Structure and Superconducting Properties of "BeB₂"

We present the crystal structure and low temperature electronic transport properties of the intermetallic commonly known as BeB_2. In contrast to the much simpler AlB_2-type structure of the 39K superconductor MgB_2, BeB_2 forms a complex structure type that is nearly unique in nature. The structure has 110.5 atoms per unit cell and a stoichiometry BeB_{2.75}. Polycrystalline Be(^{10.8}B)_{2.75} is superconducting below T_c=0.72K with a critical magnetic field H_{c2}=0.175T. Isotopically pure ^{10.0}B samples of have an enhanced$T_c=0.79K, consistent with a BCS isotope effect. Hall effect measurements suggest that the material is intrinsically compensated.