Melting and related precursor cooperative phenomena in chemically bonded assemblies

A number of experimental studies of condensed matter assemblies with different types of chemical bonding will provide the focus of this work. Condensed compounds X(CH3)4, with X = C,Si or Ge, are the first of such assemblies; two phase boundaries in the pressure temperature plane being studied: melting and a solid phase boundary heralding orientational disordering of molecules still however on a lattice. Secondly, directionally bonded d-electron transition metals such as Ni, Pd and Nb will be treated. Here, melting is the main focus, but the precursor transition is now the separation of a high-temperature ductile solid from a lower temperature mechanically brittle phase. A dislocation-mediated model of these transitions is discussed, leading into the third area of covalently bonded solids graphite and silicon. Here topological defect models again provide the focus; both dislocations and rotation-dislocations now being invoked. Some qualitative suggestions are made to interpret the melting curve of graphite subjected to high pressure. (author)