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

Abstract

AbstractA 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.