A-site ordering

In addition to B-site ordering, A-site order can occur although its observation is much more rare. A-site ordering is seen only when the large Ba + ion is present in company with a Ln + ion. Both double, LnBaT205+s, where T = Fe or Mn, and triple, LnBa2T308+s, where T = Fe, perovskites of this type have been reported recently. ... 

In these cases, A-site ordering results in a doubling or tripling of one of the perovskite unit cell axes, hence the nomenclature. Note also that the oxygen content is always less than the fidly stoichiometric level, 5 -F 5 instead of 6, and 8 -F 5 instead of 9, for the double and triple perovskites, respectively. A consequence of this is that the coordination enviromnent of at least one set of T ions is 5-fold pyramidal rather than 6-fold octahedral. 

Figure 38 Crystal structures of the A-site ordered double (a), LnBaT205, and triple (b) perovskites. Of particular interest is the occurrence of fivefold square-planar sites...

The A-site ordered double perovskite (Nag jKg jlBajBi Ojj has a superconducting transition temperature T of approximately 27 K. Tlie structure is cubic, space group Imlm (229), a=0.8550nm, with ordering of the (Na, K) and Ba atoms in layers to give a doubled perovskite unit cell. As the formula suggests, one set of A-sites (Al) is only partially occupied and contains the Na " and K " cations and a... 

In order for it to perform the reinforcement function, the geotextde must be allowed to deform to develop its strength. When stabilization of a site occurs, there is consolidation of the sod, and with this comes deformation of the geotextde. Due to the deformation of the geotextde, strength is required to ensure that a site fadure does not occur, ie, there can be a reinforcement component in the stabilization process. 

Filtration installations include wrapping the trench of a pavement-edge drain system to prevent contamination of the underdrain placement behind retaining walls and bridge abutments to prevent contamination of the sand blanket placed against the stmcture to allow dissipation of pore pressures in order to avoid failure of the stmcture as silt fences to allow surface mnoff from a site while retaining the soil suspended in the mnoff and on earth slopes beneath larger stone or other overlay materials to prevent erosion of the slope as water escapes from the interior of the slope. 

The rate of destruction of active sites and pore structure can be expressed as a mass-transfer relation for instance, as a second-order reaction... 

In order to produce a rubbery material the polymer must have a flexible baekbone, be suffieiently irregular in structure to be non-crystalline and also contain a site for cross-linking. These are of course requirements applicable equally to any potential elastomer whether or not it contains fluorine. 

Note that if sticking is controled by site-exclusion only, i.e., if S 6,T) = 5 o(P)(l — 0), this rate is that of a first-order reaction at low coverage. This simple picture breaks down when either the sticking coefficient depends dilferently on the coverage, as it does for instance for precursor-mediated adsorption, or when lateral interactions become important. It then does not make much physical sense to talk about the order of the desorption process. 

Any combination of first-order reactions can be simulated by extension of this procedure. Reversible reactions add only the feature that reacted species can be regenerated from their products. Second-order reactions introduce a new factor, for now two molecules must each be independently selected in order that reaction occur in the real situation the two molecules are in independent motion, and their collision must take place to cause reaction. We load the appropriate numbers of molecules into each of two grids. Now randomly select from the first grid, and then, separately, randomly select from the second grid. If in both selections a molecule exists at the respective selected sites, then reaction occurs and both are crossed out if only one of the two selections results in selection of a molecule, no reaction occurs. (Of course, if pseudo-first-order conditions apply, a second-order reaction can be handled just as is a first-order reaction.)... 

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