ETCHING AND NUCLEATION OF (001) ALPHA-NISO4.6H2O - EXISTENCE AND ROLE OF AN INTERPHASAL ZONE

Etch studies performed on (001) cleavage surfaces of alpha-NiSO4.6H2O reveal that some of the pits formed, named "structured" pits, show higher faceting and symmetry. Ad hoc experimentation indicates that all pits, except the structured ones, form at the sites of emergence of dislocation lines. It is suggested that the latter pits form at surface defects which originate during surface reconstruction following cleavage. Such a reorganization must extend some distance inside the crystal thus creating an interphasal zone, differing structurally from the bulk, which structured pits belong to. Comparisons between the positioning of etch pits and dehydration nuclei on matched cleavage faces indicate that the rather poor correspondence between vacuum generated nuclei and pits improves when the dehydration is performed by dry nitrogen. On the contrary a relevant correspondence between nuclei and structured pits is found when nuclei are allowed to form on already etched surfaces. These results offer the possibility of reconciling an apparent disagreement between previous reports by other authors. They also point out again the importance of the interphasal zone in the interactions of crystals with the exterior.