  The compounds Ca(Fe1-xNix)2As2 with the tetragonal ThCr2Si2-type structure
(space group I4/mmm) show a continuous transition of the interlayer As-As
distances from a non-bonding state in CaFe2As2 (dAs-As = 313 pm) to
single-bonded As2-dimers in CaNi2As2 (dAs-As = 260 pm). Magnetic measurements
reveal weak ferromagnetism which develops near the composition
Ca(Fe0.5Ni0.5)2As2, while the compounds with lower and higher nickel
concentrations both are Pauli-paramagnetic. DFT band structure calculations
reveal that the As2-dimer formation is a consequence of weaker metal-metal in
MAs4-layers (M = Fe1-xNix) of Ni-richer compounds, and depends not on
depopulation or shift of As-As antibonding states as suggested earlier. Our
results also indicate that the ferromagnetism of Ca(Fe0.5Ni0.5)2As2 and related
compounds like SrCo2(Ge0.5P0.5)2 is probably not induced by dimer breaking as
recently suggested, but arises from the high density of states generated by the
transition metal 3d bands near the Fermi level without contribution of the
dimers.
