Partially Disordered Inorganic Materials
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IN/MAY 1999
proach to the definition of partial disor der is to consider disorder in terms of the length scale and dimensionality of any disorder that may be present. The most ordered Systems that we have considered are those in which long-range, threedimensional (3D) crystalline order is perturbed only by the presence of localized defects. Materials that fall within this category include superconducting cuprates and colossal magnetoresistive manganates, in which there is substitutional disorder (e.g., La2-vSr.vCu04 and Lai-vSr.vMnOj), and grossly nonstoichiometric binary and ternary Compounds such as Ce0 2 -. v , LnH 2 + v and Li,Mn02- Other examples include lead-containing, mixed-ion ferroelectrics based upon PbTi03, where displacements of the lead ions from their expected positions arise because of the lone pair of electrons on Pb 2 + , and zeolite host-guest Systems in which adsorbed molecules are disordered within a wellordered zeolite Channel System. In all of these materials, a conventional structure determination using x-rays or neutron diffraction often fails to reveal the precise nature of the disorder. Systems with apparent long-range order can also contain defects resulting from domain formation; in ferroelectrics, for example, the sizes of individual domains ränge from as small as only 50 Ä in relaxor ferroelec trics to tens of millimerers for normal ferroelectric materials. Materials with intermediate disorder include those that have good long-range order in one or two dimensions, but which may be poorly ordered otherwise. Turbostratic materials, such as clays, graphites, and certain other layered materials, rep resent the most important groups in this area; they ränge from being highly or dered in all three dimensions (as in high charge-density clays such as vermiculite) to Systems in which the layers are completely exfoliated. Carbon nanotubes, which are ordered in just one dimension, also belong to this class. Another fas-
cinating category is that of mesoporous silicas and related materials, where the long-range pore structure is well ordered, but the local structure within the Chan nel walls is highly disordered. Beyond this level of disorder are many composite materials where the individ ual components may exhibit various degrees of long-range order, but there is no long-range order between these compo nents. Here, we think of both natural composites, such as bone and other bio minerals, and synthetic ones, such as fiber-reinforced ceramics. In this and many other situations, interfaces play a very important role. Also of great interest are the carbon molecular sieves, where substantial local order coexists with an absence of any long-range order; these represent the most disordered group that we shall consider. Glasses, such as amor phous semiconductors, may possess sub stantial short-range order (in that each atom is almost perfectly tetrahedrally coordinated), but littleorno medium-ränge and long-range order. For practical reasons, they are beyond the scope of this article. Although
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