Solid decompositions, isomorphic crystals

Each crystalline substance has a unique structure. Groups of compounds classified as isomorphous have similarities of lattice symmetry, but dimensions, and hence interionic forces, are different. Moreover, a particular substance can adopt alternative structures under changed conditions of temperature, pressure, crystallization conditions, presence of impurities, etc. Ordered packing, with symmetrical intracrystalline forces, appears to confer enhanced stability within the bulk solid so that decomposition processes usually occur at surfaces within a restricted reaction zone. Interfaces can be regarded variously as complex imperfections, zones of destabilizing strain, or (product) sites of catalytic activity. 

Triruthenium dodecacarbonyl (Ru3(CO)i2) orange solid, was first (276) prepared by decomposition of Ru(CO)5 but initially incorrectly identified as Ru2(CO)g. This error was first rectified in 1961 when Ru3(CO)i2 was shown (87) to be isomorphous to OS3(CO)i2 and the crystal structure of the latter was determined by x-ray diffraction. Later the structure of Ru3(CO)i2 was determined to be 11 (M = Ru) with the ruthenium triangle having 2.85-A edges (282). In 1966 (48, 50) an improved preparation of Ru3(CO)i2 was developed, utilizing the reductive carbonylation of hydrated ruthenium(III) chloride with carbon monoxide at 10-atm pressure in the presence of zinc, according to the following reaction ... 

There are known cases when formation of a solid solution actually removes the valence tension in the structure and stabilizes a state in which an individual compound does not exist. Thus, pure CuF is unstable against decomposition into elemental Cu and CuFa, but is quite stable as an isomorphic impurity (up to 2 %) in NaF, where A c(Cu+) = 6 according to spectroscopic data [97], although crystal-chemical rules suggest that CuF should belong to the structural type of ZnS. 

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