Preparation and characterisation of Ru doped MgB2

Highlights: •Polycrystalline ruthenium doped MgB2 samples were prepared by solid state route. •Up to 1.5 at.% Mg can be replaced by Ru in the MgB2 lattice. •Tc decreases at a rate of 1.0 K/at.% Ru upon doping. -- Abstract: Samples with Mg1−xRuxB2 nominal stoichiometry were prepared by sintering at 800 °C. The critical transition temperature decreases up to a substitution level of x ≈ 0.015. A maximum solubility limit slightly in excess of 1 at.% Ru for Mg is confirmed by energy dispersive spectroscopy measurements. From and beyond this limit, an unidentified phase appears in the X-ray diffraction patterns. Interestingly, the lattice parameters of the MgB2 phase are constant up to x = 0.015, but start to decrease for higher Ru contents. This feature could be related to the fact that a Mg–Ru impurity phase results in a Mg-deficient (Mg,Ru)B2 matrix