Intralayer carbon substitution in theMgB2superconductor

Abstract

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...

Carbon solubility and superconductivity in MgB2

“…They conclude that C is insoluble in MgB 2 , and that carbon precipitation at the grain boundaries is responsible for the decrease in the superconducting volume fraction. The results of Ahn et.al 21 are similar to that of Takenobu et.al 19 but the T c depression due to carbon substitution is much smaller than that observed by the former for x=0.1. The origin of the difference in the C substitution behaviours can be attributed to arise on account of the difference in the starting materials used in the synthesis.…”

“…In MgB 2 , the substitution of the boron sublattice with C (MgB 2-x C x ), has been carried out by several authors 19,20,21,22 , but the results are at variance with each other. While Takenobu et.…”

“…The a-axis variation is similar to the earlier report of Takenobu et. al 19 for x<0.08, but the continued decrease beyond this composition seen in Fig.2a, clearly suggests the enhanced solubility of C in our samples and the absence of phase separation. The cell volume variation (cf.…”

“…The origin of the difference in the C substitution behaviours can be attributed to arise on account of the difference in the starting materials used in the synthesis. It appears that the solubility is larger if the starting material is amorphous carbon or soot 19,21 . The initial interest in our study was motivated by recent theoretical calculations that predict the possibility of an increase in T c by chemical substitution on the Mg sublattice by a monovalent cation viz., Cu, to increase the hole concentration, along with a C substitution in the B sublattice to increase the stiffness of the Boron layer 6 .…”

“…While Takenobu et. al 19 show that T c decreases with C substitution upto the solubility limit of x=0.10, beyond which superconductivity disappears, Paranthaman et. al 20 claim that T c remains unchanged with C substitution, and that the volume fraction of the superconducting phase decreases with carbon substitution.…”