Structure Control and Synthetic Strategies

The X-ray diffraction data indicates over bonding on the titanium cations, or that the shortened Ti-O bonds are under compressive stress. The neutron diffraction data, on the other hand, gives a titanium valence closer to the formal charge of 4-1-. The structure from which these bond distances were taken, presumably, should be the least strained. 

In certain respects, synthetic reactions involving nonmolecular inorganic solids are not so different from synthetic reactions in other fields of chemistry. Specific phases, often with crystal stmctures close to that of the desired products, are chosen as the starting materials. Stmctural transformations are then carried out that modify the parent phases in some 

Two types of transformations can be very broadly distinguished. The first is the formation of a solid solution, in which solute atoms are inserted into vacancies (lattice sites or interstitial sites) or substitute for a solvent atom on a particular sublattice. Many types of synthetic processes can result in this type of transformation, including ion-exchange reactions, intercalation reactions, alloy solidification processes, and the high-temperature ceramic method. Of these, ion exchange, intercalation, and other so-called soft chemical (chimie douce) reactions produce no stmctural changes except, perhaps, an expansion or contraction of the lattice to accommodate the new species. They are said to be under topotactic, or topochemical, control. 

Consider the brownmillerite (A2B2O5) stmcture examined earlier. This structure consists of alternating layers of vertex-sharing BO3 octahedra and BO2 tetrahedra. It is immediately obvious to the synthetic strategist that one has the opportunity to place cations with specific coordination preferences exclusively in one type of layer to give A2 B0 B 02), as was previously discussed. For example, in Ca2MnGaOs, Mn exclusively occupies the BO3 layers. In this case, that would have been predicted by a consideration of the CFSE. 

In some cases, secondary forces may exert a sizable influence on the coordination environment as well. For example, it was seen earlier how the second-order JT effect frequently manifests itself as a displacement of transition metals from the center of an octahedron. The phenomenon is only observed for metals with low d electron counts. This could be used advantageously in synthetic strategies where the goal is to selectively place cations in specific sites within a stmcture. 

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