Chemistry of High-Temperature Superconductors - ACS Publications

Chapter 5 Synthesis of N e w Classes o f H i g h - T e m p e r a t u r e Superconducting Materials Francis J. DiSalvo

Downloaded by NANYANG TECH UNIV LIB on June 9, 2014 | http://pubs.acs.org Publication Date: August 28, 1987 | doi: 10.1021/bk-1987-0351.ch005

Department of Chemistry, Baker Laboratory, Cornell University, Ithaca, NY 14853

At present (May 1987), there are several known oxide compounds that are superconducting at temperatures above 30K, with some approaching 100K, and numerous reports of metastable drops in resistance in some materials at temperatures as high as room temperature. There are three types of copper oxides in this class with different but related crystal structures: La M CuO with M - Ca, Sr, or Ba [1,2]; MBa Cu O with M - trivalent metals [3-6]; and La Ba Cu O [7]. The structures are all derivatives of perovskite, which has the composition ΜΜΌ , and are variously described as having square planar coordinated copper-oxygen sheets or copper in highly distorted octahedral or square pentagonal oxygen coordination. Another oxide, BaPb Bi O3, should probably also be included in this class, even though its T is only 13K, since on the basis of its density of states, its T should only be a few degrees [8]. Perhaps coincidently, it also has the perovskite structure. Since the end of 1986, there has been an enormous amount of synthetic work on related oxides obtained by substitution of the cations or anions or by an Edisonian variation of the composition. These approaches are an important process in the search for new high T materials. If general characteristics of the high T phases can be used as a guide to future synthesis strategies, it is hoped that a more rapid route to the discovery of other high T phases can result. Such a search presupposes a "faith" that the known oxide superconductors are not unique but that other compounds with similar or enhanced properties must exist. In this paper I outline some ideas on the unusual features of the oxide superconductors and some thoughts on what other compounds might show similar characteristics. I start with a "disclaimer": while my students and I have prepared some of the high T materials by following others recipes and are just now starting to attempt to prepare new materials, I have not been one of the contributors to the large body of information that exists concerning these oxides and from which my thoughts arise. Further, I have been fortunate to receive many of the preprints that active groups have been circulating to avoid 2-x

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0097-6156/87/0351 -0049$06.00/0 © 1987 American Chemical Society

In Chemistry of High-Temperature Superconductors; Nelson, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.


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the "slowness" of the normal publication procedures. Without the openness of others, I would have l i t t l e to say. I have also somewhat a r b i t r a r i l y chosen the p a r t i c u l a r references to c i t e , choosing to pick one that i l l u s t r a t e s the point, rather than t r y i n g to give an exhaustive l i s t of a l l the preprints that I have seen on the topic. The f i r s t and most surprising feature of these materials i s apparent i n band structure calculations. Such calculations may i n f a c t not be correct i n d e t a i l , since these materials appear to be close to the composition at which a Mott t r a n s i t i o n to the i n s u l a t i n g state takes place [9]. This suggests that Coulomb correlations must be included i n a r e a l i s t i c description of the properties. However, the observation based on the band structure calculations [10-12] that the states near the Fermi l e v e l have a strongly mixed character ( of Cu d and 0 ρ states) w i l l also l i k e l y survive i n a "correct" description of the e l e c t r o n i c structure. This admixture arises from the closeness i n energy of the corresponding atomic energy levels and from the fact that the band at the Fermi l e v e l i s derived from a sigma antibonding state. This i s indeed rather unusual for a m e t a l l i c compound. The band structure r e s u l t s suggest that 50% or more of the wavefunction amplitudes are based on the oxygen! Perhaps, then, i t i s better to describe these materials as "metallic oxygen". In the vast majority of other metallic compounds the wavefunctions at the Fermi l e v e l are predominantly of cation (metal) character, t y p i c a l l y containing 10% or less anion character. In other conducting oxides, p a r t i c u l a r l y of early t r a n s i t i o n metals (such as LÎTÎ204), the states at the Fermi l e v e l are derived from nonbonding or weakly p i bonding levels, further reducing the already low oxygen character r e s u l t i n g from the large energy difference between the oxygen ρ and titanium d states. There are several ways to describe the e l e c t r o n i c state of the materials. Band structure i s one way and formal valence states i s another. While the l a t t e r i s oversimplified, i t i s also very easy and widely used. But i t can be a l i t t l e misleading. For example, the formal valence description of YBa2Cu30y i s Y B a 2 C u 2 C u 0 " 7 . This implies that the oxygen ρ l e v e l s are f u l l y occupied ( i . e . , that the oxygen i s f u l l y d i v a l e n t ) . Since the oxygen and copper l e v e l s each make a considerable contribution to the wavefunctions at the Fermi l e v e l , i t i s impossible to oxidize the copper to +3 without also oxidizing the oxygen. In f a c t some recent experiments can detect only C u , suggesting that only the oxygen i s oxidized. In that case the f r a c t i o n of c a r r i e r density that i s based on oxygen i s even higher than the band structure calculations would suggest. Therefore, when authors speak of needing "mixed valence" C u / C u i n the oxides to produce superconductivity, a broader i n t e r p r e t a t i o n of what species are being oxidized should be kept i n mind. The calculations also suggest that the e l e c t r o p o s i t i v e cations (alkaline earth or rare earth metals) have l i t t l e influence on the electronic structure near the Fermi l e v e l . I believe that t h e i r role i s two f o l d . F i r s t , they help "enforce" a p a r t i c u l a r structure; that i s , the large cations are responsible for the compounds adopting the perovskite structure. Second, the e l e c t r o p o s i t i v e metals e f f e c t i v e l y increase the o x i d i z i n g power of +3

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the oxygen. In binary copper oxides, the maximum formal valence that can be obtained using the usual high temperature s o l i d state preparation techniques i s +2. whereas i n ternary copper oxides containing, for example, a l k a l i metals, copper can be formally +3, as i n NaCu02The next unusual feature i s that they are lousy normal state metals. Their r e s i s t i v i t y at 300K i s between 10" and 10"^ Ohm-cm, two to three orders of magnitude higher than "good" metals and at l e a s t a few factors higher than i n t e r m e t a l l i c t r a n s i t i o n metal compounds [4,9,13]. This implies rather short mean free paths for the conduction electrons, perhaps as short as a l a t t i c e parameter. Yet the r e s i s t i v i t y i s quite temperature dependent, t y p i c a l l y varying l i n e a r l y with temperature. I t i s l i k e l y that the unusual behavior of the r e s i s t i v i t y i s related to the nearness to the Mott t r a n s i t i o n and may be an important key i n determining the d e t a i l e d mechanism that leads to the high T . At the same time the mechanical properties are more l i k e those of ceramics ( b r i t t l e ) than metals ( d u c t i l e ) . Recall, however, that the i n t e r m e t a l l i c compounds are also often b r i t t l e , and that some techniques have been invented to allow f l e x i b l e wires to be fabricated from them. These materials are also fast ion conductors; at l e a s t oxygen i s able to d i f f u s e rather r e a d i l y through the bulk of the compound at temperatures as low as 300C. This implies a bonding p o t e n t i a l vs p o s i t i o n for oxygen i n the l a t t i c e that i s rather f l a t , or at l e a s t b a r r i e r s that are rather small i n the d i r e c t i o n of d i f f u s i o n . This i n turn implies that oxygen vibrations at lower temperatures w i l l have a large amplitude, perhaps being rather anharmonic. Recent neutron d i f f r a c t i o n measurements indeed show that the c r y s t a l l o g r a p h i c a l l y unique oxygen atoms i n the "one dimensional" chains i n YBa2Cu30y have large thermal amplitudes of motion [5] and are the s i t e s that are reduced i n occupancy when oxygen i s removed from the l a t t i c e [14]. (Some others have also suggested that the fact that C u i s a Jahn-Teller ion w i l l also make i t s motion rather anharmonic. However, the copper i s already i n a rather d i s t o r t e d s i t e , and the neutron d i f f r a c t i o n measurements show only a normal v i b r a t i o n a l amplitude.) This ready d i f f u s i o n also has important consequences for the synthesis and processing of the material. Some attention has been drawn to the low dimensional aspects of these materials. While the high T materials have two dimensional sheets of Cu-0, and the 90K superconductor has i n addition one dimensional Cu-0 chains, Ba(Pb/Bi)03 has an almost cubic structure. Consequently, low dimensionality seems not to be a necessary condition for materials i n this class to be superconducting. In any case low dimensional structures w i l l l i k e l y r e s u l t i n other materials from low coordination number of the t r a n s i t i o n metal and by including large e l e c t r o p o s i t i v e cations. Before discussing some rather straightforward generalizations of the above properties that might be used as a guide for new synthesis, I want to say a few words about the theories that have been proposed to explain the high T 's observed. While no theory has been developed to the point that i t explains i n microscopic d e t a i l a l l the observed phenomena, i t might be useful to take a b r i e f look at them to extract e s s e n t i a l


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features. Then using these features and the above observations, we can t r y to predict what new compounds might be synthesized that reproduce some or even a l l of the desired aspects of the known high T materials. I w i l l l i m i t this discussion, both since I am not a t h e o r i s t and because other such reviews e x i s t [15]. The theories can be divided into three types, each characterized by the p r i n c i p a l i n t e r a c t i o n responsible for the high T : phonon, magnetic, or exciton. The phonon mechanisms might be divided into two categories: enhanced electron-phonon i n t e r a c t i o n of the BCS type [10] and l o c a l d i s t o r t i o n s about c a r r i e r s to produce bipolarons [16]. Both r e l y on the large coupling of the electron energy to l a t t i c e motions that r e s u l t from the large mixing of the cation and anion o r b i t a l s and the antibonding character of the wavefunctions at the Fermi l e v e l . The magnetic models may also be grouped into two categories: i n t e r a c t i o n with d i f f u s e magnon modes and i n t e r a c t i o n on a more l o c a l l e v e l to produce l o c a l i z e d s i n g l e t pairs (the RVB state, Resonating Valence Bond) [17,18]. These theories emphasize the nearness of the Mott i n s u l a t i n g state to produce l o c a l i z e d spin 1/2 C u s i t e s . Some theories emphasize the large oxygen contribution to the conductivity and the possibly l o c a l nature of the electrons on copper [19]. F i n a l l y , the exciton theories are based on a d i r e c t coupling of the electrons to an electronic e x c i t a t i o n . These might include plasmons [20] or charge transfer excitations [21]. The l a t t e r mechanism also r e l i e s on the small energy difference between the cation d and anion ρ states and on a large o s c i l l a t o r strength of the t r a n s i t i o n . Each theory then picks one aspect of the many unusual properties and attempts to explain the high T based on that feature. Now we can t r y to put this a l l together to t r y to predict where else to look for new superconducting phases. The main themes that arise from the preceding discussion can be condensed into four general c h a r a c t e r i s t i c s : 1) A large cation-anion mixing of the wavefunctions near the Fermi l e v e l , 2) m e t a l l i c conductor, but close to a Mott t r a n s i t i o n , 3)fast anion conductor, 4) the e l e c t r o p o s i t i v e cations do not play an e s s e n t i a l r o l e i n the electronic properties. For the sake of discussion, we can a r b i t r a r i l y break these materials into oxides and other anion compounds. c


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Oxides The atomic d states of copper are about leV above the ρ states of oxygen. On moving to the l e f t i n the periodic table, the d states r i s e by about 3eV when at T i . The energy of these l e v e l s i s s h i f t e d when the atom i s incorporated i n a compound. The anion l e v e l s r i s e and the cation l e v e l s f a l l i n energy, due to the charge transfer and configuration mixing inherent i n compound formation. The charge transfer and energy s h i f t s depend upon the electronegativity difference between the anion and cation and upon the cation-anion r a t i o . However, the r e a l charge transfer i s usually considerably smaller than that suggested by the formal valence. Unfortunately, these s h i f t s are d i f f i c u l t to predict before a compound i s even prepared. We can r e l y on general trends to "guess" what w i l l happen i f the cation i s d i f f e r e n t than



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copper. I f compounds are prepared from 3d elements to the l e f t of copper the cation-mixing w i l l tend to decrease, unless the t r a n s i t i o n metal i s i n a higher average oxidation state than that obtained i n the copper compounds ( which i s about +2.3). Perhaps i f n i c k e l compounds with an average valence of greater than +3 could be prepared, the mixing would again be strong. Using the same reasoning i t i s possible that other late t r a n s i t i o n elements w i l l also be suitable. These include Ag, Pd, Co, and Pt. For each element i t may require special preparation conditions to obtain the "correct" phases. That i s , unusual conditions such as preparation under high pressure or at low temperatures using solution techniques may be necessary. In oxides, condition 2 (near a Mott transition) i s l i k e l y to be met, i f the compound i s conducting at a l l . Most oxides are i n fact not metallic conductors, and, of those that are, many exhibit metal insulator transitions with changing temperature or pressure. I t may also be necessary that i n the insulating state the cation has a spin 1/2 configuration (probably necessary for the RVB mechanism). The occurrence of the state depends upon the valence of the cation and upon i t s l o c a l coordination, with highly d i s t o r t e d near-neighbor environments favoring non degenerate states and spin 1/2 configuration. A l t e r n a t i v e l y , for a given environment the cation valence necessary to produce a spin 1/2 ion can e a s i l y be determined i n the low and high c r y s t a l f i e l d strength l i m i t s . I t i s not clear to me how to design a material to be a fast ion (anion) conductor, nor i s i t clear that this i s a necessary condition for high T superconductivity. I f the large oxygen vibrations that are a consequence of the f l a t e l a s t i c potential for v i b r a t i o n produce an enhanced electron-phonon interaction, i t would seem that such large vibrations could occur without ionic d i f f u s i o n . Perhaps the f l a t e l a s t i c p o t e n t i a l i s a r e s u l t of the near degeneracy i n the cation and anion d and ρ l e v e l s . This would make charge transfer between them a low energy process, r e s u l t i n g i n a high l a t t i c e p o l a r i z a b i l i t y , or equivalently e a s i l y deformable ions. Such a picture i s usually invoked i n discussing fast ion conductors. So i t could be that most of the systems that are p o t e n t i a l l y interesting superconductors of this type w i l l also c o i n c i d e n t a l l y be superionic conductors. While a continued search for copper oxides containing mixed Cu* /Cu formal valence states w i l l be extended, perhaps a single example of another compound that should reproduce a l l the above features w i l l s u f f i c e for i l l u s t r a t i v e purposes. Consider an oxide containing Ni^/Ni" "^. The formal valence state i s higher than copper, so the Ni d states should be pulled down i n energy, hopefully enough to again produce strong mixing with the oxygen. The electron configuration i s d^, which i n a distorted cubic or square planar environment w i l l be spin 1/2. A tetrahedral Ni enviornment i n the low c r y s t a l f i e l d l i m i t produce a spin 3/2 state, but i n the large c r y s t a l f i e l d l i m i t a spin 1/2 state would again be produced. I f the compound i s metallic, the band states at the Fermi l e v e l would be anti-bonding, thus further increasing the mixing and producing large electron-phonon coupling. Such Ni oxides would therefore be almost exact analogues of the high T copper oxides. Experimentally, the preparation of n i c k e l oxides c

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with an average formal valence greater than +3 may require high oxygen pressures or low temperature techniques.


Other anions Other anions that are l i k e l y candidates include nitrogen, sulfur, and chlorine. Fluorine i s u n l i k e l y to produce considerable mixing, because the ρ levels are a few v o l t s below those of oxygen, and therefore fluorides are u n l i k e l y candidates. Further, fluorides tend to be rather ionic and very l i k e l y to be insulators (without a l i t e r a t u r e search, I can only think of a few conducting fluorides, Ag2F and graphite intercalated with fluorine or f l u o r i n e containing anions). Mixing anions may produce small s h i f t s i n the cation and other anion levels and may be a suitable way to fine tune the properties. But the anion ρ levels are s u f f i c i e n t l y d i f f e r e n t i n energy that i t i s u n l i k e l y that both anions would mix with the cation levels to the same extent; one would always dominate. Nitrides and sulfides that meet the conditions necessary for extensive mixing of the states at the Fermi l e v e l may be prepared i f the cations are chosen from elements to the l e f t of copper, since the anion ρ states are higher i n energy than those of oxygen by about 1.5eV. I f i t i s important to be near a metal-insulator t r a n s i t i o n , then choosing a compound based on a 3d t r a n s i t i o n element w i l l enhance that p r o b a b i l i t y . Empirically, such t r a n s i t i o n s , as a function of composition or temperature, occur most frequently i n that part of the periodic table. 4d and 5d compounds tend to be more metallic than the equivalent 3d compounds, since the r a d i a l extent of the d wavefunctions i s larger than for 3d cations. However, there i s no hard and fast rule that w i l l exclude other p o s s i b i l i t i e s ; witness the metalinsulator t r a n s i t i o n i n Ba(Pb/Bi)03 as a function of composition. Depending upon the oxidation state, t r a n s i t i o n elements from n i c k e l to vanadium should be considered. I f the d occupancy becomes small (say less than 4), the states at the Fermi l e v e l w i l l be predominantly non-bonding and w i l l tend to have somewhat less mixing than the unoccupied anti-bonding states at higher energy. This w i l l make the largest difference to mechanisms based on phonon or exciton interactions, but may not be important to the RVB state at a l l . Chlorides may also be good candidates, i f m e t a l l i c phases can be prepared. However, the vast majority of chlorides are insulators. The ρ states of chlorine are close to those of oxygen, perhaps 0.5eV higher. Most of the same considerations applied above to the case of n i t r i d e s would apply to the chlorides as well. Summary The unusual features of the new high temperature superconductors have been outlined. I t i s suggested that further synthetic studies, using some general guidelines coupled with a few physical property measurements, of new oxides as well as n i t r i d e s , s u l f i d e s , hydrides, and perhaps even chlorides may well lead to more of these fascinating novel superconductors.


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ACKNOWLEDGMENTS This work was supported i n part by the National Science Foundation through the Materials Science Center at Cornell University.

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RECEIVED July 6, 1987


Chemistry of High-Temperature Superconductors - ACS Publications

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