The nucleus in solid state reactions : towards a definition
Abstract The term nucleus , as applied to solid state reactions, defines a product aggregate capable of sustained and unrestricted growth by continual acquisition of material at a chemically active periphery that constitutes the reactant/product interface . Such an interface is any laminar boundary contact zone, at the surface of a nucleus, within which chemical changes and/or phase transformations preferentially occur. Nucleation , the initial establishment of a new and discrete particle of product (nucleus) within the solid reactant, is usually identified as the crystallographic transformation or structural reorganization that generates the smallest possible quantity of the product phase. Nucleation sites are the limited number of sub-microscopic domains within the reactants at which nucleation most readily occurs: these are structures of locally enhanced reactivity and usually identified as crystal imperfections at surfaces (damage, impurity, etc.). Precursor reactions, preceding the establishment of a nucleus, may yield product that is initially capable of retaining structural features of the reactant (topotaxy). Each individual assemblage, possessing enhanced reactivity but not yet transformed into product, may be termed an incipient nucleus . Nucleation is identified as the subsequent transformation by which this initial accumulation of reacted material is organized or recrystallized into the structure characteristic of the product phase(s): this product particle is the germ nucleus . Interfacial free energy properties of these very small germ nuclei are believed to reduce their reactivity so that their rates of initial development may be relatively slower than that subsequently achieved when these become the larger developing assemblages, the growth nuclei . A growth nucleus, often simply referred to as a nucleus, is the aggregate total assemblage of product or recrystallized material that accumulated following a single nucleation event. We believe that communication in the subject would be clarified if the term nucleus was always used with a descriptive adjective, examples being: nucleation site, incipient nucleus, germ nucleus, growth nucleus, nucleus interface. The nomenclature is intended to be equally applicable to reactions involving a single solid, and to reactions of a solid with another reactant, gas, liquid or solid. Product formation occurs within the active interface of reactant-product contact that defines the nucleus periphery. This interface advances through unchanged reactant, usually without restriction, and is expected to propagate to all reactant crystal edges, or its progression may be locally terminated as a consequence of coalescence with a neighbouring growth nucleus. Interface movement may proceed at different rates in different crystallographic directions and may be subject to diffusion control, of reactants to the interface or products away, as nucleus growth continues. The product assemblages that constitute a single nucleus may remain coherent or the product particles may become progressively dispersed as the nucleus grows. Interfacial strain accompanying growth may sometimes result in product detachment or even crystallite disintegration.