Crystal Chemistry of Carbon-Substituted MgB2

TL;DR

This study uses neutron diffraction to analyze C-substituted MgB2, revealing structural details, C homogeneity, and a significant Tc reduction at 10% C substitution, challenging previous estimates.

Contribution

It provides precise structural characterization of C-substituted MgB2, clarifies C concentration effects on lattice parameters and Tc, and suggests a revised stoichiometry for future research.

Findings

C substitution causes lattice contraction and slight expansion, with no C ordering.

Tc drops significantly at 10% C, more than previously reported.

Mg site occupancy is nearly full, supporting the Mg(B0.9C0.1)2 stoichiometry.

Abstract

Neutron powder diffraction has been used to characterize a sample of C-substituted MgB2 synthesized from Mg and B4C (with isotopically enriched 11B). The sample is multiphase, with the major phase [73.4(1) wt.%] being Mg(B1-xCx)2 with x=0.10(2). Minor phases include MgB2C2, Mg, and MgO. The major Mg(B1-xCx)2 phase displays diffraction peak widths as sharp as for pure MgB2, indicating good C homogeneity. There is no evidence for ordering of the substituted C atoms or distortion of the host structure other than contraction of the a axis and slight expansion of the c axis. The observed changes in lattice parameters vs. C concentration provide a means for estimating the C concentration in other Mg(B1-xCx)2 samples. The reduction in Tc resulting from 10% C substitution is much larger than previously reported, suggesting that previous reports of the C concentration in Mg(B1-xCx)2 are overestimated. The Mg site occupancy is determined to be 0.990(4) which is consistent with full Mg occupancy. Given these results, the stoichiometry Mg(B0.9C0.1)2 should be used by future attempts (band structural or otherwise) to explain (i) the dramatic suppression of Tc (Tc ~ = 22 K) and (ii) the persistence of the two-superconducting-gap feature in the specific heat data.