Superconductivity-a Chemical Phenomenon?

Superconductivity-a

Chemical Phenomenon?**

By Arndt Simon* The accepted theory of superconductivity by Bardeen, Cooper, and Schn‘effer“l is based on the assumption that electrons pair (Cooper pairs) via special interactions with the lattice. The Coulomb repulsion between electrons is overcome indirectly by electron-phonon coupling. The interaction of an ion in the crystal and a passing electron changes the vibrations of the ion. The interaction of the ion with a second electron finally leads to attraction between both electrons. In other terms, a virtual phonon is exchanged between both electrons. The superconducting ground state corresponds to pairs of electrons with opposite spin and wave vectors of equal length but opposite sign ( + k t , - k j ) . All pairs have the same energy. An energy gap separates ground and excited states, so no continuous change in momentum can occur. This abstract model, as also the nature of the “classical” superconductors-as a rule these are intermetallic phases-does not make it easy to correlate the phenomenon of superconductivity with the electronic properties of certain elements and the specific features of their chemical bonding. Moreover, it does not add to the descriptive value of the model that the electrons of a Cooper pair can be thousands of angstroms apart. The discovery of high-temperature superconductivity in oxidesi’.31has not yet forseeable practical applications, but, to the chemist, in addition it opens up insights into the relationships between the phenomenon of superconductivity and such concepts as, e.g., oxidation number, change of coordination with oxidation state etc. BaPb,-xBi,03 ( ~ ~ 0 . 31 )crystallizes in the simple (deformed) perowskite structure, i.e. (Pb,Bi)06 octahedra are linked via all apices. The compound is metallic and becomes superconducting at T,= 13 K.IZ1 Bismuth has the unusual oxidation state + 4 and exhibits the tendency to delocalize one electron in the conduction band (BiS++ e - ) or to localize one electron to form a lone pair (Bi3’). Regular octahedra and tetragonal pyramids are common coordination polyhedra for BiS+ and the lone-pair ion Bi3+, respectively. Lattice vibrations which lead to a deformation of the coordination octahedron around the Bi-atoms in 1 (longitudinal optical (LO) phonons in the direction of one of the main axes), “switch” the tendency to localize (off-center position with Bi3+, lone pair with antiparallel spin) and delocalize (Bi” in the center of the octahedron, delocalized electron pair) electrons with the characteristic frequency of this vibration (cf. Fig. 1) and correlated for all atoms in the crystal. The coupling of electrons and phonons hereby corresponds to a n oscillating change in the oxidation state of bismuth (“valence fluctuation”) by two. The aforementioned characteristics of the Cooper pairs, e.g. antiparallel spin, + k t , - k l , same energy of all pairs, large correlation lengths, immediately follow from such a tendency of the conduction electrons to localize pairwise at the Bi atoms. Of course, the oxygen atoms are vitally important for this process. Bilz et aLt4.’l pointed out that 02-exhibits a very unusual increase of its polarizability [“I (**I

Prof. Dr. A. Simon Max-Planck-lnstitut fur Feslkorperforschung Heisenbergstrasse I. D-7000 Stuttgart 80 (FRG) Discussed i n part at the International Conference on Lanthanides and Actinides (IGLA), Lisbon (Portugal), April 5 - 10, 1987.

Angew. Chem. I n , . Ed. Engl. 26 (1987) No 6

with increasing cation-anion separation, enhanced by anisotropy and covalency effects which make the strong coupling of the electronic states of the cation to the vibrations of the oxygen atoms (and vice versa) plausible.

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I

Fig. 1. Model of the pairing of electrons due to a change in the valence of the Bi atoms in I .

In La,.,Ba,l.2Cu042 (K,NiF, type), Cu has the oxidation state f2.2. The transition temperature of ca. 30 Kol increases upon substitution of Ba by Sr to ca. 36 K.‘61Apparently, La, Ba and Sr merely determine the correct coordination dimensions and electron balance with Cu. Elongated CuO, octahedra (d’, Jahn-Teller distorted, 4 x 190 and 2 x 240 pm) are linked according to C U O ~ , ~ O via, the basal 0 atoms.[’] To a first approximation, networks of Cuatoms square-planar coordinated to 0-atoms are formed in layers (Fig. 2, top) and the indicated quadrupole mode

Fig. 2. Top: Cu atoms surrounded by a square-planar array of 0-atoms. Bottom: C u 0 2 units linked via bridging 0-atoms.

leads to a n oscillation between the coordination numbers 4 and (+2)* The deformation Of the square-planar nation, characteristic for Cu3 (d8, delocalized electron

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pair) could again result in a tendency to localize two electrons with antiparallel spin (Cu +,d“’).”] The explanation given for the superconductivity in 2 is supported by the results of investigations with YBaZCu30,-, ( ~ ~ 0 . 3, 1 ) which contains Cu in a mixed valence state +2.27 and which exhibits the highest transition temperature, T, =93 K reported so far.LX1 Neutron diffraction studies on powders revealed an orthorhombic structure‘’ 101 containing Cu2 in slightly buckled layers C U O ~(Fig. , ~ 2, top) which enclose the Y atoms. Superconductivity should not arise from this part of the structure as substitution of Y by magnetic 4f elements does not destroy the superconductivity. In contrast, the other Cu atoms (Cul) form parallel ribbons CuO,,,O2 (Fig. 2, bottom), which can be described as linear CuOz units (d(Cu0)=183 pm) linked via bridging 0-atoms (194 pm). The square-planar environment of the Cu atoms is strongly deformed in these ribbons and therefore the tendency to localize an electron pair (Cu +,dIo) in the CuOz unit should be even more pronounced than in 2. This tendency can be switched on and off periodically for each atomic site via the indicated zone edge mode. Of course, such an ionic model is a crude approximation. The covalency of the C u - 0 bonds and its dynamic variation is essential. The explanation given for the pair-wise attraction of electrons at the Fermi level seems closely related to the model of the Peierls-instability and in particular the model of the “ b i p ~ l a r o n ” . [I4]’ ~But ~ the latter is distinctly different as it is based o n electronic interactions without involving specific atomic properties. Unspecific local lattice distortions lead to a pairing of electrons in the bipolaron model. Here, it is intended to draw attention to the specific bonding of certain elements which might lead to the formation of “itinerant” electron pairs on the basis of the familiar chemistry of these elements.

Regulation of the 6-s Equilibrium Conformation of Retinal in Bacteriorhodopsin by Substitution at C-5; 5-Methoxy- and 5-Ethylretinalbacteriorhodopsin** By Elisabeth Kolling, * Dieter Oesterhelt, Henning Hopf, and Norbert Krause Retinal proteins, which contain retinal as protonated Schiff base (1 in Scheme I), absorb visible light. This involves, on the one hand, the triggering of sensory processes, e.g., vision in vertebrates by the rhodopsins and photophobic/photoattractive reactions by membrane proteins of Halobacterium halobium. On the other hand, energy conversion takes place, which in bacteriorhodopsin[’]leads to proton translocation and in haIorhodopsin‘’l to chloride translocation. The absorption of the protonated Schiff base in solution is shifted bathochromically in these proteins. This is brought about by the charged environment in the protein and the specific conformation of the retinal formed by interaction with the protein. Contributing fac-

“

I

b

C

i R =om3 ‘?.En3

I,R=CH3;2,R=CH2CH3; 3 , R = O C H 3 Received: April 21, 1987 [Z 2216 1Ej German version: Angew. Chem. 99 (1987) 602

Scheme I . The electron delocalization in the protonated Schiff base of retinal is illustrated by the resonance structures a and b. A methoxy group of C-5 additionally stabilizes the positive charge by a lone pair (resonance sfructure C).

[I] W. Buckel: Supraleirung. Grundlagen und Anwendung. Physik Verlag,

Weinheim 1977. [2] A. W. Sleight, J. L. Gilsson, P. E. Bierstedt, Solid State Commun. 17 (1975) 27. 131 J . G . Bednorz, K. A. Muller, 2. Phys. B 64 (1986) 189. 141 R. Migoni, H. Bilz, D. Bauerle, Phys. Reu. Lett. 37 (1976) 1155. [S] A. Bussmann, H. Bilz, R. Roenspiess. K. Schwarz, Ferroelectric.7 25 (1980) 343. 161 R. J. Cava, R. B. von Dover, B. Batlogg, E. A. Rietman, Phys. Reu. Lett. 58 (1987) 408. (71 J. B. Goodenough, G. Demazeau, M. Pouchard, P. Hagenmuller, J . Solid State Chem. 8 (1973) 3 2 5 . [XI M. K. Wu, J. R. Ashburn, C. J. Torug, P. H. Hor, R. L. Meng, L. Gao, 2. J. Huang, Y . Q. Wang, C. W. Chu, Phys. Reu. Lett. 58 (1987) 908. 191 M. A. Beno, L. Soderholrn, D. W. Capone 11, D. G. Hinks, J. D. Jorgensen, 1. K. Schuller, C. U. Segre, K. Zhang, J. D. Grace, Appl. Phys. Lett.. in press. [lo] J. E Greedan, A. OReilly, C. V. Stager, Phys. Rev. B 3 5 , in press. [Ill K. Hesterrnann, R. Hoppe, 2. Anorg. Allg. Chem. 367 (1969) 249. [I21 C. L. Teske. Hk. Muller-Buschbaum, Z . Anorg. Allg. Chem. 379 (1970) 113. 1131 B. K. Chakraverty, J . Phys. iParrsj 42 (1981) 1351. [I41 T. M . Rice, L. Sneddon, Ph.w Reu Lett. 47(1981) 689.

[*] In this context the structures of KCuO. [ I I ] and SrCu20z1121 are instructive. In KCuO. the Cu“ ion has square coordination (4 x 184 pm). Other than in 3, C u 0 4 I ribbons are formed by edge-sharing. In contrast, SrCu201contains (interconnected) linear CuO. units (2 x 184 pm).

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tors are: a) the distance between a negative charge in the protein and the positively charged nitrogen center of lysine 216”’ of the protonated Schiff base,[‘] b) a further negative charget5’or a dipole16’above (below) the cyclohexene ring, and c) the planarization of ring and chain at the C-6-C-7 bond (6-s The steric and electronic interaction of the retinal with the protein part can be analyzed by use of retinal analogues and isosteric compounds as controls. Especially interesting in this respect are substitutions at the cyclohexene ring, which can influence the torsion angle C5-C6-C7-C8 of the 6-s bond (“6,7-torsion angle”). Therefore 5-methoxyretinal and 5-ethylretinal‘“’1 were synthesized and their influence on the absorption of the corresponding bacteriorhodopsins was investigated.

[*] Dipl.-Chem. E. Kolling, Prof. Dr. D. Oesterhelt

I**]

Max-Planck-lnstitut fur Biochemte Am Klopferspitz. D-8033 Martinsried (FRG) Prof. Dr. H. Hopf, Dr. N. Krause lnstitut fur Organische Chemie der Universitat Hagenring 40, D-3300 Braunschweig (FRG) Retinoids, Part 9.-Part 8: H. Hopf, N . Krause, Tetrahedron Lett. 27 (1986) 6177.

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