Effects of unreacted Mg impurities on the transport properties of MgB2
  We synthesized polycrystalline MgB2 from a stoichiometric mixture of Mg and the 11 B isotope under different conditions. All the samples showed bulk superconductivity with Tc = 38~39 K. The samples containing the least amount of unreacted Mg showed the highest Tc and the sharpest transition width (Delta_Tc). A residual resistivity ratio (RRR) of ~ 5.8, and a magnetoresistance (MR), at 40 K, of 12% were obtained for these samples. Moreover, there was no upturn of resistivity in a low temperature region at 10 Tesla. The samples containing appreciable amounts of unreacted Mg showed quite different behaviors; the values of Delta_Tc, RRR, and MR were much larger. An upturn appeared in resistivity of the samples below about 50 K at 10 T and is thought to be due to the unreacted Mg. 
C. U. Jung, Heon-Jung Kim, Min-Seok Park, Mun-Seog Kim, J. Y. Kim,   Zhonglian Du, Sung-Ik Lee, K. H. Kim, J. B. Betts, M. J...