Nonstoichiometric compounds: A critique of current structural views
The current status of experimental knowledge and theoretical views about nonstoichiometric compounds and heteroionic solid solutions is briefly surveyed, particularly in relation to structural aspects. It is emphasised that there is a little understood transition from the state adequately described by point defect theory, valid for very small deviations from ideal stoichiometry, to the highly organised local structure of crystals with a high formal concentration of defects. The role of short range ordering effects, of microdomains of modified, ordered structure and of defect clusters in different systems is considered. Orderdisorder transformations undergone by nonstoichiometric phases at high temperatures present a special problem. The high temperature phases have very high formal defect concentrations, and the site correlation coefficients found experimentally differ but little from those predicted for completely random structures, although the high defect concentrations imply that all the factors promoting short range ordering must be strong.
Theoretical developments are considered. Treatments of pair interaction effects, and calculations of the energetics of defect interaction and the reconstructive formation of defect clusters hold some promise for theoretical development.
Volume 134, 2022
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