X-Ray Studies of Metallic Single Crystals

Citation

Jacobs, Robert Byron (1935) X-Ray Studies of Metallic Single Crystals. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/4fms-9r20. https://resolver.caltech.edu/CaltechTHESIS:10142024-203231460

Abstract

Problem (1)

Some improvements in the use of the Siegbahn precision spectrometer employing two crystals (a specimen to be measured and a standard comparison crystal) are described. These enabled the author to ascertain changes in lattice spacing smaller than one part in ten thousand. Two series of bismuth single crystals containing impurities up to five atomic percent were studied. These were bismuth-tin and bismuth-lead. Roughly, up to these concentrations of impurities the change in lattice spacing went linearly with the amount of added impurity. However, the accuracy was still not sufficient to decide whether or not the first few foreign atoms (first tenth of a percent or so) go into the crystal lattice as do the subsequent atoms, there being reasons to believe that they do not

Problem (2)

A new X-ray cryostat is described which enables one to keep a crystal at any desired temperature down to about 25 degrees absolute using liquid hydrogen. The method of temperature control is by regulation of the flow of the heat-transfer gas (helium) which is circulated through the apparatus. A fast ionization method for measuring small changes in the Bragg angle of reflection and hence small changes in the crystal lattice is explained. This makes it possible to measure coefficients of expansion by the x-ray method in these low temperature regions. A series of measurements (in three orders for purposes of counterchecking) of lattice changes and coefficients of expansion are given covering the temperature range from 25 to 535 degrees absolute. A test of the Grueneisen rule is made, values of the specific heat at low temperature being extrapolations from the extremely low temperature values taken at Leiden. The rule is found to have only limited applicability, failing above 100 degrees absolute, and failing to show similar discontinuities in specific heat values where discontinuities occur in expansion coefficient values. The question as to whether or not there is a difference between x-ray and macroscopic expansion values is believed to be settled.

Problem (3)

A method for using the apparatus described in problem (2) for intensity measurements is outlined, the chief feature being the automatic integration of the intensity under a given line. Measurements are taken throughout the same temperature region as in problem (2), namely from 25 - 535 degrees absolute. The exponential form of the Debye heat factor is found to be obeyed below 480° absolute. Above that point there is noticeable hysteresis and an increased intensity with increasing temperature. No discontinuities in intensity occur where they are found in the coefficient of expansion measμrements.

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