X-RAY DIFFRACTION STUDIES OF THERMAL MOTIONS IN CRYSTALS. Annual Report No. 1 for June 1, 1958 to May 31, 1959
The general problem is the study, by x-ray means, of thermal vibrations of the atoms in crystals and their interactions with other phenomena of interest in crystal physics. The first experimental studies deal with the temperature dependences of the motions in AgCl as revealed by their effect on the Bragg intensities. Extended consideration was given in the background of the problem as found in the theory of and the literature on thermal motions of atoms, or lattice vibrations, as related in x-ray measurements of Bragg intensities. A method for determining temperature factors (hence Debye- THETA 's) from temperature dependences of x-ray intensities is derived and experimentally illustrated. A second method is suggested. Effects on the data of such things as thermal diffuse scattering (TDS), temperature dependence of primary extinction, and defect concentration are considered in some detail. Possibilities for assessing or studying the same factors through their effects on the apparent Bragg peaks are noted. Equipment design and performance, design of experiments, experimental techniques, and characteristics of the specimen, etc. are discussed in detail, especially in concention with accuracy and reproducibility. Particular attention has been given to means for shaping the specimens without distorting them. A conceptual device referred to as the acceptance region model was developed and utilized to determine the requirements on and the factors affecting the x-ray beam geometries. Intensity data were collected for (hhh), (hhO), and (hOO) reflections of AgCl at 10-degree intervals at 90 to 300 deg K. Variations in diffraction peak breadths with temperatures were observed and are discussed. Reproducibility in the intensity vs. temperature measurements was of about 0.5 to 0.75%. The amplitudes of thermal motions of the silver and chlorine atoms were found to be similar, and in agreement with observations by others. This result is interpreted in mean that the acoustic (rather than optical) modes of lattice vibrations have dominant importance to x-ray observations of the present type in AgCl. An inflection in the semi-log plot of intensity vs. temperature was found in the case of the (111) reflection. The possibility that the inflection may indicate a temperature induced change in bond character will be investigated. The errors introduced by TDS and alpha -doublet separation (both of which are temperature dependent) in the measures of Bragg intensities vs. temperature were partially overcome by a technique involving extrapolation to sin theta = 0. Evidence for anisotropy in the thermal motions of the atoms in AgCl was not conclusive, and further effort will be expended definitely to support or to reject present indications of anisotropy. Existence of anisotropy would probably indicate anharmonic thermal vibrations. Approximate Debye- THETA 's have been determined from the intensity vs. temperature data which are in reasonable agreement with literature values obtained by other means. The general technique of extracting crystal-physics information from careful measurements of the temperature dependence of the Bragg intensities continues to show promise. (auth)
- Research Organization:
- Georgia Inst. of Tech., Atlanta. Engineering Experiment Station
- DOE Contract Number:
- NONR 991(00) AND 991(06)
- NSA Number:
- NSA-14-001911
- OSTI ID:
- 4235044
- Report Number(s):
- NP-8023; Project No. A-389
- Resource Relation:
- Other Information: Project No. A-389. Orig. Receipt Date: 31-DEC-60
- Country of Publication:
- United States
- Language:
- English
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