The recent discovery of two types of metallic copper oxide compounds which are superconducting to above 90/sup 0/K has renewed interest in the search for new high temperature superconducting materials. It is significant that both classes of compounds, La/sub 2-x/Sr/sub x/CuO/sub 4-y/ and YBa/sub 2/Cu/sub 3/O/sub 7-delta/ are intimately related to the extensively studied perovskite family. Both compounds contain highly oxidized, covalently bonded Cu-O sublattices, however, they differ in geometry. In this paper we discuss the relationship of these features to the superconducting properties. 30 refs., 6 figs

Beno, M.

Hinks, D.G.

Jorgensen, J.D.

Schuller, I.K.

Segre, C.U.

Soderholm, L.

Zhang, K.

UNT Digital Library

STRUCTURAL RELATIONSHIPS IN HIGH TEMPERATURE SUPERCONDUCTORSIvan K. S c h u l l e r , C. U. Segre , D. G. Kinks, J . D. Jorgensen, L. Soderholm,M. Beno, and K. ZhangC O N F - 8 7 0 7 1 0 9 — 2DE88 0 0 2 9 2 4September 1987DISCLAIMERThis report was prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither the United States Government nor any agency thereof, nor any of theiremployees, makes any warranty, express or implied, or assumes any legal liability or responsi-bility for the accuracy, completeness, or usefulness of any information, apparatus, product, orprocess disclosed, or represents that its use would not infringe privately owned rights. Refer-ence herein to any specific commercial product, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom-mendation, or favoring by the United States Government or any agency thereof. The viewsand opinions of authors expressed herein do not necessarily state or reflect those of theUnited States Government or any agency thereol.Submitted for Proceedings ofthe Adriatico Research Conference on HighTemperature Superconductors, July 6-8, Trieste, Italy, World ScientificPublishing Co. (1987).**Work supported by the U.S. Department of Energy, BES-Materials and ChemicalSciences, under Contract #W-31-109-ENG-38.Permanent address: Dept. of Physics, Illinois Institute of Technology,Chicago, IL 60616, USA.MASTEROF THIS DOCUMENT IS UNLIMITEDf ha aibmltwd m»nmcripi tin b m luihortdby • contractor of tht U. 5. Govtrnmantundar contract No. W-31-10»ENG-38.Accordingly, th* U. S. Govarnmtnt maim •noMKlinn*. n>yalty-lr»» lictnw to pubtiil)or nproduc* tht pubtiinad form of thisor allow OHMTI to do JO, torStructural Relationship* In High Temperature SuperconductorsIvan K. Schuller, C.U.Segret, D.G. Hinks, J.D. Jorgensen,L Soderholm, M. Beno and K. Zhang*Argonne National Laboratory, Argonne, IL, 60439, USAProceedings of the Adriatic© Research Conference onHigh Temperature SuperconductorsJuly 6-8,1987, Trieste ITALYWorld Scientific Publishing Co. (1987)Ttw wbmitwd menuacript hM bwn Milhot*dby • contractor of tt» U. S> Cowrrtrnvntunhr cwitracf No. W J1 109-EKG 38(Ucertlnaly. |h« U. S. Cu.lirWTwnt M i n i •nommcluint. royalty-frM llcinn to publiih« nproduc* lt>4 publi*»d form o« tMlcontritwfiwi. or lllow o«»™ » do ID. «OfU. S. Gov«*iwiMm puqMMtf.Structural Relationships In High Temperature SuperconductorsIvan K. Schuller, CLLSegre*, D.G. Hlnks, J.D. Jorgensen,L. Soderholm, M. Beno and K. Zhang+Argonne National Laboratory. Argonne, IL, 60439, USAThe recent discovery of two types of metallic copper oxide compounds which aresuperconducting to above 90K has renewed Interest in the search for new hightemperature superconducting materials. It Is significant that both classes of compounds,La2_xSrxCu04 „ and YBagCugO^ are Intimately related to the extensively studiedperovsklte family. Both compounds contain highly oxidized, covalently bonded Cu-Osublattices, however, they differ In geometry. In this paper we discuss the relationshipof these features to the superconducting properties.Superconductivity above 30K was first discovered in La^BajjCuO^y with thel<2NiF4 tetragonal structure1 ~^. Subsequent work revealed that Tc could be raised to37K by replacement of Ba by Sr5!. The undoped La2Cu04 compound, which forms in anorthorhombically distorted variant of the r^NiF4 structure Is non-metallic at lowtemperatures and shows no superconductivity. Recently it has been shown that in thestoichiometric material, the Cu ions order antiferror nagnetfcally6"8!. There are alsoreports that this compound can be made superconducting by special post-synthesisprocedures9!, however the superconductivity might not be bulk and the reason for itsexistence is not yet understood. As Sr is substituted for La, superconductivity appearswith Tc increasing until La1 8 5Sr0 1 5Cu04 . y . The basic structure of tetragonalLa2_xMxCuO4.y (M-Sr, Ba) is shown In Figure 11°1, however, at low temperaturesthere is evidence for a slight orthorhombic distortion11!. This distortion has little effecton the structural properties we will be discussing below. The most prominent feature Inthis structure is the presence of two-dimensionaS (2D) Cu-O planes with short (1.888A) distances within the plane. The Cu atoms are octahedrally coordinated with tworelatively distant oxygen atoms above and below the planes (2.416 A). All of theLa2_xSrxCuO4_y compounds have very low carrier concentrations12i13l. consistent withLa/BaFigure 1. The structure of L a g ^ ^ ^ yThe Sr-aubstituted compouod has the samestructure (ref. 10).each Sr atom producing a single hole. From a chemical standpoint, the addition of Sr canbe considered as increasing the formal oxidation state of the copper. Assuming C"2, La+ 3and Sr*2 , and a composition of La1 Q^STQ 1 5 C U 0 4 for the compound with the maximumTc, the calculated formal charge state of Cu is +2.15. Partial substitution for the Cuatoms in this structure rapidly depresses T c to OK within a few percent, while magneticrare earth substitution for La leads to a slow decrease in T c1 4 l Full substitution of rareearth ions is not possible in this compound because of a structural phase transition whichcompletely destroys superconductivity15'.The current confirmed record-holder for T c (-90K) Is YBa2Cu3O7^, which wasdiscovered as a minority phase in attempts to raise the Tc of La2.xBaxCu04.y byreplacing La with Y 1 6 l Subsequently, the single phase composition was found to beYBa2Cu3O7^1 7"1^ with the orthorhomblc structure shown In Figure 2 ^ l Thisstructure is composod of dimpled 2D Cu-0 planes and one-dimensional (1D) Cu-0 chainsseparated by fairly large distances. The Cu-O bond lengths are 1.928 A and 1.962 Awithin the 2D planes and 1.943 A along the ID chains. Furthermore, the Cu1-O4distance above and below the chains is quite short (1.850 A). Various neutrondiffraction studies find the exact composition to be YBa^UgOg g^ corresponding to aformal charge state of +2.23 for Cu2 0 '2 3! . At high temperatures, this compoundundergoes a phase transition to a tetragonal structure (Figure 3) 2A^si. in-situFigure 2. The ortfiorbombic structureof superconducting YBa 2 Cu 3 O 7 ^(ref. 20).Figure 3. The high-temperature,non-superconducting tetragonalstructure of YBa2Cu3O7.5 (ref. 26).neutron diffraction shows that as the temperature is increased, the oxygens in the 1Dchains disorder into the formerly vacant positions along the a-axls26l Figure 4 showsthe site occupation of oxygen in the basal plane as a function of temperature under 100%O2. At 750°C, site occupations become equal and the compound becomes tetragonal. Thetotal oxygen stolchlometry of this compound depends on temperature and oxygen partialto0.8-0.7-0.60.4-0.3-0.2-0.1-0.0-0.1100% oxygen(0.V2.0) site(1/2,0,0) site— i —200—1 ' 1—eoo BOOTemperature f t )—I—400Figure 4. Oxygen site occupancy as afunction of temperature in the basal plane, of YBa 2 Cu 3 0 7 ^ under 100% oxygen(ref. 26).woopressure as shown in Figure 6. The temperature at which the structural phasetransition occurs Is Indicated by the vertical lines and Is found to correspond to an oxygencontent of 6.5. Independent of oxygen partial pressure. It should be noted that thisstolchiometry corresponds to a formal charge state of +2.00 for Cu. Studies of quenchedsamples have shown that T c decreases monotonlcally to zero at the phase transition27!.7JO&96,86.7-6.66.56.16.0o - 100% oxygeno - 20% oxygena - 2% oxygenFigure 5. Total oxygen content in YBa2CugOxas a function of temperature for varyingpartial pressures of oxygen. The verticaldashed lines mark the temperature of theorthorhombic-to-tetragonal phase transition(ref.26).0 200 400 600 800 1000Temperature fC)Substitution of Y by trfvalent rare earth elements does not affect Tc despite the rareearth magnetic moments being introduced into the structure28'29!. Of the three rareearths which have stable tetravalent states (Ce, Pr and Tb), only Pr can be substitutedfor Y In this compound30!. Figure 6 shows the temperature dependent relativeresistivity for various concentrations of Pr in V1 .xPrxBa2Cu307^. If Pr enters as atetravalent ton, the observed transition from metallic to semiconducting behavior couldbe understood as a reduction of the formal charge state of Cu to below +2.00.Structural Considerations and SuperconductivityThe origin of superconductivity In the La2.xSr](CuO4_y compounds is clearly in the20 Cu-O planes, while there Is some question as to whether the 1D chains or the nearly2D planes In Y B a ^ g C y ^ are more important for superconductivity. It is surprisingthat the addition of 1D chains more than doubles the superconducting transitiontemperature. Some insight can be gained by considering the results of magnetic rareearth substitution into both these structures. In La2_xSrxCu04_y, the magnetic ion liesadjacent to the planes carrying the superconductivity (see Figure 1. La/Ba positions)and depresses Tc . On Ihe other hand, the YBa2Cu3O7^ structure is quite Insensitive toSO 100 150 200 250 300Figure 6. Relative resistivityas a function of temperaturefor compunds in the seriesWx(ref.30).magnetic rare earth Ions whose location is adjacent to the 20 planes but far from the 1Dchains (see Figure 2, Y positions). This result seems to indicate that superconductivitymay reside In the 1D chains. However, h Is also possible that the chains may only serveto stabilize a structure in which two Cu-O 2D planes are much closer together (3.50 A)than the equally spaced planes in La2_xSrxCu04_y (6.65 A). The effect of dimensionalityon superconductivity in these materials is far from being understood and will require agreat deal of additional experimental and theoretical effort.Superficially, one of the most striking differences in the two compounds is thatLa2.KSrxCu04,y is basically a solid solution, with La and Sr sharing the samecrystallographlc suWattice. In contrast, WB^Cu^Oj^ Is a stofchiometric compound withIndependent Y and Ba suWatttoes. When one considers the defect nature of thesecompounds, however, they are electronically quite similar, being low carrier density"doped semiconductors". Both L^CuC^ and YBagCugOg^, which contain only divalentcopper, exhibit semiconducting behavior at low temperatures10'. In both thesematerials, metallic-like behavior and superconductivity occur when the copper isoxidized to greater than a divalent state. In La2_xSrxCu04_y this is accomplished bydoping with divalent Sr while in YBagCugO^, the same effect is obtained throughincreasing the oxygen content Because of the strong covalent nature of the Cu-Osubiattices in these materials, it may be inappropriate to consider formal oxidationstates of the copper. Indeed, while these compounds are oxidized In a chemical sense, theholes Introduced by doping are deteallzed over the entire Cu-0 sublattices. In light ofthis "doped semiconductor" picture, it Is not surprising that defects play a major role Indetermining the electrical, magnetic and superconducting properties of these compounds.A good example is the "unintentionally doped" L^CuO,, compound which exhibits a widerange of magnetic and superconducting properties caused by the incorporation of as yetunidentified electrically active defects.AcknowledgementsThis work is supported by Basic Energy Sciences, Divisions of Chemical Sciences andMaterials Sciences, under Contract #VV-31 -109 -ENG-38 .^Permanent address: Department of Physics, Illinois Institute of Technology, Chicago, JL,60616, USA.ReferencesII Bednorz. J.G. and Muller. K.A.. Z. Phys. fi££. 189 (1986).2 l Uchida, S.. Takagi. H.. Kitazawa, K. and Tanaka. S., Jap. J. Appl. Phys. Lett. 2fi. L1(1987).31 Chu, C.W.. Hor. P.H.. Meng. R.L., Gao, L. Huang, Z J . and Wang. Q.Y.. Phys. Rev. Lett 58,405 (1987).4 l Tagaki, H., Uchkfe. S.. Kitazawa, K. and Tanaka. S.f Jap. J. Appl. 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Structural relationships in high temperature superconductors

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Structural relationships in high temperature superconductors

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