Diffuse Phase Transitions: Specific-Heat Anomaly

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Abstract
A theory for diffuse phase transitions is developed. The isothermal-isobaric partition function is expressed in terms of molecular partition functions and related to the thermodynamic properties of the system. The theory has been applied in detail to the lambda-point anomaly found in ammonium hexafluosilicate at 38.6°K, and it is found that the specific-heat curve can be generated using only one numerical constant, which relates to the free energy of formation of a boundary phase. Results are also presented for five other systems. In the region of the lambda point, the theory predicts that both phases coexist, and that the lines characteristic of both phases would be visible on x-ray diagrams. A quantitative prediction is made of the broadening of the x-ray diffraction lines (for powder diagrams) at the lambda point.

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