Thesolid-state reaction route is the most widely used method for the preparation ofpolycrystallinesolids from a mixture of solid starting materials. Solids do notreact together at room temperature over normal time scales and it is necessary to heat them to much higher temperatures, often to 1000 to 1500 °C, in order for the reaction to occur at an appreciable rate. The factors on which the feasibility and rate of a solid state reaction depend include, reaction conditions, structural properties of the reactants,surface area of the solids, theirreactivity and thethermodynamic free energy change associated with the reaction.[1][2]
These are the solid reactants from which it is proposed to prepare a solidcrystalline compound. The selection of reactant chemicals depends on the reaction conditions and expected nature of the product. The reactants are dried thoroughly prior to weighing. As increase in surface area enhances the reaction rate, fine grained materials should be used if possible.
After the reactants have been weighed out in the required amounts, they are mixed. For manual mixing of small quantities, usually anagatemortar and pestle are employed. Sufficient amount of somevolatile organic liquid – preferablyacetone oralcohol – is added to the mixture to aid homogenization. This forms a paste which is mixed thoroughly. During the process of grinding and mixing, the organic liquid gradually volatilizes and has usually evaporated completely after 10 to 15 minutes. For quantities much larger than ~20g, mechanical mixing is usually adopted using aball mill.
For the subsequent reaction at high temperatures, it is necessary to choose a suitable container material which ischemically inert to thereactants under the heating conditions used. Thenoble metals,platinum andgold, are usually suitable. Containers may becrucibles or boats made from foil. For low temperature reactions, other metals likeNickel (below 600–700 °C) can be used.
The heating programme to be used depends very much on the form and reactivity of the reactants. In the control of either temperature or atmosphere, nature of the reactant chemicals are considered in detail. A goodfurnace is used for heat treatment.Pelleting of samples is preferred prior to heating, since it increases the area of contact between the grains.
The product materials are analyzed using various characterization techniques such asX-ray diffraction (XRD),scanning electron microscopy (SEM),transmission electron microscopy (TEM), etc.