Effect of Rb₂CO₃ and Cs₂CO₃ on MgB₂ in polycrystalline bulk samples

MgB₂ polycrystalline bulk samples were prepared by means of reaction of Mg and B powders mixed with various amounts of Rb₂CO₃ or Cs₂CO₃ powders. The a-axis lattice parameter is decreasing in parallel with the increase of the carbonate addition level, more in the case of Cs₂CO₃ than Rb₂CO₃, while the c-axis parameter marginally increases. The extent of the a-axis contraction appears to be larger than expected from carbon doping only. The superconducting transition temperature of MgB₂ decreases upon addition of the carbonate compounds. EDS analysis shows that some Cs and maybe Rb are still present in the samples after reaction. Whereas no evidence was found for segregation of these elements in impurity phases, it is not possible to ensure that Rb and/or Cs actually enter the MgB₂ lattice due to the low amounts used in this study. The critical current density of MgB₂ was improved both in self field and under low applied magnetic field for T ≤ 30 K, with optimum results for 1 mol.% Rb₂CO₃ addition and 1 mol.% Cs₂CO₃ addition respectively. The absolute value of the pinning force is hardly improved. The flux pinning mechanism at low field is similar for all samples and corresponds to the point pinning model behavior but at reduced fields b > 1 the pinning force dependence points towards an increasing contribution of surface pinning. This indicates that the jc improvement at low fields is due to other effects than enhanced flux pinning. It is possible that Rb₂CO₃ and Cs₂CO₃ induce a transient liquid phase during the reaction between the Mg and B powders, which improves the crystallinity and thus the critical current density at low field.