(WHY SOLID STATE REACTIONS ARE DIFFICULT??)
“Solid State reactions are difficult because nucleation process is difficult in solid state”
This can be understood by taking an example of MgAl2O4 spinel. MgO and Al2O3 are brought in contact at one face and after providing the appropriate heat , they both form a reactant-product interphase of MgAl2O4 which is in form of nuclei but we know that nucleation process in this case is rather difficult. ( the bond must break and reorganization of atoms should take place in order to form product). Normally the ions of magnesium and aluminium are embedded in their lattice and are not able to jump to and fro from their site to adjacent empty site in the normal condition but when extra temperature or high temperature is applied, the ions thereby getting sufficient thermal energy get escape from their fixed sites and diffuse through the crystal.
Therefore, the nucleation of MgAl2O4 spinel involves some reorganization of oxide ion at the site of potential nucleus together and interchange of Mg+2 and Al+3 ions across the interface between MgO and Al2O3 crystals. In order to increase the thickness of the product layer counter diffusion of Mg+2 and Al+3 must occur through the existing MgAl2O4 product layer to the new interface. At this time two reactions may take place:
1) Between MgO and MgAl2O4 2) Between Al2O3 and MgAl2O4
If we assume that the rate limiting step for the reaction is the diffusion of Mg+2 and Al+3 ions to and from these interfaces which is slow even at high temperature, as the rate decreases the growth of layer( thickness) increases.
“Nucleation is facilitated if there is a structural similarity between reactant and product because this reduces the amount of structural reorganization also the activation energy required for nucleation to occur”…
There is usually clear orientation relationship between the structure of reactant and product.The two types of orientated reactions and transformations have been identified:
- Topotactic Reactions
- Epitactic Reactions
Both type of oriented reactions require structural relationship between the two phases but in epitactic reaction there is a restriction to the actual inter phase between the two crystals hence (hypothetically) the two structures may have a common arrangement of oxide ions at the interphasebut the oxide arrangement to the either side have a different way.Therefore, “epitactic reactions require only a 2D structure, similar at the crystal interphase. The topotactic reactions are more specific than epitactic reactions because they requirestructureal similarities not only at the interphase but also that similarities should continue in the bulk of both crystalline phase”.