We present a study of the structural and physical properties of directly hole doped LaFe1-xMnxAsO (x = 0.0-0.2) and the influence of charge compensation / electron-doping by additional F doping in LaFe0.9Mn0.1AsO1-yFy (y = 0.1-0.5). High quality poly
crystalline samples were prepared using a solid state metathesis reaction. The unit cell increases upon Mn doping, but decreases again when additional F is inserted. The semiconducting character of LaFe1-xMnxAsO decreases with additional F doping. Muon spin relaxation (muSR) measurements reveal short range magnetic order in LaFe1-xMnxAsO and a suppression of magnetism by additional electron-doping with fluoride in LaFe0.9Mn0.1AsO1-yFy. Superconductivity remains absent even though the electronic preconditions are fulfilled in electron-doped LaFe0.9Mn0.1AsO1-yFy at x > 0.1, which is suggestive of effective pair breaking by Mn in this system.
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 CaNi2
As2 (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.
