High-resolution synchrotron X-ray powder diffraction measurements have been carried out on superconducting MgB2-xCx (0 ≤ x ≤ 0.1) phases. The carbon miscibility in the hexagonal AlB2-type structure is very small, x < 0.04. At higher carbon contents, macroscopic phase separation occurs leading to the coexistence of at least two phases, isostructural with the parent MgB2 compound. The coexisting phases differ mainly in their in-plane lattice constants, suggesting that boron is substituted by carbon in the boron layers, but in different concentrations. The multiphase samples still show a clear one-step superconducting transition and full shielding, χ = -1/4π.
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