Structure Maps of Close-Packed Layered Compounds and DNA

Close-packed layered compounds such as FexGa2S3+x and NxC3Al4+x (x=0–4) are analyzed as chains of covalent-bonded S=Ga(–S–Fe)x–S–Ga=S or AlC(–Al=N)x–Al=C=Al (–N=Al)xʹʹ–CAl with delocalized bonds. The rules for valence compounds allow RM, R(MR)n (n=1,2,3), RMMR, and different combinations and substitutions. The conformation of the chains is characterized by the Jagodzinski–Wyckoff symbols c and h and a similar notation s and t at the occupation of tetrahedral sites. The interactions between the different atoms are analyzed by structure maps with the self-coordination numbers of nearest and next-nearest neighbors as parameters. A similar analysis of 18 different DNA or RNA reveals four different groups within different fields of the structure map similar to solid solutions or nonperiodic polytypes of layered compounds.