Charge reservoirs

In the materials that follow, the structures are all layered. This structural feature has lead to a description of the doping in terms of charge reservoirs, a different approach to that described previously, and which is detailed below. Structurally the phases are all related to the perovskite-layered structures (Figs. 4.27, 4.28, 4.29, and 4.30). The similarity can be appreciated by comparison of the idealized structures and formulas of some of these materials, Bi2Sr2CuOg... 

Figure 8.6 Schematic depiction of a cuprate superconductor. Doping into the charge reservoir layers results in the transfer of holes to the superconducting layers.

The concept of hole doping into charge reservoir layers can be explained with a number of examples. The materials are treated as if they are ionic. The simplest example is provided by the series of phases with a charge reservoir (AO AO), typified by La2Cu04, already described. 

Charge Reservoir Charge Reservoir Formula Superconducting Slab Formula Ideabzed Series Formula" Examples ... 

The charge reservoir layer is nonstoichiometric. This is not indicated in the idealized formulas given. 

The composition is variable due to the nonstoichiometric nature of the charge reservoir slabs 8 indicates this and may be either positive or negative. 

These charges exactly balance, and the material would be an insulator, a normal ionic compound. Hole doping can be achieved by adding lower valence cation such as Sr2+ or oxygen interstitials to the charge reservoir. For Sr2+ aceptor dopants ... 

The charge difference between the charge reservoir and the superconducting layers must be achieved by the addition of balancing charges, xh in this case. For oxygen interstitials ... 

The idea of charge reservoirs can be applied to the other members of the series. 

The charge reservoir layer in the cuprate superconductor HgBa2CaCu206+s is ... 

The nature of the charge reservoirs in the cuprates determines the carrier concentration and the ease of charge-transfer to the CUO2 sheets. Covalent charge reservoirs can... 

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