Influence of radius ratio on the structure of intermetallic compounds of the AB 3 type

Abstract

Attempts have been made to investigate the factors governing the relative stability of the various structure types found by mixing different kinds of atoms, A or B, in a large number of AB 3 intermetallic compounds related to Cu 3Au. A geometric factor, leading to the radius-ratio dependent hexagonality of stacking character, as recognized by Van Vucht and Buschow in rare-earth trialuminides, seems to be predominant in many cases and may be used in predicting ternary phases. This factor has also served as the basis on which trial structures are proposed for three new phases encountered as transition structures between the Cu 3Au and the Ni 3Sn types, namely, (Ba 0.95,Sr 0.05)Pb 3 with a rhombohedral unit cell (a = 12.180 A ̊ , α = 34°43') ; (Ti 0.97,Nb 0.03)Ni 3 with a hexagonal unit cell (a = 5.10 A ̊ , c = 20.80 A ̊ , c/a = 4.08) , and Ti(Ni 0.11,Pt 0.89) 3 with a hexagonal unit cell (a = 5.482 A ̊ , c = 15.65 A ̊ , c/a = 2.85) . The geometric factor may even be seen as influencing a transition of the layer type to be stacked. A second factor, the electronegativity difference of the atoms A and B, is recognized as the principal reason why the radius-ratio rule cannot be given in a quantitative manner. In many cases a third factor, the valence-electron concentration prevails.