  The crystal structure and physical properties of BaFe2As2, BaCo2As2, and
BaNi2As2 single crystals are surveyed. BaFe2As2 gives a magnetic and structural
transition at TN = 132(1) K, BaCo2As2 is a paramagnetic metal, while BaNi2As2
has a structural phase transition at T0 = 131 K, followed by superconductivity
below Tc = 0.69 K. The bulk superconductivity in Co-doped BaFe2As2 below Tc =
22 K is demonstrated by resistivity, magnetic susceptibility, and specific heat
data. In contrast to the cuprates, the Fe-based system appears to tolerate
considerable disorder in the transition metal layers. First principles
calculations for BaFe1.84Co0.16As2 indicate the inter-band scattering due to Co
is weak.
