The ordering of the σ phases Cr₂Ru and Cr₂Os

Abstract

Two a-phase structures, Cr2Ru and Cr2Os, have been studied using X-ray single-crystal diffraction techniques. The ordering determined has been compared with those of other a phases and A15 structures. Although atoms of both kinds occupy every atomic position, a significant departure from complete disorder was detected. This is particularly important in the 14-fold J atomic positions which are associated with bond angles of 180°: the marked preference of the smaller Cr atoms for these sites indicates that their occupancy appears to be determined by the electronic A.]

Site occupation in the Cr-Ru and Cr-Osσphases

“…Such is the case for the phase with approximate composition Cr 2 Ru and Cr 2 Os where the early TM Cr is smaller in size than the late TMs Ru and Os. Experimental data 25,26 on these systems gives a more or less random site occupation, in contradiction with the earlier stated empirical rules. 11 Here, we will seek to clarify these results theoretically employing ab initio density-functional calculations and statistical thermodynamic lattice-gas models.…”

“…25. The experimental determinations are close, all sites agreeing within a few percent with the exception of the 8i 2 site where Alte da Veiga et al 26 give an occupation that is 9a / o higher in Os than Joubert. 25 The experimental measurements were carried out on samples annealed at 1673 ͑Ref.…”

“…Figure 5 shows that the site occupation in Cr 0.668 Ru 0.332 is rather intermixed at temperatures of interest. The horizontal lines indicate the site occupation as an average of three experimental measurements, 25,26,42 see Table IV in Ref. 25.…”

“…The site occupation in Cr 2 Ru and Cr 2 Os as computed here shows a rather intermixed behavior just as has been found experimentally. 25,26,42 There is no strong correlation with either coordination number or approximate point symmetry.…”