Study of intermetallic compounds isostructural to MgB₂

The small intergrain effect of MgB₂ on supercurrent makes it one of the most promising candidates for superconducting conductors due to its easier processing and the associated lower manufacturing cost. Unfortunately, the superconducting transition temperature T_c is only 40 K. However, band-structure calculations predict that a higher T_c than that of MgB₂ is possible for isostructural and isovalent intermetallic compounds with greater lattice parameters or greater unit cell volumes. The prediction appears to be consistent with the negative pressure effect on T_c observed. The substitution of the larger Ca-ions for the smaller Mg-ions has thus been suggested to raise T_c, but not yet realized. Alternatively, we have synthesized and studied a series of binary and pseudobinary intermetallic compounds, AGa₂, AGa_2-xSi_x, and AAl_2-xSi_x, where A = Ca, Sr, or Ba, which are isostructural to MgB₂ and have greater lattice parameters than MgB₂. In spite of the greater lattice parameters, AGa₂ are not superconducting. However, all pseudobinary compounds AGa_2-xSi₂ and AAl_2-xSi₂ for 0.6 < x < 1.5 are superconducting. Superconductivity is reported for the first time in SrAlSi, with T_c of 5.1 K. Although T_c varies with x in a similar fashion for all members of the series, no specific correlation between T_c and lattice parameters or ionic mass is observed. The maximum T_c of these compound series with different A's varies between 5.5 and 7.8 K, much lower than that of MgB₂. The results strongly suggest the unique role of B in the superconductivity of C32 intermetallic compounds. They also demonstrate that factors additional to the lattice parameters and densities of states must play an important role, and that the rigid-band model is not sufficient to account for the observations.