We report that the ternary MgB 2-x C x compounds adopt an isostructural AlB 2 -type hexagonal structure in a relatively small range of nominal carbon concentration, x< 0.1. The lattice parameter a decreases almost linearly with increasing carbon content x, while the c parameter remains unchanged, indicating that carbon is exclusively substituted in the boron honeycomb layer without affecting the interlayer interactions. The superconducting transition temperature T c , determined by magnetometry experiments, also decreases quasilinearly as a function of the carbon concentration. The structural and electronic behavior of MgB 2-x C x displays a remarkable similarity with the isoelectronic Mg 1-x Al x B 2 despite the different substitution sites.Discovery of superconductivity in MgB 2 at T c = 39 K (1) is attracting wide attention because of the simplicity in the chemical composition, the crystal and electronic structure of the system and its highly promising potential applications. Detailed information on the properties of MgB 2 , particularly related to the nature of superconductivity, is being currently rapidly accumulated by means of structural and electronic probes on the parent compound, MgB 2 . An alternative approach is to synthesize related materials by partial chemical substitution on either the Mg or the B interleaved layers and follow the evolution of the properties.MgB 2 adopts a hexagonal structure (AlB 2 -type, space group P6/mmm) (2) which is analogous to intercalated graphite with all hexagonal prismatic sites of the primitive graphitic structure (found in hexagonal BN) completely filled and resulting in two interleaved B and Mg layers. In addition, allowing for full charge transfer from Mg to the boron 2D sheets, the latter are themselves isoelectronic with graphite. Also, it has been known that various metal borides form an isostructural series of compounds (2). Moreover, theoretical calculations predict that substitution of Mg results in 2 significant changes of the density of states at the Fermi level without introducing any disorder, potentially allowing access to increased T c (3-5). These structural features have motivated attempts to substitute Mg with alkali (6), alkaline-earth, group III metals (7,8), and other elements. However, a significant difference of MgB 2 from graphite intercalation compounds is that both the structural and electronic properties are substantially more isotropic. For example, high pressure synchrotron X-ray diffraction experiments revealed that the isothermal compressibility of MgB 2 is only moderately anisotropic between the boron intra-and inter-layer directions (9,10). Also, band structure calculations showed that the electronic states of this superconductor are essentially three-dimensional (3,4,5,11). Thus, the substitution of Mg sites is not entirely analogous to the case of intercalation of the strongly bonded strictly 2D graphitic sheets (12). Instead, appropriate substitution on the more weakly bonded boron sheets offers an alternative route of modu...
