We introduce carbon onto boron sites in ${\mathrm{MgB}}_2.$ The resulting changes in crystal lattice constants and superconducting transition temperature $T_c$ are characterized by x-ray-diffraction, magnetic susceptibility, and electrical resistance measurements. The consequence of approximately 10% carbon doping of boron sites is a 1% contraction of the intraplane lattice dimension (with no appreciable change in the interplane dimension) and a lowering of $T_c$ by approximately 7 K. The relative contributions to the shift in $T_c$ from lattice contraction and charge transfer are evaluated.

• Received 8 August 2001

DOI:https://doi.org/10.1103/PhysRevB.65.052505