Structure and Disorder of the Laves Phases in the Co-Nb System
- PDF / 894,134 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 23 Downloads / 198 Views
1128-U08-08
Structure and Disorder of the Laves Phases in the Co-Nb System Guido Kreiner1, Daniel Grüner2, Yuri Grin1, Frank Stein3, Martin Palm3 and Alim Ormeci1 1
Max-Planck-Institut für Chemische Physik fester Stoffe, Dresden, Germany Department of Physical, Inorganic and Structural Chemistry, Arrhenius Laboratory, Stockholm University, Stockholm, Sweden 3 Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf, Germany
2
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. INTRODUCTION Laves phases [1-4] have been studied intensively to understand the fundamental aspects of phase stability. However, simple factors governing the crystal structure type of geometric (rA/rB) and electronic (valence electron concentration, vec, and electronegativity difference, ∆χ) nature have proven to be helpful in predicting the occurrence and stability of the Laves phases in strictly limited cases [5] only. In general, phase stability and properties of Laves phases are difficult to forecast, especially the origin of the homogeneity ranges and disorder phenomena. In order to understand the nature of Laves phases, structure and disorder phenomena of Laves phases in the Co-Nb system have been studied in this work. The binary system Co−Nb is particularly suitable to throw light on the stability of the different polytypes due to the coexistence of the C14, C15 and C36 Laves phases. Recently, the phase diagram of the Co-Nb system was reinvestigated [6]. The C36 and the C14 phases form strongly off-stoichiometric at lower and higher Nb concentration, respectively, and only at high temperatures. Both phases exhibit a small homogeneity range while the room temperature phase C15 shows a large asymmetrical homogeneity range enclosing the stoichiometric composition NbCo2. Here, we report on the interplay between chemical disorder, site preferences and the homogeneity ranges and on locally driven and cooperative deviations from the idealized crystal structures.
EXPERIMENTAL DETAILS Samples of Nb and Co were prepared by arc-melting from the elements
Data Loading...
