Optical Properties of Ordered and Disordered Systems.
Abstract
An experimental investigation of the effects of generalized order-disorder transitions on the optical properties and electronic levels in three metallic systems is described. The three metallic systems studied were (1) atomically ordered and disordered Cu_{rm 1-x} Pt_{rm x} alloys (x <=q 0.25), (2) metallic glass a-Fe_{0.8}B _{0.2} and (3) layered and laser quenched Ni-Ti alloys. Measurements of the optical reflectivity in the energy range 1 eV to 5 eV and the electron energy loss spectra in the energy range 3 eV to 90 eV were undertaken to study electronic properties. Sample characterization was achieved by X-ray and electron diffraction for structure determination and X-ray energy dispersive spectroscopy for composition micro-analysis. In dilute (x <=q 0.10) Cu _{rm 1-x}Pt _{rm x} disordered alloys, an indirect evidence suggesting the formation of Pt derived impurity states (virtual bound states) was seen. In alloys with 0.175 <=q x <=q 0.25 certain order-disorder related effects were identified. These included (1) an overall lower reflectivity of the ordered phase compared to the disordered phase and (2) a structure in varepsilon_2( omega) at about 6 eV seen only in the disordered phase. Origins of these effects are discussed in detail. The electron energy loss spectra of the metallic glass a-Fe_{0.8} B _{0.2} agree well with the results of other investigators. Certain features (at 2.5 eV and 4.0 eV) in the loss spectra attributed to volume excitations by other investigators were seen to be due to surface excitations. Directly measured and computed reflectivities agree well with published results; a significant difference between our computed varepsilon_2(omega ) and that of other workers was seen. The energy loss spectra of layered NiTi was seen to be different from a simple superposition of the spectra of Ni and Ti. In addition, a close resemblance between the EELS of layered Ni-Ti and crystalline NiTi alloy was seen. The processes of alloying and amorphization appeared to have no effect on the dominant loss feature in the EELS of these systems. However, the M_{2,3 } core losses of Ti appeared to have undergone a large change in the laser quenched alloy, indicating a drastic change in the joint density of states associated with these transitions.
- Publication:
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Ph.D. Thesis
- Pub Date:
- 1987
- Bibcode:
- 1987PhDT.......134K
- Keywords:
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- Physics: Condensed Matter