The first order phase transition and superconductivity in BaNi2As2 single crystals

We report the synthesis and physical properties of single crystals of stoichiometric BaNi2As2 that crystallizes in the ThCr2Si2 structure with lattice parameters a = 4.112(4) angstrom and c = 11.54(2) angstrom. The resistivity and heat capacity show a first order phase transition at T-0 = 130 K with a thermal hysteresis of 7 K. The Hall coefficient is weakly temperature dependent from room temperature to 2 K where it has a value of -4 x 10(-10) Omega cm Oe(-1). The resistivity, ac susceptibility, and heat capacity provide evidence for bulk superconductivity at T-c = 0.7 K. The Sommerfeld coefficient at Tc is 11.6 +/- 0.9 mJ mol(-1) K-2. The upper critical field is anisotropic with initial slopes of dH(c2)(c)/ dT = -0.19 T K-1 and dH(c2)(ab)/ dT = -0.40 T K-1, as determined from the resistivity.