Superconductivity of MgB₂ after Mechanical Milling

A MgB₂ powder was mechanically milled for different times. X-ray diffraction indicated the formation of an intermediate phase with an unknown structure before formation of amorphous phase. The intermediate structure was superconducting below 18 K, while the amorphous phase was non-superconducting till 4.2 K. After heat treatment at 700 °C, all the mechanically milled samples formed back into the hexagonal MgB₂ structure with T_c = 39 K. X-ray and Raman investigations suggested that the intermediate structure was metastable with a lattice compression, which was probably associated with the reduction in transition temperature.