Solids mixed valence halides

Solids with octahedral, tetrahedral, square planar, and linear metal coordination geometries, including many different types of polyhedra connectivity modes, are amenable to dimensional reduction. Tulsky and Long compiled an enormous database of over 300 different allowed combinations of M and X and over 10,000 combinations of A, M, and X. The formalism may be extendable to quaternary phases as well. However, frameworks containing anion-anion linkages, anions other than halides, oxide, or chalcogenides, nonstoichiometric phases, and mixed-valence compounds were excluded from their initial study. 

The silver halides are semiconductors, that is, their electronic structure is best described by the solid state model in which atomic orbitals overlap extensively to form occupied valence bands together with conduction bands that are unoccupied in the absence of external perturbation (i.e. in the dark at normal temperatures). All the halides absorb light in the UV region of the electromagnetic spectrum, with AgBr, Agl and mixed salts also absorbing into the visible out to ca. 500 nm (Fig. 11.7). 

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