The C14 phase Nb(Cr 1Ϫx Co x ) 2 forms within a broad homogeneity range of 0.127(3) Յ x Յ 0.937(8) at 1100°C. The distribution of chromium and cobalt atoms on the crystallographic positions 2a and 6h has been studied by single crystal structure analysis and a statistical mechanics approach using energies from 637 first-principles full potential electronic structure calculations. For the first time a preferential occupation of the minority component on the 2a site has been observed. The calculations support a preferential site occupation with a reversal close to equal chromium and cobalt content.

YFe 2 Al 10 ,orthorhombic, Cmcm (no. 63), a =8.9654(2) Å,

The ternary Nb-Cr-Al phase diagram exhibits extended phase fields of the cubic C15 and the hexagonal C14 Laves phases Nb(AlxCr1-x)2. A number of Nb-Cr-Al alloys were prepared by levitation melting and annealed at temperatures between 1150 and 1450 °C for up to 1500 h. Isothermal sections of the ternary Nb-Cr-Al phase diagram at 1150, 1300 and 1450 °C were obtained from electron probe microanalysis, X-ray powder diffraction and metallographic investigations in order to study the effect of Al on the stability and structure of the Laves phases. The C14 Laves phase in the Nb-Cr-Al system can dissolve up to 45 at.% Al by substituting Cr with Al on the two different crystallographic B-sites 2a and 6h of the C14 AB2 unit cell. The site occupations of the Al and Cr atoms on these two B-sites were determined by Rietveld analysis using the program FullProf. The experimental site occupation factors were compared to site occupation factors computed by a statistical mechanics approach based on first-principles electronic structure calculations. The experimental as well as the calculated site occupation factors indicate a preferred occupation of the 2a site by Al.