The heat capacity of ammonium chloride near the order-disorder transition

A theory is presented to explain the behavior of ammonium chloride near the first order phase transition at 240K in terms of· a physical model that has singular and nonsingu1ar parts of the free energy coupled together. The singularity is normally associated with a second order phase transition, but it can lead to a first order transition in the model discussed. The heat capacity at constant pressure is measured in the neighborhood of the transition in order to test the predictiors of the theory. An A.C. technique is used to measure the heat capacity of ammonium chloride single crystals, and a discussion of the experimental factors involved in making the measurement is given. The heat capacities measured showed a strong sample dependence. Values are obtained for parameters occurring in the theory and are compared with those obtained from other experiments on ammonium chloride and from theoretical predictions. In particular, the volume dependence of the interaction responsible for the transition is in qualitative agreement with a prediction based upon an octopo1e-octopo1e, electrostatic interaction. The critical exponents a+ and a are found to be in the neighborhood of .7, which is quite different from. the predictions of Ising model calculations.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Physics

Year dc:date

2011

Author dc:creator

• Schwartz, Paul Michael

Contributors dc:contributor

• Slichter, C.P.

• heat capacity
• ammonium chloride
• order-disorder transition
• first order phase transition

Statement dc:rights

• 1969 Paul Michael Schwartz

Language dc:language

en

Identifier

6075554

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/25796