La2CuO4+y Phase Diagram
Stage 6 Stage 4 Stage 3 Stage 2 Oxygen Doping C~' and Staging #)
Fig. 113. T-x phase diagram for La cuprates. After Wells et al. (1997).
In the phase diagram of fig. l 13 several types of stripes have been found, following the ingenious discovery of stripes by Tranquada et al. (1995) and the concept of quantum wires (Bianconi and Missori 1994). Whereas for the La cuprates a f o l k l o r e of stripes have been reported by many authors, up to very recently, this, type of phase separation has not been experimentally proven for 123. Very recently, however, two papers (Sharma et al.
2000, Mook et al. 2000) have presented evidence that stripes do exist also in 123-Ox (sect. 7.4).
Summary
After five decades of semiconductor research in pursuit of the ideally homogeneous and perfect material, the picture of HTc materials emanates as intrinsically inhomogeneous.
This review has tried to show some of these aspects:
(A) There is a wealth of data showing the existence of important lattice effects and intrinsic inhomogeneities in 123-Ox.
(1) Five phase transitions exist:
- The well-known T - O transition at x = 6.4, which according to Kartha et al.
(1995) and Krumhansl (1992) is a first-order martensitic transformation. Also a neutron diffraction refinement (Radaelli et al. 1992) concludes that it is a first-order structural transition (sect. 3.2.1). The onset of superconductivity (I-M transition) takes place inside the composition range of the T - O transition (fig. 25d, e) and the latter triggers the former, as originally proposed by Cava et al. (1990). The ideas of a universal hole count for all cuprates to become superconducting (Tallon et al.) are doubtful for 123-Ox because their proof was based on questionable samples (sect. 5.3, 8.4). Recent structural data on very slowly cooled equilibrium samples (Conder et al. 1999; sect. 3.2.2) support clearly the abrupt change of the c-axis and apical bond found earlier by Cava et al. (1990). Charge transfer from the chains to the planes is coupled to this c-axis contraction. A giant dTJdP effect is measured at this composition which seems to be correlated with the oxygen ordering taking place at this transition (sect. 5.4).
Raman investigations show another transition or cross-over at the N~el temperature (x=6.22). Deconvolution of the apex-phonon width shows the coexistence of four phases in the nonstoichiometric range of 123. It is possible that physical phase separation starts at x = 6.22 with the formation of hole-doped islands in an AF matrix (sect. 3.3.2), the percolation to the superconducting phase taking place at x ~ 6.40.
- A fourth latent transition or cross-over takes place at x=6.75. At this composition the a- and b-lattice parameters deviate from linearity, and orthorhombicity, mimicking Tc, starts saturating (figs. 25a-c). It is possible, therefore, that this composition marks the start of the negative interactions leading to a decreasing T~ in the overdoped phase. Structurally, these changes are mainly in the planes, but have appreciable influence on the apex phonon frequency which for x > 6.75 becomes independent of doping (although its bond length is further decreasing). The most pronounced physical effect is the giant dTJdP increase (sect. 5.4), which is also connected with thermal expansion anomalies o f the a-axis near T~ (sect. 5.4.2).
- A fifth transition exists at the onset of the overdoped phase, x - 6.95. This is a displacive transformation (Kaldis et al. 1997b) followed by a decrease of orthorhombicity and a stepwise decrease of the b-axis (fig. 25b), but most importantly by an abrupt increase of dimpling (sect. 6). This transition is
OXYGEN NONSTOICHIOMETRY AND LATTICE EFFECTS IN Y B a 2 C u 3 0 ~ 175 the boundary of the phase-separated overdoped phase. This is supported by the splitting of the diamagnetic transition to Tc2, onset (almost independent of oxygen doping) and To1, and also by Raman measurements. The physical phase separation is probably due to overdoped islands in a matrix of the optimally doped phase. Thus, both ends of the superconducting field seem to bephase-separated (6.15 < x < 6.40, and 6.95 < x < 7.00). The Tc2,onset appears with the same value in the Ca-doped 123 (see below). The transitions at x = 6.75 and 6.95 lead to changes parallel to the c-axis, but are induced by changes of orthorhombicity. The latter is triggered by displacement of 0 2 , 0 3 parallel to the c-axis and away from Cu2 (which moves out of the planes).
(2) Several lattice distortions have been found in the underdoped and optimally doped fields.
- PDF (sect. 4.2) shows that in the local structure, shifts of 0.1 A of the oxygen sites appear along the c-axis in domains of~10 A. The question arises whether this is the result of the presence of polarons or some kind of stripes, as was shown to be the case for the La cuprates.
- Electron diffraction shows correlated displacements of Cu2 and O1 in the a-b plane (sect. 4.3), leading to local perturbation of charge density roughly parallel to the c-axis, supporting the results of PDE These perturbations form a network of cells in the a-b plane with nanoscopic dimensions, comparable with the coherence length. The reason for these distortions is the strain field, which has been shown to exist also with the bond valence sum method in the 123-perovskite lattice (sect. 3.2.1.4). These results support the nanoscale phase separation model of Phillips and Jung (2001a,b) (sect. 4.3.1).
- Y-EXAFS shows displacements of the 0 2 , 0 3 and Cu2 atoms parallel to the c-axis, inducing distortions of the dimpling. Lattice distortions appear also both at Tc and the spin-gap opening temperature.
(B) A large number of experiments support the existence of several fields of phase separation. We have mentioned under (1) and (2) the physical phase separation at the two boundaries of the superconducting regime. In the underdoped range a kind of phase-rule obeying phase separation in mesoscopic scale can be considered, due to the existence of a series of in-phase phonon-frequency steps ("phases"), appearing at compositions where also the chain superstructures appear. This puts the question about the existence of dimpling-chain superstructures, where the chain-ordering phenomena would be correlated with the dimpling Raman shifts parallel to the c-axis (sect. 5.5.2). (figs. 110, 111).
(C) A bag full of interesting and unexpected complexities is the case of Ca-doped 123-Ox.
The structural data and the Tc change give the impression of classical overdoping:
c-axis and apical bond increasing, To decreasing with Ca doping. A thorough inspection of the magnetization data of fully oxy.,genated samples shows that there also is a strong diamagnetic splitting and, therefore, phase separation following the pattern of the O-overdoped 123, and supported by the appearance of new Raman modes (sect. 8.2). Tcz, onset has the same value as for optimally O-doped 12306.92
and is almost independent of Ca doping, Td decreases strongly with Ca doping.
The question about the local structure opens Pandora's box. Y-EXAFS sees, up to Reff ~ 5 A, almost no changes in the spectra and at higher Ca doping something like amorphisation. A model accommodating these difficulties (sect. 8.4) is the monodisperse dissolution of Ca-123 molecules in the lattice (R6hler et al. 1999b).
This is expected to lead to localization of the holes introduced by Ca, the carriers screened by the locally distorted Y-123 cells surrounding the Ca-123. Then, the overdoping does not come from the holes doped by Ca but by the decreasing Y-123 contributions to the superconducting carriers with increasing Ca content.
Not only may all of these be wrong in our picture of Ca-123. The assumption of linear dependence of O content on Ca% is also wrong (sect. 8.1). Up to 17% Ca very little oxygen is lost, not supporting the classical picture of substitution, except if the Ca holes are not available to the lattice. And last but not least, Ca-12306 which has been used to support the adoption o f the parabolic dependence of Tc on the carrier concentration for 123 (Tallon et al. 1995) is not superconducting!
12306 doped with 5 different Ca compositions (0-22%) was found not to be superconducting (fig. 51) as discussed in sect. 8.4 (Merz et al. 1998).
Summing up we can say that from the structural point of view the lattice strains are very important, leading to most of the local structure effects. The above is a list of experimental facts, which depending on the theoretical model can be interpreted differently. The ultimate goal of this review is to bring these facts to the attention of the reader (or remind some of them). Because whatever their interpretation may be, these experimental facts should not be overlooked.
Acknowledgments
Many thanks are due to Professor J. R6hler for critically reading the manuscript, to Professors K.A. Milller, H. Keller, E. Liarokapis, and J. R6hler for many fruitful discussions, and to Professor E. Liarokapis and Drs G. B6ttger, K. Conder, P. Fischer, A.W. Hewat, J. Karpinski, I. Mangelshots-Loquet, D. Palles, Ch. Rossel, S. Rusiecki, H. Schwer, A. Shengelaya and D. Zech for longstanding collaborations and/or permission to use some yet unpublished data. For the discussion of the phase relationships of 123-Ox and the scouting for phase-separation phenomena the input of the Raman investigations as a function of nonstoichiometry was found to be an invaluable asset. My particular thanks go to Professor E. Liarokapis and his collaborators for these important contributions. Last but not least the technical support of E. Jilek, S. Mollet and A. Wisard over the years is gratefully acknowledged.
Permissions for publication of figures and/or preprints and reprints have been received from Dr. N.H. Andersen, Prof. E Bridges, Dr. R Burlet, Prof. R. Cava, Prof. T. Egami, Dr. J. Etheridge, Dr. W.H. Fietz, Prof. L.H. Greene, Dr. J.D. Jorgensen, Dr. C. Meingast, Dr. S.B. Qadri, Prof. R.R Sharma, and Dr. J. Tallon. The author acknowledges this with pleasure.
OXYGEN NONSTOICHIOMETRY AND LATTICE EFFECTS IN YBa2Cu30 x 177 The experimental work from the author's laboratory reported here has been supported in the past by the Swiss National Science Foundation (National Program of Supercon- ductivity) and the ETH. The author wishes to aknowledge this crucial support.
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