Perovskites and cuprate superconductors

The industrial fabrication of electronic devices containing perovskite-type metal oxides traditionally involves the preparation of powdered materials which are then cast as required. However, there is great interest at the research level in developing techniques for thin film deposition and in this section we consider the use of CVD methods. 

Reaction 27.17 is one conventional method of preparing BaTi03. A second route (used industrially) involves the preparation of BaTiO(ox)2-4H20 (ox = oxalate) from BaCl2, TiCl4, H2O and H2OX, followed by thermal decomposition (scheme 27.18). 

So far, we have illustrated the formation of binary (e.g. GaAs, TiC) and ternary (e.g. GaAsj BaTi03) systems through the combination of two or three volatile precursors in the CVD reactor. A problem that may be encountered is how to control the stoichiometry of the deposited material in some cases, controlling the ratios of the precursors works satisfactorily, but in other cases, better control is 

The explosion of interest in cuprate superconductors (see Section 27.4) during the last two decades has led to active research interest into ways of depositing these materials as thin Aims. For example, CVD precursors and conditions for the deposition of YBa2Cu30v have included BaL2, CuL2 and YL3 (L = 27.6) with He/02 carrier gas, and an 

LaA103 substrate at 970 K. Progress to date, however, has not reached a level that makes CVD commercially viable. 

H2O and H2OX, followed by thermal decomposition (scheme 28.19). 

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The superconducting oxides include both perovskites and Ruddlesden-Popper compounds which have an orthorhombic arrangement of cubic cells, alternatively of the perovskite and sodium chloride structures. The common feature of all of these is the presence of copper as a major component. The first ceramic superconductor was a lanthanum-strontium substituted cuprate (Lai Sr Cu04 z), which is a perovskite, but subsequently the inter-oxide compound Y203 2BaO 3CuO, commonly referred to as a 123 compound, was shown to have superior performance. The speculation concerning the conduction mechanism is that this involves either Cu3+-Cu2+ positive hole... 

All the high Tc superconductors discovered so far, with one exception, contain weakly coupled copper oxide, Cu02, planes. The highest critical temperatures are found for cuprates containing a Group 2 metal (Ca, Ba, Sr) and a heavy metal such as Tl, Bi, or Hg. The structures of all the cuprate superconductors are based on, or related to, the perovskite structure. The one report (in 2000) of a non-cuprate high T superconductor is of surface superconductivity in Na WOs. The structure of NUxWOs is also based on the perovskite structure. 

Already in the seminal paper of Bednorz and Muller [1], the guide to look for systems with a high superconductive transition temperature (Tc), has been the presence of strong electron-phonon interactions. Such interaction has been known to exist in a wide class of perovskite type oxides. The authors mention [1] the vibronic Jahn-Teller polaron effect [2] in this context. They also emphasize the fact that the Cu2+-ion is a well-known Jahn-Teller system and this circumstance preserves significance in the physics of cuprate superconductors [3-7]. As a microscopic cause for ferroelectric ordering the interband vibronic hybridisation has been supposed [8-11] enlargening the view on perovskites as Jahn-Teller systems. 

The examples chosen indicate that devices based on epitaxial systems of cuprate superconductor films have to be fabricated at atomic or close to atomic precision. The orthorhombic structure ofYBayCusOy and the fact that the three perovskitic sub-units making up the unit cell have different dimensions induces problems in grain boundary formation which affect the weak links in step-edge junctions and lead to fault structures if YBayCuyOy is deposited on simple perovskitic substrates such as SrTiOy. 

Other high-temperature superconductors can be described in similar fashion, e.g. Tl2Ca2Ba2Cu30io (containing Tl, Ca and Ba centres) is composed of layer sequence 27.4. The non-Cu02 oxide layers in the cuprate superconductors are isostructural with layers from an NaCl structure, and so the structures are sometimes described in terms of perovskite and rock salt layers. 

There are a number of metallic perovskite-related cuprates that do not show superconductivity, typified by La BaCUjOjj, containing apex-linked CuO square pyramids and CuOg octahedra. A comparison of these materials with the superconductors makes it apparent that superconductivity in the cuprates depends on two-dimensional sheets of CuO planes. In these cuprates, there is a considerable spin-charge correlation which appears to be important in the appearance of a superconducting region. 

Figure 93 Basic structural types of cuprate superconductors, (a) Perovskite structure (cubic) (b) Infinite- layered structure (tetragonal) (c) Rocksalt prototype structure (reduced cell) (d) Composite layer between infinite layer and rocksalt building blocks.

The non-Cu02 oxide layers in the cuprate superconductors are isostructural with layers from an NaCl structure, and so the structures are sometimes described in terms of perovskite and rock salt layers. 

The erystal La2-xBaiCu04 was the first high-temperature superconductor which was discovered in 1986 [1], Since then a whole series of imusual magnetic and transport properties was foimd in lanthanum cuprate perovskites and related compounds. In particular, one of the most interesting features of the inelastic neutron scattering in these crystals is that for hole concentrations x > 0.04, low temperatures and small energy transfers the scattering is peaked at incommensurate momenta (, -5 + ), in the reciprocal lattice units... 

Among the high-temperature superconductors one finds various cuprates (i.e., ternary oxides of copper and barium) having a layered structure of the perovskite type, as well as more complicated oxides on the basis of copper oxide which also include oxides of yttrium, calcium, strontium, bismuth, thallium, and/or other metals. Today, all these oxide systems are studied closely by a variety of specialists, including physicists, chemists, physical chemists, and theoreticians attempting to elucidate the essence of this phenomenon. Studies of electrochemical aspects contribute markedly to progress in HTSCs. 

CUPRATE HIGH-TEMPERATURE SUPERCONDUCTORS 8.6.1 Perovskite-Related Structures and Series... 

The structures of ternary oxides such as spinels, perovskites, pyrochlores, layered cuprates (high-7 c superconductors), and other lamellar oxides are fascinating subjects by themselves and are beyond the scope of the present discussion. 

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