Crystal chemistry of carbon-substituted MgB2
Abstract Neutron powder diffraction has been used to characterize a sample of C-substituted MgB 2 synthesized from Mg and B 4 C (with isotopically enriched 11 B). The sample is multiphase, with the major phase [73.4(1) wt%] being Mg(B 1− x C x ) 2 with x =0.10(2). Minor phases include MgB 2 C 2 , Mg, and MgO. The major Mg(B 1− x C x ) 2 phase displays diffraction peak widths as sharp as for pure MgB 2 , 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(B 1− x C x ) 2 samples. The reduction in T c resulting from 10% C substitution is much larger than previously reported, suggesting that previous reports of the C concentration in Mg(B 1− x C x ) 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(B 0.9 C 0.1 ) 2 should be used by future attempts (band structural or otherwise) to explain (i) the dramatic suppression of T c ( T c ≈22 K) and (ii) the persistence of the two-superconducting-gap feature in the specific heat data.