The thermal expansion of ammonium chloride near the order-disorder transition
Fredericks, George Ernest
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Permalink
https://hdl.handle.net/2142/25801
Description
Title
The thermal expansion of ammonium chloride near the order-disorder transition
Author(s)
Fredericks, George Ernest
Issue Date
1969
Doctoral Committee Chair(s)
Slichter, C.P.
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Date of Ingest
2011-07-13T19:23:14Z
Keyword(s)
thermal expansion
ammonium chloride
order-disorder transition
first order transition
elastic lattice system
Language
en
Abstract
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.
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