Influence of Rb, Cs and Ba on superconductivity of magnesium diboride

Magnesium diboride has been thermally treated in the presence of Rb, Cs, and Ba. Magnetic susceptibility shows possible onsets of superconductivity in the resulting samples at 52 K (Rb), 58 K (Cs) and 45 K (Ba). Room-temperature $^{11}$B NMR indicates to cubic symmetry of the electric field gradient at boron site for the samples reacted with Rb and Cs, in contrast to the axial symmetry in the initial MgB$_2$ and in the sample treated with Ba.