Effect of Al Substitution on the Mg Sites in Mg 11 B 2 Prepared Under High-Pressure Conditions

Abstract

The effect of aluminum substitution on superconductivity in Mg11B2 prepared under a high pressure (HP) of P≈3 GPa was compared with that in (Mg1−x Al x )B2 prepared at ambient pressure (AP). In both the HP and the AP samples, the superconducting transition temperature (T c) tended to decrease in the same way due to the substitution, but the width of the superconducting transition (90%–10%), ΔT c, decreased more drastically in the HP sample (HPS) than it did in the AP sample (APS). Moreover, bulk superconductivity lasted up to x=0.3 for the HPS, which is not the case for the APS. A systematic change in the lattice constants due to the substitution of Al for Mg was observed by X-ray diffraction. A scanning electron microscopic image showed that the grains were compactly connected in the HPS, thus explaining the reduced normal-state resistivity.