Resonant X-ray diffraction of α-phase Mo0.15Ru0.85Si and crystal stability calculation in Mo–Ru–Si system (FeSi and CsCl types)

Resonant X-ray diffraction at the Mo K edge on α-phase Mo7Ru43Si50 (FeSi type) has been performed in the aim of increasing the scattering contrast between Ru and Mo. The X-ray composition obtained from multipattern refinement with the Rietveld method (Mo7.8(1)Ru42.2(1)Si50) is in agreement with EPMA results (Mo6.9(2)Ru42.8(2)Si50.3(2)), thus validating the method for further crystallographic studies in the Mo–Ru–Si system. R calculations show that the stability of both structural forms of RuSi is related to the presence of either an energy gap (FeSi) or a very low density of states (DOS) at the Fermi level (CsCl). This particular electronic structure behavior is absent in other CsCl-type compounds, since they exhibit a markedly large DOS at EF. Conversely, the corresponding DOS values at EF are found to be smaller in existing FeSi-type phases (TcSi and RhSi). The KKR–CPA results of FeSi-type Ru1−xMoxSi (0 < x < 0.3) alloys clearly reveal that the DOS at EF strongly increases with Mo content, which comes from rigid band structure and decreasing number of electrons as well as higher d-Mo states replacing d-Ru ones. Such a DOS behavior suggests crystal instability with increasing Mo content, which is in line with the limited Mo solubility in Ru1−xMoxSi.