Pressure Dependence of Glass Transition in As2Te3 Glass

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• 作者：K. Ramesh
• 刊名：Journal of Physical Chemistry B
• 出版年：2014
• 出版时间：July 24, 2014
• 年：2014
• 卷：118
• 期：29
• 页码：8848-8853
• 全文大小：257K
• ISSN：1520-5207

文摘

Amorphous solids prepared from their melt state exhibit glass transition phenomenon upon heating. Viscosity, specific heat, and thermal expansion coefficient of the amorphous solids show rapid changes at the glass transition temperature (T_g). Generally, application of high pressure increases the T_g and this increase (a positive dT_g/dP) has been understood adequately with free volume and entropy models which are purely thermodynamic in origin. In this study, the electrical resistivity of semiconducting As₂Te₃ glass at high pressures as a function of temperature has been measured in a Bridgman anvil apparatus. Electrical resistivity showed a pronounced change at T_g. The T_g estimated from the slope change in the resistivity鈥搕emperature plot shows a decreasing trend (negative dT_g/dP). The dT_g/dP was found to be 鈭?.36 掳C/kbar for a linear fit and 鈭?.99 掳C/kbar for a polynomial fit in the pressure range 1 bar to 9 kbar. Chalcogenide glasses like Se, As₂Se₃, and As₃₀Se₃₀Te₄₀ show a positive dT_g/dP which is very well understood in terms of the thermodynamic models. The negative dT_g/dP (which is generally uncommon in liquids) observed for As₂Te₃ glass is against the predictions of the thermodynamic models. The Adam鈥揋ibbs model of viscosity suggests a direct relationship between the isothermal pressure derivative of viscosity and the relaxational expansion coefficient. When the sign of the thermal expansion coefficient is negative, dT_g/dP = 螖k/螖伪 will be less than zero, which can result in a negative dT_g/dP. In general, chalcogenides rich in tellurium show a negative thermal expansion coefficient (NTE) in the supercooled and stable liquid states. Hence, the negative dT_g/dP observed in this study can be understood on the basis of the Adams鈥揋ibbs model. An electronic model proposed by deNeufville and Rockstad finds a linear relation between T_g and the optical band gap (E_g) for covalent semiconducting glasses when they are grouped according to their average coordination number. The electrical band gap (螖E) of As₂Te₃ glass decreases with pressure. The optical and electrical band gaps are related as E_g = 2螖E; thus, a negative dT_g/dP is expected when As₂Te₃ glass is subjected to high pressures. In this sense, As₂Te₃ is a unique glass where its variation of T_g with pressure can be understood by both electronic and thermodynamic models.