Journal of Alloys and Compounds 429 (2007) 10–18 On the ternary Laves phases {Sc,Ti} 2 M 3 Si (M = Cr, Mn, Fe, Co, Ni) with MgZn 2 -type X.L. Yan a , Xing-Qiu Chen a , A. Grytsiv a , V.T. Witusiewicz b , P. Rogl a,∗ , R. Podloucky a , G. Giester c a Institut f ¨ ur Physikalische Chemie, Universit¨ at Wien, W¨ ahringerstrasse 42, A-1090 Wien, Austria b ACCESS e.V. RWTH Aachen, Intzestrasse 5, D-52072 Aachen, Germany c Institut f ¨ ur Mineralogie und Kristallographie, Universit¨ at Wien, Althanstrasse 14, A-1090 Wien, Austria Received 24 February 2006; received in revised form 24 March 2006; accepted 27 March 2006 Available online 2 May 2006 Abstract {Sc,Ti} 2 M 3 Si 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 2 Co 3 Si and Rietveld refinements of X-ray powder diffraction data for Sc 2 M 3 Si (M = Fe, Co) and Ti 2 M 3 Si (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 5 Si 3 , have been calculated to compare with available calorimetric data for multicomponent-alloys design. The enthalpy of formation of Ti 2 Fe 3 Si 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 2 Cr 3 Si” 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} 2 M 3 Si. 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 2 Fe 3 Si, TiFe 2 , Ti 2 Fe 3 Si) indicate a significant drop of the local magnetic moment of Fe upon substitution of Fe by Si. © 2006 Elsevier B.V. All rights reserved. Keywords: Transition metal alloys and compounds; Crystal structure; Enthalpy; Calorimetry; Computer simulations 1. Introduction The technological and scientific importance of silicon con- taining Laves phases serves as a basis for continuous scientific interest. A number of early studies [1–12] are concerned with the Laves phases in the ternary systems {Sc,Ti}–M–Si (M = Cr, Mn, Fe, Co, Ni). The stabilization of the Laves phases with MgZn 2 - type structure by metal–silicon substitution was attributed to a reduction of the effective electron concentration [12]. Particular interest was devoted to the composition {Sc,Ti}(M 0.75 Si 0.25 ) 2 for which an ordered variant of the MgZn 2 -type structure (Mg 2 Cu 3 Si-type) was frequently reported [4,11,10]. Recently, excellent mechanical characteristics [13], magnetic and elec- ∗ Corresponding author. Tel.: +43 1 4277 52456; fax: +43 1 4277 9524. E-mail address: peter.franz.rogl@univie.ac.at (P. Rogl). trical properties [14,15] of {Sc,Ti}(M 1-x Si x ) 2 Laves phases provided a stimulus for material researchers for detailed inves- tigation of the phase diagrams and the crystal structure of these phases [14,16–18]. Despite of the achievements obtained, some contradictions can be found. For example, Ti 2 Ni 3 Si with MgZn 2 -type was found in alloys annealed at 750 [5], 1000 [1] and 1100 ◦ C [2,17], as well as in specimens prepared by a laser cladding process, but no Laves phase was claimed to exist in the 900 ◦ C isothermal section of Ti–Ni–Si [16]. Early studies of the scandium silicides report that Sc 2 Co 3 Si [10] and Sc 2 Fe 3 Si [11] crystallize in the Mg 2 Cu 3 Si-type structure, which is a super- structure of MgZn 2 with an ordered distribution of Fe and Si in the 2a and 6h sites, respectively. But a recent detailed reinvesti- gation of Sc 2 M 3 Si (M = Fe, Co, Ni) shows that M and Si atoms are randomly distributed in the 6h and 2a sites [14]. Sc 2 Fe 3 Si is the only compound for which all crystallographic properties are reported [14]. However, crystallographic data on Laves phases 0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.03.086