparent. Transition-metal oxides I. Introduction have been used since ancient times as pigments because of strong optical-frequency localized electronic absorptions between differ- URING the long history of electronic oxide ceramics before ent excited states of the electrons in the transition-metal d orbitals, 1986, even the most astute observer would have found few D which lead to color. As electronic conductors, however, i.e., as clues predicting the revolution in materials science and physics competitors for copper wire, with few exceptions, they have never that those materials would eventually unleash. Oxide ceramics are been of significant technological interest. Their principal consid- an important part of electronics technologies because of their eration as electrical conductors has come primarily from their electrically insulating, optical, and dielectric properties, but very stability under high temperature, chemically extreme conditions rarely for their ability to conduct electrical current. Insulators such where metals are not expected to be thermodynamically stable, as, as steatite, dielectrics such as SiO and Ta O , piezoelectrics such 2 2 5 for example, is found in electrodes for magneto-hydrodynamic as lead zirconate titanate (PZT), optical materials such as LiNbO , 3 generators. and the transparent semiconductor indium tin oxide (ITO) can be This whole picture was stood up side down in late 1986 with the considered as representative of the large class of metal oxides with discovery of superconductivity at surprisingly high temperatures in real and potential applications in electronics technologies. Al- an electronically conducting oxide based on lanthanum, barium, though such materials are sophisticated and complex when con- copper, and oxygen. Superconductivity is the passage of electrical sidered from any point of view, their technological usefulness in current with zero resistance below a certain critical temperature electronics generally results (with the exception of ITO and a (T ). The best superconductors known at the time, to be described handful of other materials) from their ability to block or highly c below, were metallic compounds, not ceramics, as one might resist the passage of electrical current and, as an important expect as a consequence of the (no longer obvious) general fact consequence in some cases, their transparency at optical wave- that metals are good electrical conductors and oxides are not. Good lengths. superconductors do not have to be good metallic conductors above These important characteristics of most oxides are related to a their superconducting transition temperature, however, and the fundamental chemical difference between the metallic elements and oxygen: the difference in their electronegativities. These fundamental characteristics that result in superconductivity at high temperatures in the copper oxides have little to do with any type of conventional understanding of the electronic properties of materi- als. In fact, more than a decade after their discovery, there continues to be no universally accepted theory for why these H. U. Anderson—contributing editor materials are superconductors. No one working in the field in the early days could possibly have foreseen that this would be the case after so much work and thought had been devoted to the problem. Manuscript No. 189179. Received August 5, 1999; approved October 15, 1999. All reviews on topics that are the subject of active on-going The authors research program in superconducting materials at Princeton University research are necessarily snapshots in time. The same has been true is supported by the U.S. Department of Energy under Grant No. DE-FG02-98- ER45706. for reviews of the status of research in high-temperature (high-Tc) *Member, American Ceramic Society. superconductivity. Many of the reviews of this topic published in Centennial Feature 5 6 Journal of the American Ceramic Society—Cava Vol. 83, No. 1 the past decade are of considerable value, and they are not be The second, and most interesting factor, especially from the duplicated here. The references in those reviews,1–10 especially physicists’ point of view, derives from the electronic configuration Ref. 1, serve to provide the primary sources for much of the of the Cu2ϩ ions that form the basis for the superconducting information summarized in the present article. References to compounds. The electronic configuration for Cu2ϩ is 3d 9: nine of more-recent work or work complementary to what is found in the ten available d-orbital energy states are filled. In the Cu–O those reviews are included here. The occasion of this centennial coordination polyhedra (octahedra, pyramids, and squares) that review series in ceramic materials is used to present a more tutorial form in the cuprate superconductors, these energy levels are and historical perspective of the topic, with strong emphasis on the nondegenerate. The t2g orbitals, which are directed between the structural and chemical aspects of the story. Although widely oxygen atoms, are at low energy and, therefore, are completely viewed as a revolution in condensed matter physics, it is equally filled with electrons (six). The shapes of the Cu–O coordination valid to view the breakthrough of high-Tc superconductivity in polyhedra are such that there are four near in-plane oxygen copper oxides as a revolution in materials science and ceramic neighbors and one (pyramidal) or two (octahedral) apical oxygens materials. The pace of discovery of new materials has slowed from more distant when present. This makes the energies of the orbitals its peak in the late 1980s, making a catalog of the known oxide with “z” components (i.e., toward the apices) lower because of superconductors and their “history” as understood at the turn of the lower repulsion from the oxygen orbitals. The result is that the millenium somewhat longer lived than it would have been a nine-electron configuration has a single unpaired electron in the 2 2 decade ago, and that comprises a significant proportion of this dx – y orbital (Fig. 1b), which points toward the in-plane review. In this context, a set of uniform representations of oxide oxygens. The single unpaired electron carries a spin of 1/2. This superconductors emphasizing their relation to the important per- low spin value allows for the possibility of nonclassical (e.g., ovskite structure type, in terms of the packing of metal–oxygen quantum mechanical) interactions between spins in the solid, polyhedra, is presented. which is different from the more familiar case of the spins in The scientific issues under study at the present time primarily ferrites, for example, where the interactions between the larger 3ϩ involve the precise determination of the details of the electronic spins (e.g., spin 5/2 for Fe in Fe3O4) can be described by more and magnetic characteristics of the oxide superconductors in an conventional physics. attempt to get at the vexing microscopic heart of the problem of In isolated atoms, these orbitals are discrete energy states, but, why superconductivity occurs in the cuprates. The appearance of in solids, where the atoms are close together, the orbitals interact, superconductivity at high temperatures in such oxides has revealed and the sharp atomic energy states become bands of energies. The a glaring hole in our understanding of the way magnetism and similar energy of the O 2p states and the Cu 3d states leads to the situation shown in Fig. 1(c). The electrons fill half of the Cu electrical conductivity are intertwined and related in transition- 2 2 metal-based compounds in general—both an embarrassing and dx – y -derived band, and the high-energy part of the oxygen- wonderful event in materials science and condensed matter phys- derived band has almost the same energy as the highest occupied ics. Magnetism and superconductivity continue to be the “Jekyll copper states. This is more complicated than in the usual electronic oxides, but the complexity continues. This type of picture would and Hyde” of oxide superconductors—they are clearly intimately 2ϩ related and may be different sides of the same personality—but, predict that Cu oxides with