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.The KKR 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.