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The Role of off-Diagonal Disorder on Alloy Phase Stability: An Application To The Ni-Pt System

Published online by Cambridge University Press:  26 February 2011

M. Sluiter  and
P.E.A. Turchi
M. Sluiter
Affiliation:
Lawrence Livermore National Laboratory, L-268, Livermore, CA 94550
P.E.A. Turchi
Affiliation:
Lawrence Livermore National Laboratory, L-268, Livermore, CA 94550
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Extract

The occurrence of ordering in the Ni-Pt alloy system seems to violate theoretical predictions based on a tight-binding description of the electronic structure [1,2,3]. In these descriptions of transition metal alloys, ordering is predicted if the d-band is about half filled, and phase separation is predicted if the d-band is almost empty or almost full. This so-called bandfilling argument clearly fails in the case of Ni-Pt alloys, which order despite an almost filled d-band. The band filling rule is derived under the assumption that off-diagonal disorder (ODD), that is, the difference in d-bandwidths of the constituents, can be ignored. In Ni-Pt that is not the case, the d-bandwidths of the pure elements Ni and Pt are 0.32 Ry and 0.57 Ry, respectively [4]. However, it has been argued that taking ODD into account would result in an even stronger phase separation tendency in the theoretical prediction [2]. Magnetism is not a factor in equiatomic Ni-Pt alloys, and thus can not be invoked to correct the erroneous prediction. Other factors, such as spin orbit coupling in the relativistic description of Pt can change the prediction only if very large spin orbit splitting is assumed on the Pt atom [5].

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 213: Symposium Q – High-Temperature Ordered Intermetallic Alloys IV , 1990 , 101
Copyright
Copyright © Materials Research Society 1991

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References

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