About SnF2 stannous fluoride. IV. Kinetics of the α → γ and β, γ → α transitions |
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Authors: | Georges Dénès |
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Institution: | Université de Rennes, Laboratoire de Chimie Minérale D, Laboratoire Associé au C.N.R.S. No. 254, Avenue du Général Leclerc, 35042 Rennes Cedex, France |
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Abstract: | The kinetics of the reconstructive first order phase transitions α → γ (upon heating) and β, γ → α (upon cooling) of SnF2 were studied by quantitative X-ray powder diffraction. The main feature of the kinetics is that these transformations are incomplete over a wide temperature range. Consequently it is not possible to fit the experimental data using the usual solid state rate expressions. Although these transitions are not reversible (large hysteresis) the empirical use of a kinetic model of a reversible transformation provides a good fit. The α → γ transformation was also investigated by D.T.A.; this transition was found to be greatly influenced by particle size: the transition temperature increases with grain size, and its enthalpy decreases. The X-ray investigation revealed that the process of the α → γ transformation involves an intermediate noncrystalline phase named the “transition phase.” It is suggested that the transition proceeds through a process of fragmentation-reconstruction. |
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