Polysilicon hydride
Polysilicon hydrides are polymers containing only silicon and hydrogen. They have the formula (SiHn)x where 0.2 ≤ n ≤ 2.5 and x is the number of monomer units. The polysilicon hydrides are generally colorless or pale-yellow/ocher powders that are easily hydrolyzed and ignite readily in air. The surfaces of silicon prepared by MOCVD using silane (SiH4) consist of a polysilicon hydride.
Polysilicon hydrides are much less thermally stable than the corresponding alkanes (CnH2n+2). They are kinetically labile, with their decomposition reaction rate increasing with increases in the number of silicon atoms in the molecule. Consequently, the preparation and isolation of polysilicon hydrides is difficult for species containing more than a few silicon atoms. Greater catenation of the Si atoms can be obtained with the halides (SinX2n+2 with n = 14 for the fluorides. Thus the polymeric silicon hydrides are formed along with smaller silicon hydride oligomers and hydrogen gas from the spontaneous but slow decomposition, as well as from the accelerated thermolysis, of acyclic and cyclic liquid silanes that are higher in molecular weight than monosilane (SiH4) and disilane (Si2H6). Polysilicon hydrides are intermediates in the high-temperature conversion of mono- and disilane to silicon and hydrogen gas. In the following idealized sequence cyclopentasilane is the polysilicon hydride intermediate:
Polymeric silicon hydrides by be prepared by hydrolysis of certain silicides. Acid hydrolysis of calcium monosilicide (CaSi) produces (SiH2)x. CaSi consists of zigzag silicon chain with the formula (Si2−)n. This chain is preserved in the hydrolysis. This reaction was reported in 1921 by the German chemists Lothar Woehler (1870–1952) and Friedrich Mueller.
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