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https://hdl.handle.net/2142/30463
Description
Title
Long-range forces in sodium chloride
Author(s)
Marshall, Russell Frank
Issue Date
1971
Director of Research (if dissertation) or Advisor (if thesis)
Granato, A.V.
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
sodium chloride
NaCl crystals
third order elastic constants
Language
en
Abstract
The third order elastic constants C144 and C166 of NaCl have been
determined more accurately than heretofore. The results are
C144 = .185 ± .015 and C166 = -.635 ± .015
in units of 10 12 dynes/cm2.
The NaCl structure provides one with a unique opportunity for the measurement of long range forces in crystals, since the third-order elastic constants C144 , C123 , and C456 are not affected by nearest and next-nearest neighbor central forces. These constants are, however, very small compared to the others and are difficult to measure accurately. A new inertial loading technique has been developed which measures C144 in NaCl much more directly. Inertial stresses are produced by rotating a specimen at high speeds, while the temperature of the specimen and changes in velocity of a ten-MHz sound signal coupled into the specimen are continuously monitored. In order to use this technique, it was necessary to solve the problem of an elastic rotating cubic solid. An experiment using uniaxial loading in two modes specifically optimized toward measuring C144 was also developed and used. Finally, some results of a new and different experiment by Dunham and Huntington on the mixing of ultrasonic beams in solids was combined with the present experimental results to improve the accuracy of the result.
The determined value of C144 is about 30% lower than that expected assuming Coulombic forces only. This discrepancy is greater than the determined experimental error of the measurement, and cannot be attributed to van der Waals forces, thermal vibration effects, central-force repulsion between third-nearest neighbors, or Coulomb effects due to the finite sizes of charged ions. No explanation of the discrepancy has been found, but the possibility of a weak non-central force between ions cannot be excluded. The sound velocity measurement is very sensitive to temperature and by itself provides a practical technique for the measurement to within millidegrees of the temperature of a rotating body for which the third order elastic constants are known.
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