Structure, bonding, and properties of RCr6Ge6 intermetallics (R[dbnd]Gd–Lu)

Abstract

We report the discovery and characterization of three novel ternary compounds: TmCr6Ge6, YbCr6Ge6, and LuCr6Ge6, which complete the series of RCr6Ge6 intermetallics with kagome lattice. Energy dispersive X-ray analysis revealed the off-stoichiometric composition of RCr6Ge6 (RGd, Tb, Dy, Ho, Er, Tm, Yb, and Lu). The powder XRD analysis with subsequent Rietveld refinement showed that investigated compounds crystallize in the SmMn6Sn6 structure type (space group P6/mmm), a disordered variant of the MgFe6Ge6-type, with a small number of vacancies at the rare earth (1a) and germanium (2e) sites and two additional 1b and 2e sites partially occupied by additional rare earth and germanium atoms, respectively. A tendency to structure ordering is observed with decreasing rare earth ionic radii. Electrical resistivity measurements indicate a metallic conductivity type for all studied compounds. Magnetic characterization reveals paramagnetic behavior for TmCr6Ge6 and LuCr6Ge6, respectively, and suggests YbCr6Ge6 as a ferrimagnetic material. The DFT modeling of RCr6Ge6 shows a distinct high electron localization between Ge1 atoms, corresponding to strong covalent bonding. The calculated enthalpies of formation range from −18.56 kJ/mol for Gd to −19.60 for Tm compounds, while the distribution of the density of electronic states (DOS) confirms the metallic type of conductivity for all the series. The calculated elastic properties show brittle behavior and a high hardness level for the whole series. DFT modeling allowed us to reveal mechanisms of RCr6Ge6 formation and the origin of disorder in their crystal structure.