u‰‰ŽÒF Raquel A. RibeiroŽiL“‡‘åŠw‘‡‰ÈŠw•”j “úŽžF ‚P‚QŒŽ‚P‚W“úi–Øj@ŒßŒã‚SŽž‚O‚O•ª‚©‚ç êŠF ÂŽRŠw‰@‘åŠw@—HŠw•”i‘Š–ÍŒ´ƒLƒƒƒ“ƒpƒXj‚k“‚UŠK@‚k‚U‚O‚RŽº ‘è–ÚFu"Pure" and carbon doped MgB2v —vŽ|F We show a study to better understand the large variation in residual resistance ratio (RRR) and 0 values reported in literatures. For such we prepared samples with several types of boron and different stoichiometries. We observed through magnetization and resistivity measurements that there is a monotonic improvement in Tc as the boron purity is increased. Several attempts are being made to introduce doping elements in the structure of MgB2, to help understand their influence on several physical properties of MgB2 such as the lattice parameters, density of states, critical currents and fields, transition temperatures, and so on. We used the compound B4C as a reagent in an attempt to introduce carbon in the boron sites. We tried different reaction temperatures and times to optimize the synthetic route so as to eliminate the impurity phases. We find that within limitations ofour synthesis route the Mg(B0.8C0.2)2 samples reacted at 1100-1200‹C for 24 hours are the closest to being single phase samples and have sharper superconducting transitions. The transition for these samples shifted down by ~17 K (Tc ~ 22 K) with respect to pure MgB2. The a]lattice parameter is approximately 1.2% smaller than for pure MgB2 whereas the c]lattice parameter remains practically unchang --------------------------------- ‹¤ÃF@ÂŽRŠw‰@‘åŠw@‚Q‚P¢‹ICOEƒvƒƒOƒ‰ƒ€