Order-disorder in alloys

Ultra-violet photoemission spectroscopy has been used in this project to investigate the changes in photoemission spectra across the order/disorder transition in Cu-Au alloys. Many obvious changes are observed in the valence band spectra of the different phases, particularly in the area of Au-related states. In Cu3Au, the binding energy of Au-4f core levels in the ordered phase are shifted from the pure metal value by an average of 0.46 +- 0.05eV to increased binding energy. In CuAu, the corresponding shift value in the ordered phase is 0.33 +- 0.05eV, also to increased binding energy. In both disordered Cu3Au and CuAu the value of the Au4f core level shift is reduced by 0.12eV from that of their ordered states. Some of the results are new, and others are in good agreement with previous studies.

The embedded cluster Monte Carlo (ECMC) method which combines the Korringa-Kohn-Rostoker coherent potential approximation embedded cluster method (KKR-CPA-ECM) and the Monte Carlo method has been developed in order to study phase diagrams of binary alloys. The KKR-CPA-ECM provides interchange energies to the Monte Carlo code. In this thesis, a pair-interaction (PI) method is used to provide interchange energies to the Monte Carlo code. The code of the PI method is obtained based on the KKR-CPA-ECM code. The interchange energies of Cu0.5 Zn0.5 alloys are calculated with the PI method. The critical temperature and the phase boundary of Cu-Zn alloys are obtained by carrying out both Monte Carlo calculations with above interchange energies and the ECMC calculations. A comparison between the results of both methods is made.

We attempted to isolate the individual Cu and Au contributions to the local density of states in equi-atomic CuAu using photoemission measurements with synchrotron radiation over the range 21 to 160eV. We compared our experimentally measured Au-5d/Cu-3d photoemission cross sections ratio with those of Yeh and Lindau (Atomic Data Nucl. Data Tables 32, 1, (1985)) and of Shirley et al (Physica Scripta 16, 398, (1977)) and found agreement with the former, suggesting difficulty in extracting the Au local density of states. Hence, while others have suggested it is possible to isolate the Au local density of states at 50eV using similar methods, we found we could not. The density of states calculation and the photocurrent calculations at 50eV confirmed this by showing significant overlap of the Au and Cu states. Since the Au-5d states experience a Cooper minimum around 160eV, the experimental spectrum of CuAu at this photon energy contains very little Au contribution, thus allowing us to "experimentally determine" the Cu local density of states. The photocurrent calculations at 160eV confirmed the lack of Au contribution and comparison of experimental spectra with the photocurrent calculations at 160eV showed good agreement.

The techniques of x-ray and ultra-violet photoelectron spectroscopies have been used to investigate the electronic structures in ordered (L10-structure) and disordered (fcc) CuAu. Large changes are observed in the valence band regions of the different phases, particularly in the vicinity of the Au-related states. The binding energies of the Cu-2p and Au-4f core levels in the ordered phase are shifted from the pure metal value by 0.27 +- 0.05 eV and 0.30 +- 0.05 eV, respectively. A small reduction of the Au-4f core level shift is found in the disordered phase. Details of the valence band structure and the shifts for the ordered phase are in accord with recently published calculations of the density of states. Investigations of the composition of the surface region using Auger electron spectroscopy indicate that there is excess Au in the surface region, in agreement with previous studies.