Degree Type

Dissertation

Date of Award

1989

Degree Name

Doctor of Philosophy

Department

Chemistry

First Advisor

H. F. Franzen

Abstract

The nature of the order-disorder transitions in a series of NaCl-type materials was investigated using high-temperature powder x-ray diffraction. The Landau theory of symmetry and phase transitions was applied to interpret the continuous thermal symmetry-breaking transitions, and all possible thermal symmetry-breaking transitions starting from the rock-salt type structure were determined. Known examples together with those which have been observed in this work are reported. The diffraction data were analyzed by modified Rietveld-type full profile refinement;In defect NaCl-type materials, Zr[subscript] 1-xS, NbN[subscript] 1-x and Y[subscript] 1-xSe, the order-disorder transitions occur with a variety of ordered superstructures: (a) In the zirconium-sulfur case, Zr[subscript]0.75S, the order-disorder transition occurred at about 1250° to form a superstructure with R3m symmetry by vacancy ordering in alternate planes perpendicular to a body diagonal of the cubic Fm3m cell. (b) In the nitrogen deficient niobium nitride system, the NaCl-type structure of NbN[subscript]0.77 at high temperature transforms continuously to form a tetragonal phase at low temperature by the combination of vacancy ordering and displacive motion. The space group I4/mmm was found. (c) The vacancies in defect yttrium monoselenide order at about 275°, and this ordering transition corresponding to the L point of the Brillouin zone yielded a cubic 2 x 2 x 2 superstructure. Two space groups were found to be possible for this superstructure: Fm3m and Fd3m. Refined values for individual isotropic thermal parameters indicate that the structure with Fd3m symmetry is the correct one;Some other nonstoichiometric compounds with vacancy ordering were investigated: (a) In the Y-Se system a new phase, Y[subscript] 5- xSe[subscript]7, was found and the crystal structure was determined by single-crystal and powder x-ray diffraction studies. (b) A new intermediate phase in the Lu-S system, Lu[subscript] 2+xS[subscript]3, between the sulfur-rich end of the monosulfide homogeneity range and Lu[subscript]2S[subscript]3, was found to be isostructural with Sc[subscript]2S[subscript]3. (c) The previously reported structure of nitrogen deficient (N/Cr = 0.46) Cr[subscript]2N was confirmed and refined based on single-crystal data. (d) A series of samples in the composition range TiO[subscript]1.0~ TiO[subscript]1.5 was examined by high-temperature x-ray diffraction.

DOI

https://doi.org/10.31274/rtd-180813-11665

Publisher

Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/

Copyright Owner

Sung Jin Kim

Language

en

Proquest ID

AAI9003539

File Format

application/pdf

File Size

111 pages

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