The thermal expansion of ammonium chloride near the order-disorder transition

A theory is presented to explain the behavior of the volume of ammonium chloride near the first order transition at 242°K. This theory is able to account for a first order transition by considering the coupling between an elastic lattice system and the order-disorder system. The lattice system gives rise to non-singular terms in the Helmholz Free Energy of the total system. The order-disorder system gives rise to a singular term in the free energy. The coupling of the two subsystems then produces the possibility of the first order transition. The thermal expansion at constant pressure of ammonium chloride was measured in order to test the predictions of the theory. A three terminal bridge measurement of an electrical capacitance determined by the length of single crystals of ammonium chloride was used to measure the thermal expansion of ammonium chloride. A discussion of the experimental arrangement is given. The measured thermal expansion showed sample dependence. Fits of the theory to the data are presented. These fits show qualitative agreement between theory and experiment, but systematic deviations outside the experimental scatter were found, especially near the transition. Values of the critical exponents alpha+ and alpha- range from about .80 to .97, This is in striking disagreement with theoretical calculations using the Ising model.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Physics

Year dc:date

2011

Author dc:creator

• Fredericks, George Ernest

Contributors dc:contributor

• Slichter, C.P.

• thermal expansion
• ammonium chloride
• order-disorder transition
• first order transition
• elastic lattice system

Statement dc:rights

• 1969_George Ernest Fredericks

Language dc:language

en

Identifier

6072766

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/25801