Heat Capacity of Tetramethylsilane in the Region from 2 to 26 K and Premelting Range

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The heat capacity of tetramethylsilane was measured at temperatures from 2 to 26 K and in the premelting temperature range for two crystalline forms. There are two crystalline forms in solid tetramethylsilane. The heat of fusion and triple point temperature of a stable form were found to be 1611.2±0.9 cal/mol and 174.049 K, while those of a metastable form to be 1396.0±0.8 cal/mol and 170.981 K. Tetramethylsilane was highly purified by preparative gas chromatography. The amount of impurity in the specimen of Si(CH3)4 was estimated to be 0.0046 mol% from measurement of the melting point range for two crystalline forms. The Debye characteristic temperature at 0 K, θD(0), was derived from low temperature heat capacity data, and the θD(T)⁄θD(0) curve was compared with those for C(CH3)4 and CF4. Despite high purity, Cp values for two crystalline forms in the region below melting points increases abnormally with the rise of temperature. The phenomena were interpreted as due to effects of formation of vacancies in the crystals and of melting accompanied by molecular orientation transition; analysis of heat capacity curves for the two crystalline forms was made in the region befow the melting point.
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