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Structure and Disorder of the Laves Phases in the Co-Nb System

Published online by Cambridge University Press:  01 February 2011

Guido Michael Kreiner ,
Daniel Grüner ,
Yuri Grin ,
Frank Stein ,
Martin Palm  and
Alim Ormeci
Guido Michael Kreiner
Affiliation:
[email protected], MPI für Chemische Physik fester Stoffe, Dresden, United States
Daniel Grüner
Affiliation:
[email protected], Arrhenius Laboratory, Stockholm University, Physical, Inorganic and Structural Chemistry, Stockholm, Sweden
Yuri Grin
Affiliation:
[email protected], MPI für Chemische Physik fester Stoffe, Dresden, Germany
Frank Stein
Affiliation:
[email protected], MPI für Eisenforschung, Düsseldorf, Germany
Martin Palm
Affiliation:
[email protected], MPI für Eisenforschung, Düsseldorf, Germany
Alim Ormeci
Affiliation:
[email protected], MPI für Chemische Physik fester Stoffe, Dresden, United States
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Abstract

A special feature of the Co-Nb system is the occurrence of the three different types of Laves phase with the ideal composition NbCo2. The C36 and the C14 phases are stable only at high temperatures and exhibit small homogeneity ranges, whereas the C15 phase forms with a broad homogeneity range enclosing the ideal composition. In case of C36 and Co-rich C15 the additional Co atoms substitute Nb atoms (Nb1-xCox)Co2. In the C36 phase the Co atoms preferentially occupy one of the two crystallographic Nb sites and are locally displaced by approx. 20 pm from the original Nb positions allowing the formation of favorable short Nb-Co bonds. In Nb-rich C14 only one of two crystallographic sites is occupied by Nb. The Kagomé layers of the Co atoms are distorted in the crystal structures of the hexagonal Laves phases. The deviation from the idealized crystal structure is mainly governed by the valence electron concentration. Quantum mechanical calculations show that the distortion is already an inherent feature of the point defect-free structures.

Keywords

crystallographic structurealloydefects
Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 1128: Symposium U – Advanced Intermetallic-Based Alloys for Extreme Environment and Energy Applications , 2008 , 1128-U08-08
Copyright
Copyright © Materials Research Society 2009

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