  Superconductors comprising 5d transition metals of Ir and Pt have been widely
explored because they have the potential of unique superconductivity caused by
the strong spin-orbit interaction (SOI). We successfully synthesized BaIr2, the
last Laves phase remaining unsynthesized in the MgCu2-type AM2 (A = Ca, Sr, Ba;
M = Rh, Pd, Ir, Pt). BaIr2 was crystallized at 925 C under a pressure of 3.3
GPa via a solid-state reaction between Ba and Ir powders; it was found to have
the longest a-lattice constant of 8.038(1) A among AM2. BaIr2 exhibited bulk
superconductivity at a transition temperature (Tc) of 2.7 K. BaIr2 was found to
have a type-II superconductor with an upper critical field of 67.7 kOe, which
was above the Pauli paramagnetic limit (50 kOe). The electron-phonon coupling
constant and normalized specific heat jump were measured to be 0.63 and 1.2,
respectively, indicating that BaIr2 is a weak-coupling superconductor. The
electronic-structure calculations for BaIr2 revealed that the Ir-5d states are
dominant at the Fermi energy (EF) and the density of states at the EF is
strongly affected by SOI as in the case of CaIr2 and SrIr2.
