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A series of solid solutions, CuFe 2- x Co x Ge 2 ( x = 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 2 Ge 2 structure type, which can be considered as a three-dimensional framework built of fused MGe 6 octahedra and MGe 5 trigonal bipyramids (M = Fe and Co), with channels filled by rows of Cu atoms. As the Co content ( x ) increases, the unit cell volume decreases in an anisotropic fashion: the b and c lattice parameters decrease while the a parameter increases. The changes in all the parameters are nearly linear, thus following Vegard's law. CuFe 2 Ge 2 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 ( T N ) gradually decreases, and only one AFM transition is observed for x ≥ 0.2. The magnetic behavior of unsubstituted CuFe 2 Ge 2 was found to be sensitive to the preparation method. The temperature-dependent zero-field 57 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 T N value.

X-ray absorption spectroscopy (XAS) was used to elucidate changes in the electronic structure caused by the pressure-induced structural collapse in EuCo 2 P 2 . The spectral changes observed at the L 3 -edge of Eu and K-edges of Co and P suggest electron density redistribution, which contradicts the formal charges calculated from the commonly used Zintl-Klemm concept. Quantum-chemical calculations show that, despite the increase in the oxidation state of Eu and the formation of a weak P-P bond in the high-pressure phase, the electron transfer from the Eu 4f orbitals to the hybridized 5d and 6s states causes strengthening of the Eu-P and P-P bonds. These changes explain the increased electron density on P atoms, deduced from the P K-edge XAS spectra. This work shows that the formal electron counting schemes do not provide an adequate description of changes associated with phase transitions in metallic systems with substantial mixing of the electronic states., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Rare-earth cobalt pnictides, RCo 2 Pn 2 (Pn = P, As), belong to the ThCr 2 Si 2 structure type, which is ubiquitous among intermetallic compounds. The structural and magnetic properties of simple ternary RCo 2 P 2 phosphides, which combine partially delocalized (itinerant) 3d magnetic moments of cobalt and localized 4f magnetic moments of lanthanides, were investigated extensively in 1980-1990s, predominantly by the Jeitschko group. Those studies established that LaCo 2 P 2 shows ferromagnetic (FM) ordering of Co moments, while the other members of the series, with R = Ce, Pr, Nd, or Sm, exhibit antiferromagnetic (AFM) ordering in both R and Co magnetic sublattices. This observation also correlated with the larger separation between the [Co 2 P 2 ] layers in the crystal structure of LaCo 2 P 2 as compared to the decreased interlayer distances in the other structures of the RCo 2 P 2 series. Our work over the past decade has focused on unraveling the rich magnetic behavior that can be observed in these systems when internal chemical and external physical factors are used to perturb their crystal and electronic structures. We began our foray into these materials by demonstrating that the preservation of FM ordering of Co 3d moments in the mixed La 1-x R' x Co 2 P 2 phases also forces the R 4f moments to adopt FM arrangement, although antiparallel to the Co moments. As an example, in La 0.75 Pr 0.25 Co 2 P 2 such mutual influence of the 3d and 4f moments leads to a cascade of magnetic phase transitions. All these changes were traced back to the modification of the crystal structure and, consequently, the electronic band structure of these materials. The substitution of smaller R 3+ ions for the La 3+ ions leads to structural compression along the tetragonal c axis, perpendicular to the [Co 2 P 2 ] layers, and an increase in the Co-Co distances within the layer. This structural effect is translated into more localized Co magnetic moments, stronger magnetic exchange between Co sites, and higher ordering temperatures. A more dramatic change in properties is observed in EuCo 2 Pn 2 , which exhibit AFM ordering of the localized 4f moments of Eu 2+ ions and only paramagnetic behavior in the Co sublattice. Under applied pressure, these compounds undergo structural collapse, which causes a dramatic decrease in the separation between the [Co 2 Pn 2 ] layers, an increase in the oxidation state of Eu, and magnetic ordering of Co moments. We further demonstrated that similar effects can be stimulated by chemical compression, which is achieved by doping Eu into the more constrained lattice sites, for example, in PrCo 2 P 2 or CaCo 2 As 2 . In both cases, the induced mixed valence of Eu results in the change from AFM to FM ordering in the Co sublattice. A series of solid solutions Ca 1-x Eu x Co 2 As 2 shows a fascinating evolution of magnetic behavior from AFM ordering of Co 3d moments to simultaneous FM ordering of Co 3d and Eu 4f moments to AFM ordering of Eu 4f moments as one proceeds from CaCo 2 As 2 to EuCo 2 As 2 . Importantly, all these changes in magnetic properties are well justified by the analysis of electronic density of states and crystal orbital Hamilton population, providing the understanding of how chemical factors can be leveraged, in general, to modify properties of itinerant magnets.