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f Overview of High-Temperature Superconductivity: Theory,Q Surfaces, Interfaces and Bulk Systems

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ID. Sahu, A. Langner, Thomas F. George, J. H. Weaver, H. M. Meyer III,0 David L. Nelson and Aaron Wold

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Physical Chemistry of High-TemDerature SuperconductorsEdited by D. L. Nelson and T. F. GeorgeAmerican Chemical Society Symposium Series

Departments of Chemistry and PhysicsState University of New York at BuffaloBuffalo, New York 14260

June 1988

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An overview of the theoretical and experimental aspects of the recentlydiscoveredsuperconducting compounds is presented. This overview is divided into three sections. Inthe first section a review of some of the theoretical and computational works is presentedunder the subsections entitled pairing mechanisms, electronic structure calculations andthermophysical properties. In the second section surface and interface chemistry issuesrelated to the fabrication and use of high-temperature superconductors for high-performanceapplications are presented. Specific issues that are discussed include metallization andthe formation of stable ohmic contacts, and chemically-stable overlayers that are suitable 6for passivation, protection and encapsulation of superconducting material structures thatcan then be used under a wide range of environmental conditions. Lastly, issues arediscussed that are related to each of the bulk high-temperature superconducting ceramicoxides which have received so much attention the past two years. These include %T BCa2 Cu 0 with a critical temperature of 125 K, which is the current record..

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Physical Chemistry of High-Temperature Superconductors

Edited by D. L. Nelson and T. F. George

American Chemical Society Symposium Series

Overview of High-Temperature Superconductivity: Theory, Surfaces,Interfaces and Bulk Systems

D. Sahu, A. Langner and Thomas F. George

Departments of Chemistry and Physics239 Fronczak Hall

State University of New York at BuffaloBuffalo, New York 14260

J. H. Weaver and H. M. Meyer III

Department of Chemical Engineering and Materials ScienceUniversity of Minnesota

Minneapolis, Minnesota 55455

David L. Nelson

Chemistry DivisionOffice of Naval Research

Arlington, Virginia 22217-5000

Aaron Wold

Department of Chemistry, Brown UniversityProvidence, Rhode Island 02912

An overview of the theoretical and experimentalaspects of the recently-discovered superconductingcompounds is presented. This overview is divided intothree sections. In the first section a review of someof the theoretical and computational works ispresented under the subsections entitled pairingmechanisms, electronic structure calculations andthermophysical properties. In the second sectionsurface and interface chemistry issues related to thefabrication and use of high-temperaturesuperconductors for high-performance applications arepresented. Specific issues that are discussed includemetallization and the formation of stable ohmiccon.acts, and chemically-stable overlayers that aresuitable for passivation, protection and encapsulationof superconducting material structures that can thenbe used under a wide range of environmentalconditions. Lastly, issues are discussed that arerelated to each of the bulk high-cemperaturesuperconducting ceramic oxides which have received somuch attention the past two years. These includeT1Ba 2 Ca2Cu3010 with a critical temperature of 125 K,which is-the current record.

F |

Theor

Since the discovery of high-temperature superconductivity in the CuOceramics in the year 1986, there has been an explosion oftheoretical works relating to these systems. It is not possible, ina short review of theoretical work like this, to examine theessential ideas of even a small fraction of this large body ofliterature. We select, quite arbitrarily, some of the work we areaware of and discuss the essential ideas of these. We apologize, inadvance, to all the research workers whose work we have notreviewed.

This theoretical overview is divided into three sections: (1)pairing mechanism, (2) electronic structure calculations andfinally, (3) thermophysical properties. In the section on pairingmechanism we review the basic microscopic mechanisms that might beresponsible for producing superconducting states. This is afundamental question and the success of any theoretical modeladdressing it should be judged in relation to the experiments itexplains and the phenomena it predicts. The second part, dealingwith electronic structure calculations, is quite important in thatsuch calculations give detailed information about conductionprocesses, density of states and anisotropies in k-space. Finally,in the section on thermophysical properties we review works relatingto phonon dispersion and soft modes, dynamics of tetragonal-to-orthorhombic phase transitions and the temperature and concentrationdependence of the stability of these phases.

(1) Pairing Mechanism:

In the conventional superconductors a pair of elecLrons ofopposite spin and momentum form a bound state which leads to acoherent and highly correlated many-body superconducting state. Theattractive interaction between the electrons of the pair is mediatedby lattice vibrations (phonons) and the electrons overcome theirstrong Coulomb repulsion by staying away from each other in time(retardation). The basic question in the new superconductors is -

What is the pairing mechanism? For a review of some of the earlypairing models proposed for the high-T materials we refer thereader to the work by Rice [1].

One of the leading theories for the high-T superconductors isthe resonating-valence-band (RVB) model proposedcby Anderson [2] andhis coworkers [3]. According to this model, the pairing mechanismis magnetic in origin and not of the conventional BCS type. Thestarting point of the RVB theory is a two-dimensional Hubbard modelat half-filling with strong on-site Coulomb repulsion U and anattractive inter-site hopping energy t. Without oxygen doping(i.e., dopant concentration 6 - 0), the ground state of the abovemodel is expected to be a long-range anti-ferromagnetic (AF) state[4], but Anderson argues that frustration might favor a RVB stateover an AF ground state. The basic idea of the RVB theory is thatstrong electron-electron correlations result in a separation ofcharge degrees of freedom from spin degrees of freedom. At lowdoping (6 0 0) and temperature, the quasi-particle excitations arebelieved to be "holons" (i.e., charge-carrying spinless particles)and "spinons" (i.e., spin-4 chargeless particles).

Superconductivity is due to formation of a condensate consisting ofholon pairs. Since Bose-Einstein condensation is not possible in astrictly two-dimensional system, the interplanar couplings areimportant in giving rise to superconductivity.

The RVB theory has been amplified or modified in several ways[5,6,7], the details of which we won't go into here. Rice and Wang[8] have proposed a model in which superconductivity is due tocondensation of a Rair of bosons which gives rise to quasi-particleexcitation energies which are similar to those of RVB theory, butdifferent from BCS theory. Rice and Wang, however, favor a phononinteraction which mediates the attraction between the boson pair inquestion. Coffey and Cox [9] have given a nice summary of theessential points of the RVB theory in Section II of their paper.

Another theory that has been proposed is the spin-bagmechanism of Schrieffer's group [10]. The starting point of theirtheory is the strong AF spin order in the neighborhood of thesuperconducting transition (T ) and two-dimensional spin-correlations over a length L (-20 0CA) in the neighborhood of theNeel transition (TN >> Tc). Doping creates extra holes in thesystem and these are assumedCto be localized over a length I (<<L).The spin of the hole couples to the spin density of the AFbackground through an exchange interaction. Within a given domain ahole produces an effective potential well or bag in its vicinity inwhich the hole is self-consistently trapped. The pairing is due toan effective attractive interaction between two holes whichovercomes the short range Coulomb repulsion.

The importance of spins in producing a superconducting statehas also been emphasized in a model proposed by Emery [111. He hasproposed that oxygen doping in the 214-material creates holes at theoxygen sites and that there is a narrow band of oxygen holes which NSPECTED

couple strongly to the local spin configuration of the Cu-sites.This strong coupling is responsible for an attractive interactionbetween the oxygen holes.

We mention in passing that other mechanisms such as plasmons[12] and excitons [13] have also been proposed as possible j _"-candidates for being the condensate of the superconducting state.

Recently doubts have been expressed about the importance ofthe magnetic origin of the pairing interaction. The bismuthsuperconductors [14,15] (BaKBiO3) which are closely related instructure to the earlier superconductors and which are free ofcopper provide counter examples (16] to the magnetic origin ofsuperconductivity. Another counter example [161 is the 123-materialin which Cu is substituted 100% by Ag bringing T down to 40 K.This has led some people to propose that local structure might playa role in producing a superconducting state. A recent double-well '.rmodel (17] of oxygen motion indicates trends in this direction; ithas been shown that a strong T-dependent electron-phonon couplingparameter could produce Tc - 100 K.

(2) Electronic Structure Calculations:

The electronic structure [18-25] of the undoped compounds is ofimmense interest in getting a clue to the origin ofsuperconductivity. These parent compounds are La CuO4 (denoted by214), YBa 2 Cu3 07 (denoted by 123) and Bi2Sr2taCu2 08 (denoted by

'r I , ' N I " r* -. - - * '*I

2212). The important question is: Why is the ground state of thesecompounds an antiferromagnetic (AF) insulator? The band structurecalculations, so far performed within the local densityapproximation, , give results which are in contrast to theexperimental situation - they all produce a metallic ground state.The reason for this discrepancy is believed to be the strongelectron correlations in the CuO planes which are not adequatelytaken into account in a band picture. It would be interesting tohave electronic structure calculations which take these correlationsinto account. In this regard, spin-polarized band structurecalculations are expected to yield improved results overconventional band structure calculations. However, the results [23-25] of spin-polarized band structure calculations do not seem to bedefinitive.

We would like to conclude this section by stating some of theimportant conclusions that have emerged from the band structurecalculations. In these calculations [18,19] the importance of thetwo2diiensional nature of the CuO planes was emphasized. The copperd(x -y ) orbitals and the neighboring oxygen p(x,y) orbitalsinteract to produce bonding a and antibonding a* orbitals. Similarresults [20-22] were also obtained for 123- and 2212-compounds.Another important result is that the antibonding band was positionedcloser to the Fermi energy E In the new 2212-compounds, a pair ofslightly filled Bi 6p bands provide additional carriers in the Bi-Oplanes [21]. A remarkable feature in these compounds is the chargeseparation between the two Bi-O planes [22].

(3) Thermophysical Properties:

In this brief discussion of the thermophysical properties of thehigh-T oxide superconductors we restrict ourselves to theoreticalinvestigations of the lattice dynamics of these systems. Inparticular, we review the work on phonon modes and oxygen vacancyordering and their influence on the structural transitions of the214 and 123 superconductors. Knowledge of the phonon' spectrum andits dependence on the oxygen distribution may prove to be of primeimportance in elucidating the mechanism of high-Tsuperconductivity. Several scenerios of how electron-phononinteractions can lead to high transition temperatures have beenproposed including oxygen motion in double wells [17], interlayercoupling [26,27] and coupling to soft quasicyclic modes associatedwith underconsttained nearest-neighbor rearrangements (28].

There have been a limited number of theoretical investigationson phonon frequencies and eigenvectors of the La2 x(BaSr)x CuO4''-X

(29-31] and YBa 2Cu 0O7 g (32-35] superconductors. Unscreened latticedynamical models [30, 3], yielding only the bare phonon frequencies,gave fair agreement with experimentally determined total phonondensity of states, mean square atomic vibrational amplitudes andDebye temperatures. Weber [29] has shown that the effect ofscreening, spectrum renormalization, due to conduction electronsgives rise to large Kohn anomalies near the Brillouin-zone boundaryinvolving oxygen breathing modes. It was pointed out by Chaplot[35] that an additional effect of renormalization is to hybridizethe high-frequency modes, dominated by oxygen, with the medium-

p

frequency, heavier-nuclei modes thus making necessary an effective-mass description for the analysis of the isotope effect. Cohen, etal. (31] employing the potential-induced breathing model were ableto predict the tetragonal-to-orthorhombic phase transition inLa CuO4 as arising from an instability, softening, in the Btilting mode of the 14/mmm tetragonal structure. The onset o 6th itransition has been recently derived from symmetry principles.

As is now well established, YBa 2Cu307 undergoes atetragonal-to-orthorhombic phase transition for 1 > 1> 0 which is aconsequence of oxygen vacancy ordering in the Cu-O basal plane.Several approaches have been undertaken to model this transition anddescribe the phase diagram in (T,6)-space. Most treatments are ofthe 2-D lattice gas type, employing first and second nearest-neighbor interactions for oxygen and vracancies and solved by meanfield techniques [37-45]. Another approach, which has beensuccessful in predicting the microstructure of the multiphaseregion, utilizes the method of concentration waves [46]; here theoxide is treated as an interstitial compound of ordered oxygen atomsand vacancies on a simple lattice [47]. The model of Mattis [48]deserves mention since it specifically accounts for the copper-oxygen bonds and thus enables predictions to be made concerning thedielectric response of the various phases. The picture that emergesfrom these studies is that at high temperatures, T > 750 K, and/orstoichiometries 6 < 0.5 a tetragonal phase exists with randomordering of oxygen and vacancies in the Cu-0 basal plane. Formaterial with 6 - 0 the superconducting orthorhombic phase is stablewith oxygen (atoms) and vacancies forming alternate chains along thecrystal b-direction. At low temperatures and intermediatestoichiometries phase separation occurs with micro regions oftetragonal phase, orthorhombic phase and a second cell-doubledorthorhombic phase [45]. Phase transitions between the tetragonaland orthorhombic phases are of a second-order disorder-order type.

As a final note we mention the work on establishing thetemperature domain in which thermal fluctuations and criticalbehavior dominate. Many of the theories described in this review arebased on mean field techniques which become invalid whenfluctuations are important. Estimates of the critical region,Ginzburg criterion, are in the range = (T -T)/T - 0.1 - 0.7 [49-51]. However, it has been pointed out that ihe breakdown of mean-field behavior is progressive (51]; the ability of mean-field theoryto predict non-universal quantities 6p~efactors, GL-parameters andTc) is lost within a region f - (C ) , Brout criterion (52], whichmay cover most of the superconducting regime.

Surfaces and Interfaces

The full integration of bulk and thin film high-temperaturesuperconductors into existing and new technologies of highcommercial value appears limited by a number of surface andinterface materials issues. Many of these issues can be stated ingeneral terms because they are shared by each type of ceramicsuperconductor (2-1-4, 1-2-3, or 2-1-2-2). Others are more specificto the material under study, e.g., the toxic character of the Ti 2-1-2-2 compound. As new materials with even higher criticaltemperatures are developed, analogous problems will be encountered.

- . -7 P ~ V..-" ~ ? I .

Hence, the knowledge base developed for one class of ceramicmaterial may also apply to others.

First, there are issues related to materials synthesis so thatstructures can be fabricated with predetermined shapes, sizes, andcurrent carrying ability. These range from macroscopic tomicroscopic. Second, there are challenges related to thefabrication of superconducting thin films on a variety ofsubstrates, with Si being an obvious choice from the perspective ofmicroelectronic devices. Third, there are issues related to theformation of stable ohmic contacts, particularly for small samplesand thin films. Fourth, there are problems related to thepassivation, protection or encapsulation of small structures such asfibers or thin films, so that the superconducting oxides can be usedunder a wide range of environments. These and a great many otherissues raise challenging chemical questions. Spectacular progresshas been made in the few months that the new superconductors havebeen in existence, and we can anticipate that rapid progress will bemade in the near future. These efforts that address fundamentalissues will bring us one step closer to realizing breakthroughs intechnologies of high commercial value.

It might be thought that surface and interface issues shouldbe separated from those involved in the synthesis of thesuperconductors themselves. This is certainly not the case,however, because many of the most exciting opportunities for thesematerials will be in high performance applications. In theseapplications, the size of the superconducting elements will becomparable to those of the other components. Scaling down orshrinking the size of a structure exacerbates problems related tointerfacial phenomena. Indeed, the challenges of forming,contacting, and protecting a superconducting strip 1 pm wide and 0.1pm thick point to the intermingling of a wide range of materialsissues.

To date, issues related to bulk synthesis and surface orinterface characterization have also been intimately tied. This canbe seen by noting that the starting point for surface or interfaceresearch is a well-characterized bulk material. Unfortunately forthe surface scientist, early samples did not spring completelycharacterized from the firing furnaces, to paraphrase the springing-forth of Minerva completely armed from the head of Jupiter.Instead, early samples were sintered, were 70-90% dense, andindividual grains were clad with other phases [53,54]. Difficulties "in characterizing these interfaces and identifying their intrinsicproperties are reflected in the literature of the last year for the1-2-3 and 2-1-4 materials, and the last few months for the 2-1-2-2materials. Only recently have bulk samples of sufficient qualitybeen available so that fracturing could provide a clean surface(531. Today, it is relatively routine to find single crystalshaving dimensions of greater than 1 mm, and several small companiesare preparing to sell single crystals as large a 1 cm x I cm so thatfull characterization can be achieved. %

Early studies indicated that polycrystalline, sintered samplesof the 1-2-3 and 2-1-4 materials degraded rapidly upon exposure to arange of environments, including H 0 CO CO 0 and solvents [55-571. More recent work suggests tha? degradation is much slower andthat some of the early problems were related to the intergranular

phases (e.g. carbonates or cuprates). Early work also showed thatexposure of the 1-2-3's to high-intensity ultraviolet and X-rayphoton beams produces substantial changes in the surfaces [58].Again, recent work has indicated that most of the changes arerelated to the presence of second phases and were not intrinsic tothe superconductors. At the same time, studies by Rosenberg andcoworkers [59] pointed out some of the details of photon stimulateddesorption. Electron beams of high current or high energy alsoinduce damage. Exposure of these materials to energetic ions, suchas used in Ar sputtering, leads to surface modification, structuralchanges, and the loss of superconductivity [53J. This damage pointsto the fragile character of these superconductors but also indicatesthat it may be possible to selectively alter portions of a thinfilm, for example writing nonsuperconducting lines on asuperconducting template.

An issue that has arisen repeatedly has been the possiblity ofoxygen loss through the surface at room temperature. To ourknowledge, there is no clear evidence that oxygen is lost understatic vacuum conditions. Instead, the exposure of freshly preparedsurfaces to ultrahigh vacuum leads to the chemisorption of residualgases from the vacuum system. Recent work with single crystals hasprovided evidence for the rearrangement of surface atoms aftercleaving in vacuum [60]. This has been attributed to transgranularfracture and the exposure of energetically unfavorable surfaces.Indeed, such effects can be understood in terms of the highlyaninotropic unit cell, but the restructuring does not necessarilyresult in oxygen loss.

Attempts to form contacts to surfaces or to investigate theelectrical properties of the high-temperature superconductors haveoften been complicated by nonreproducibility. This can be relatedto the chemical processes that occur at these surfaces. As detailedstudies of representative interfaces have shown, there is a strongtendency for reactive metals to leach oxygen from the superconductorto form new metal oxide bonding configurations [53,61]. The resultof this interfacial chemistry is a heterogeneous transition regionbetween the buried superconductor and the metal film. Inparticular, a cross section through an interface based on the 1-2-3,2-1-4 or 2-1-2-2 superconductors would show the superconductor; aregion where oxygen has been removed, where the structure is likelyto be disrupted, and which is not superconducting; a region wherethe metal adatoms have formed electrically-resistive oxides; and themetal overlayer, possibly containing oxygen and dissociatedsuperconductor atoms in solution and at the surface. Such aninterface is shown schematically in Fig. 1. These interfaces aremetastable because thermal processing will enhance oxygen transportto the metal layer and will increase the amount of substratedisruption. Certainly, these interfaces would not form the ohmiccontacts desired in device applications. Lil-:wise, the cladding ofsuperconducting filaments with copper sheaths does not seempropitious since there will be interfacial interactions and theformation of a nonsuperconducting layer. The scale of thisdisrupted region is at least 50 A for room temperature metaldeposition and is likely to be much larger if the interface isprocessed at a higher temperature.

S

Ohmic contacts to these superconductors can be formed with Agand Au overlayers [62]. These metals show minimal tendency to formoxides, and their deposition does not seriously disrupt thesuperconductor. For contacts, then, these metals appear to be thematerials of choice. Two caveats should be noted, however. First,processing may lead to Au clustering, as has been observed for Aulayers on many substrates. Second, if the surface to be metallizedhas been exposed to the air, it will be coated with hydrocarbons,water vapor and the like, and its superconducting properties willmost likely be compromised. Attempts using Ar-ion sputtering to Sremove these contaminants prior to metallization have beensuccessful, but almost certainly at the expense of the structuralintegrity of the surface. It remains to be seen whether the amountof surface damage (and loss of superconductivity) compromises theeffective use of this approach to the fabrication of ohmic contactsfor devices.

The protection of these superconductors is also crucial ifthey are to be integrated with other technologies. Much less isknown about such passivation issues, although attempts are presentlyunder way to develop protective overlayers. Two types of materialsthat have been examined so far and appear promising are metal oxidesand non-metallic insulators [63]. Metal oxides have been formed bythe deposition of metal atoms in an activated oxygen atmosphere toform a metal oxide precursor that does not leach oxygen from thesubstrate. The oxide layer then serves as a diffusion barrieragainst oxygen loss or atomic intermixing. To date, efforts havebeen successful with the oxides of Al and Bi, and it is likely thatother oxides will prove to be effective as passivation layers and/ordiffusion barriers. Those investigated so far are insulators, butfuture work may lead to conducting oxides that can serve as bothcontacts and passivators.

The second type of material that shows promise for passivationand electrical isolation is CaF2 [63]. This large-bandgap ionicinsulator has a high static dielectric constant, can be readilyevaporated from thermal sources in molecular form, and shows no Stendency to modify the superconductor. As such, it may be useful asa dielectric layer in advanced devices.

We also note that both organic and inorganic polymers may beuseful as encapsulants. In fact, fully protective coatings for hightemperture superconductors may need to be developed to meet thespecific demands of large- and small-scale applications. Theseprotective coatings will likely involve a multilayer structure thatwould consist of thin- or thick-films of metals, low dielectricceramics, and possibly organic and inorganic polymeric films. Thesemultifunctional coatings would be effective as a diffusion barrier,would withstand environmental stress and temperature cycling, andwould maintain strong adhesion. Specific opportunities exist forthe synthesis of precursor molecules, such as organometallics, for 5chemical vapor deposition (CVD), or sol/gel approaches for lowtemperature fabrication of these multilayer protective coatings.

Bulk SuDerconducting Ceramic Oxides

Superconducting oxides have been known since 1964, but untilrecently the intermetallic compounds showed higher superconducting

temperatures. In 1975 research scientists at E. I. DuPont de ' e

Nemours [64] discovered superconductivity in the system BaPb I Bi Ox 03with a T Cof 13 K. The structure for the superconduacting .compositions in this system is only slightly distorted from the

ideal cubic perovskite structure. It is generally accepted t at a 'disproport onation of then Bi(IV) occurs, namely, 2Bi(IV)(6s)

1 0

B(III)(6s ) + Bn(V)(6s ) at approximately 30 percent Bi. Sleightfound that the best superconductors were single phases prepared byquenching from a rather restricted sngle-phase region, and hence

these phases are actually metastable materials. At equilibriumconditions, wo phases with different values of x would exist; the

phase with a lower value of x would be metallic and with a highervalue of x would be a semiconductor. It is important to keep inmind that the actual assignment of formal valence states is aconvenient way of electron accounting; the actual states includeappreciable admixing of anion functions. The system BaPb Bi 0c oot o-x thshould be studied further since it contains compositons showing hehighest T for any superconductor not containing a transition

element. c Recently, for example, Cava and Batlogg [14] have shownthat Ba K BiO gave a T of almost 30 K, which is considerably

higher tkan the 13 K reported for BaPb 5Bi 50 .'@LacCuO was reported by oncounting cca [65 to show an

orthorhonbic distortion of the K2NiF4 structure with a - 5.363 A,.}2b - 5.409 A and c - 13.17 A. It was also reported [66,67] that.'LaCuO4 has a variable concentration of anion vacancies which may beresresented as LafCuOth Superconductivity has been reported for

some preparations oi-" a2CUDO4 . However, there appears to be some ;

question as to the stoichiometry of these products since only asmall portion of the material seems to exhibit superconductivity

[68). The extent of the anion vacancies has been recently reexaminedt69t, and the magnitude of this deficiency is less than can be

unambiguously ascertained by direct thermogravimetric analysis whichhas a limit of accuracy in x of 0.01 for the composition La 2 CuOHowever, significant shifts in the Ndel temperatures confirm a smatvariation in anion vacancy concentrations.

In the La A CuO. phases (A - Ca,Sr,Ba) the substitution ofthe alkalineearr cation for the rare earth depresses the

tetragonal-to-orthorhombic transition temperature. The transition

disappears completely at x > 0.2, which is about the composition forwhich superconductivity is no longer observed. Compositions of•La^ A CuO. can also be prepared [70,71] where A is Cd(ll) or ..Pba)l ptowever, these phases are not superconducting, and itappearT that tho he basi .divalent cations are necessary to allow

Cu(I) to coexist with 0 . The existence of Cu() and Cu(aII) inLam bSr uCuO is consistent with the ESR spectra, which shows theabsncemof square planar Cu(II), and the Pauli-paramagnetic behaviorover the temperature range from 77 to 300 K. Since the Pauli-paramagnetic behavior of La Sr CuO is consistent with

delocalized electrons, this would- Aso'Znd Cate a high probabilityfor the existence of Cu(I)-Cu(III) formed as a result ofd lsproportionaton of Cu(II) 72]. Subramanan et al. [73 haverecently substituted both sodm and potassium into La CuOra givingrise to the composition a A0CuO. (A - Na,K). However, o nly thesodium substituted sample esi arite n superconducting behavior.

appers hat he ore asi divlen catonsare ecesarytoIllo

The compound Ba YCu3 07 shows a superconducting transition of- 93 K and crystallizes as a defect perovskite. The unit cell ofBa2YCu 307 is orthorhombic (Pmmm) with a - 3.8198(1) A, b - 3.8849(1)A and c - 11.6762(3) A. The structure may be considered as anoxygen-deficient perovskite with tripled unit cells due to Ba-Yordering along the c-axis. For BaYCu0 , the oxygens occupy 7/9 ofthe anion sites. One third of the copper is in 4-fold coordinationand 2/3 are five-fold coordinated. A reversible structuraltransformation occurs with changing oxygen stoichiometry going fromorthorhombic at x - 7.0 to tetragonal at x - 6.0 [74]. The value x- 7.0 is achieved by annealing in oxygen at 400-5000C, and thiscomposition shows the sharpest superconducting transition. It wasshown by Davison et al. [72] that these materials are readilyattacked by water and carbon dioxide in air to produce carbonates.

Recently Maeda et al. [75] reported that a superconductingtransition of 120 K was obtained in the Bi/Sr/Ca/Cu/0 system. Thestructure was determined for the composition Bi2Sr2CaCu 208 byseveral laboratories [76-78].

In most of the studies reported to date on the Bi/Sr/Ca/Cu/0system measurements were made on single crystals selected frommultiphase products. The group at DuPont selected platy crystalshaving the composition Bi Sr, Ca Cu 0 (0.9 > x > 0.4) whichshowed a T - 95 K. Crystals o 'i ^r 2 9Ycu 0 for x - 0.5 gaveorthorhombic cell constants a - 539;_ , g - 584 A, c - 30.904 A[76]. These cell dimensions are consistent with the results ofother investigators [77,78]. The structure consists of pairs ofCuO sheets interleaved by Ca(Sr), alternating with double bismuth-oxige layers. Sunshine et al. [78] have indicated that the additionof Pb to this system raises the T above 100 K. There are now threegroups of superconducting oxides wIch contain th ?mixed Cu(II)-Cu(III) oxidation states, namly La2 A CuO4 where A - Ba,Sr,Ca;R.Ba Cu 0 where R is almost any lanitani~e; and Bi Sr Ca Cu 2 0 R

2 heng and Herman have recently reported 79xonx a 1jh-temperature superconducting phase in the system Tl/Ba/Ca/Cu/0. Twophases were identified by Hazen et al. [80], namely Tl2Ba2CaCu 209and T12Ba2Ca2Cu 3O1 . Sleight et al. [76,81] have also reported onthe structure of T1BaCaCu 0 as well as Tl Ba CuO6 . In addition,the superconductor Ti Ba 8a8Cu 0 0 has been prepared by the DuPont

2 2310group [82] and shows tne highest T of any known bulksuperconductor, namely - 125 K. c

A series of oxides yith high Tc values has now been studiedfor the type (A 0) 2A Ca 1Cu 0 , where A(III) is Bi or Tl,A(II) is Ba or Sr, and n is te number f Cu-O sheets stacked. Todate, n - 3 is the maximum number of stacked Cu-0 sheets examinedconsecutively. There appears to be a general trend whereby T cincreases as n increases. Unfortunately, these phases involverather complex ordering, crystals of the phases are grown in sealedgold tubes, and excess reactants are always present. The toxicityas well as volatility of thallium, coupled with problems inobtaining reasonable quantities of homogeneous single-phasematerial, presents a challenge to the synthetic chemist. It willalso be interesting to see if these materials are truly more szableover time than the La2 .xAxCUO4 or RBa2Cu307 phases.

W~~~~ ~~~~ P 9,r' %',l~ ~ 'a

Acknowledgments

Support by the Office of Naval Research is gratefully acknowledged.

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t.

lip

N

jjjV

Fig. I Schematic showing passivation of high-Tc superconductorsurface with Ag, Au, and composite materials (left)compared to disruptive reaction for chemically activeoverlayers (right).

l1

.?

Overlayers/Contacts on

Superconductors

Passivation Reaction

metal

metal metal oxide______________extended

interfacesuperconductor disrupted substrate

superconductort

Ag, Au Ti, Fe, Cu, La

Bip 39 Alp03 Al, In, Pd, Ri

CaF. Ge

1111 1 1 , N 11111111111111r Q = 111

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