Ferromagnetism and the formation of interlayer As2-dimers in Ca(Fe1-xNix)2As2

The compounds Ca(Fe1-xNix)(2)As-2 with the tetragonal ThCr2Si2-type structure (space group 14/mmm) show a continuous transition of the interlayer As-As distances from a non-bonding state in CaFe2As2 (d(As-As) = 313 pm) to single-bonded As-2 dimers in CaNi2As2 (d(As-As) = 260 pm). Magnetic measurements have revealed weak ferromagnetism which develops near the composition Ca(Fe0.5Ni0.5)(2)As-2, while the compounds with lower and higher nickel concentrations are both Pauli-paramagnetic. DFT band structure calculations have shown that the As-2 dimer formation is a consequence of weaker metal-metal bonding in the MAs4/4 layers (M = Fe1-xNix) of the Ni-rich compounds, and depends not on depopulation or shift of As-As sigma* antibonding states as previously suggested. Our results also show that the ferromagnetism of Ca(Fe0.5Ni0.5)(2)As-2 and related compounds like SrCo2(Ge0.5P0.5)(2) is not as recently suggested induced by dimer breaking, but arises from the high density of states generated by the transition metal 3d bands near the Fermi level without contribution from dimers.