On the ternary Laves phases {Sc,Ti} 2M 3Si (M = Cr, Mn, Fe, Co, Ni) with MgZn 2-type

Abstract

{Sc,Ti} 2M 3Si alloys (M = Cr, Mn, Fe, Co, Ni) were investigated by X-ray powder and single crystal diffraction, optical microscopy and electronic density-functional theory. Structure determination from X-ray CCD-data from a single crystal of Ti 2Co 3Si and Rietveld refinements of X-ray powder diffraction data for Sc 2M 3Si (M = Fe, Co) and Ti 2M 3Si (M = Cr, Mn, Fe, Co, Ni) revealed in all cases isotypism with the C14 MgZn 2-type Laves phase: Sc (or Ti) atoms fully occupy the 4f sites, whereas M and Si atoms share the 6h and 2a sites in various ratios, in good agreement with ab inito results. Furthermore, the series of enthalpies of formation for these compounds as well as for the two binary intermetallics, ScFe 2 and Sc 5Si 3, have been calculated to compare with available calorimetric data for multicomponent-alloys design. The enthalpy of formation of Ti 2Fe 3Si of −55.9 ± 1.6 kJ/mol of atoms measured in an isoperibolic calorimeter agrees well with the calculated value of −52.6 kJ/mol of atoms. The chemical instability of “Sc 2Cr 3Si” with the MgZn 2-type structure was theoretically confirmed by the calculation considering the competing phases in equilibrium. The general trends of thermodynamic stabilities versus atomic number of M were discussed for {Sc,Ti} 2M 3Si. Our structural models confirm the fact that M/Si substitution of 16.7 at.% Si in the binary Laves phase stabilizes for all these ternary compounds a MgZn 2-type structure irrespective of the crystal structure of the binary parent Laves phase. Finally, spin-polarized calculations for four selected compounds (ScFe 2, Sc 2Fe 3Si, TiFe 2, Ti 2Fe 3Si) indicate a significant drop of the local magnetic moment of Fe upon substitution of Fe by Si.