High T c Bismuth and Thallium Copper Oxide Superconductors
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LLETIN/JANUARY 1989
K, and this has been obtained for several distinctly different compositions (Table I). Structure and Composition The structures of the (AO)mM2Ca„.] Cu„0 2n+2 family with m=\ and 2 are shown for «=1, 2 and 3 in Figure 4 in the article by I.K. Schuller and J.D. Jorgensen in this issue of the MRS BULLETIN. The collage of these structures shown in Figure 1 demonstrates how easily all these structures can intergrow
with each other. Such intergrowth is so facile that "single crystals" are not necessarily homogeneous with regard to either structure or composition. When the A site is fully occupied by Bi, only the AO double layer (m=2) structures are known. This is presumably because the lone pair of electrons for Bi"1 needs to be positioned in the interlayer spacing of the AO double layer. This in turn results in a large (~3.2 A) interlayer spacing and micaceous properties. Other members of this family do not show such micaceous behavior.5,9 The number of consecutively stacked C u 0 2 layers achieves infinity in (Ca,Sr)Cu02.13 However, the maximum n value in the (A0)mM2Ca„.1Cu„02„+2 series is currently four in reasonably well characterized materials. The Cu0 2 sheets of the copper oxide superconductors are the best determined features of these structures. Both the Cu and O sites are essentially always occupied in an ideal fashion. The Cu0 2 sheets of copper-oxidebased superconductors are generally not flat but are instead buckled in one of two ways. This buckling is due to Cu-OCu bond angles bending away from 180° which in turn may be related to the n antibonding interaction of this bond.14 The buckling of the Cu0 2 sheets can lead to oxygen alternately above and below the plane of copper atoms as in
Table 1. ( A O ^ M a C a ^ C U n O ^ + j Phases Compound
TC(K)
n
m
TIBa2YCu207 TIBa2Cu05 TIBa2CaCu207 TIBa2Ca2Cu309 TIBa2Ca3Cu40,, (Tl, Bi)Sr2Cu05 (Tl, Bi)Sr2CaCu207 (Tl, Pb)Sr2CaCu207 (Tl, Pb)Sr2Ca2Cu309 TI2Ba2Cu06 TI2Ba2CaCu208 TI2Ba2Ca2Cu3O,0 TI2Ba2Ca3Cu40,o Bi2Sr2Cu06 Bi2Sr2CaCu208 Bi2Sr2Ca2Cu30,o Bi2Sr2Ca3Cu4012 Bi2Sr2YCu206
NSC NSC 90 110 122 50 90 90 122 90 110 122 119 12 90 110 90 NSC
2 1 2 3 4 1 2 2 3 1 2 3 4 1 2 3 4 2
2 2 2 2 2 2 2 2 2
45
High Tc Bismuth and Thallium Copper Oxide Superconductors
orthorhombic La 2 Cu0 4 . Alternately, the C u 0 2 sheets can buckle with all oxygen atoms in one plane which is slightly displaced from the plane of copper atoms as in YBa 2 Cu 3 0 7 . Both types of buckling are found in the (AO)mM2Ca„.1Cu„02„+2 family. For example, the La 2 Cu0 4 type buckling is found in Bi 2 Sr 2 Cu0 6 , and the YBa 2 Cu 3 0 7 type buckling is found in Tl 2 Ba 2 CaCu 2 0 8 . Some members of the (T10) m M 2 Ca„.,Cu n 0 2 „ + 2 family h a v e C u 0 2 sheets that are essentially flat. 815 The only soft mode behavior establ i s h e d for t h e c o p p e r - o x i d e - b a s e d superconductors is associated with the bending of Cu-O-Cu bonds. This soft mode behavior is expected to be most pronounced when the Cu-G-Cu bond is 180° in the static description. That high T c (90-120
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