Evolution of Bonding and Magnetism via Changes in Valence Electron Count in CuFe 2- x Co x Ge 2

A series of solid solutions, CuFe Co Ge ( = 0, 0.2, 0.4, 0.8, and 1.0), have been synthesized by arc-melting and characterized by powder X-ray and neutron diffraction, magnetic measurements, Mössbauer spectroscopy, and electronic band structure calculations. All compounds crystallize in the CuFe Ge structure type, which can be considered as a three-dimensional framework built of fused MGe octahedra and MGe trigonal bipyramids (M = Fe and Co), with channels filled by rows of Cu atoms. As the Co content ( ) increases, the unit cell volume decreases in an anisotropic fashion: the and lattice parameters decrease while the parameter increases. The changes in all the parameters are nearly linear, thus following Vegard's law. CuFe Ge exhibits two successive antiferromagnetic (AFM) orderings, corresponding to the formation of a commensurate AFM structure, followed by an incommensurate AFM structure observed at lower temperatures. As the Co content increases, the AFM ordering temperature ( ) gradually decreases, and only one AFM transition is observed for ≥ 0.2. The magnetic behavior of unsubstituted CuFe Ge was found to be sensitive to the preparation method. The temperature-dependent zero-field Fe Mössbauer spectra reveal two hyperfine split components that evolve in agreement with the two consecutive AFM orderings observed in magnetic measurements. In contrast, the field-dependent spectra obtained for fields ≥2 T reveal a parallel arrangement of the moments associated with the two crystallographically unique metal sites. Electronic band structure calculations and chemical bonding analysis reveal a mix of strong M-M antibonding and non-bonding states at the Fermi level, in support of the overall AFM ordering observed in zero field. The substitution of Co for Fe reduces the population of the M-M antibonding states and the overall density of states at the Fermi level, thus suppressing the value.