Another significant advantage of the molecular precursor approach is the production of metastable materials. Since the molecule-to-material transformations operate far from equilibrium conditions, kinetically rather than thermodynami-cally favored products are formed. Barron et al. have shown that the easy formation of metastable cubic GaS is determined by the cubane structure of the precursor compound, [('Bu)GaS]4, which is preserved under MOCVD conditions. The structure of the precursor was determined and found to be identical at 25 and 250 °C by single-crystal X-ray diffraction and gas-phase electron diffraction studies [149, 330]. Their work on several other gallium thiolates and selenolates is an elegant effort to elucidate the role of precursor structure in determining the phase of the material. The different GaS compounds (Scheme 6) were found to produce different solid-state phases in the films deposited by their chemical vapor deposition), which may be related to the symmetry of the core structures. For example, chemical vapor deposition) of the dimer [(Bui)2Ga(SBui)]2 (I), possessing a Ga2S2 core of C2„ symmetry, yields poorly crystalline hexagonal GaS. On the other hand, the highly symmetrical Ga4S4 core of [('Bu)GaS]4 (II) provides a template for a high-symmetry solid phase (cubic GaS). In a similar manner, the lack of any but C3 symmetry in heptameric [('Bu)GaS]7 (III) should result, if the core is retained during the deposition, in an amorphous film. The Ga7S7 core of [('Bu)GaS]7 has a C3 axis with no possibility of ordered close packing. Thus, during CVD a random array of Ga and S atoms is incorporated into the film. Indeed, an amorphous film is obtained when [('Bu)GaS]7 is used in the MOCVD process [18].

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