Europium-rich ternary phases with noble metals Eu2M2X (M = Pd, Pt; X = Al, Ga, Ge, Cd), and their structural relationship to the Laves phase Ca2Pt3Al1–xAgx

The combination of divalent Eu with high magnetic moment and strong spin-orbit coupling (SOC) on Pd or Pt atoms in a single substance, makes Eu-Pd/Pt-X ternary phases attractive for materials with exotic physical properties. For that reason, compositions according to Eu2M2X (M = Pd, Pt; X = Al, Ga, Ge, Cd) were explored. X-ray diffraction analyses revealed that the three Pd analogues Eu2Pd2X with X = Al, Ga, and Ge, crystallize isostructural to Ca2Pd2Ge, a ternary ordered variant of the Zr2Al3-type (orthorhombic Fdd2). In contrast, Eu2Pt2Al adopts the Ca2Ir2Si type rival structure (monoclinic C2/c). The two structure types are closely related as they both feature linear chains of the noble metals, with the X atoms bridging a the metal chains from adjacent layers. However, a striking difference is noticeable within the metal chains: The Pd linear chains consist of equidistant PdPd atoms (2.96 Å), whereas in the Pt chains are pairwise distorted with alternating shorter (Pt–Pt = 2.80 Å) and longer (2.98 Å) contacts. The pairwise distortion is, at first glance, is attributed to stronger relativistic influence on Pt atom. In addition, both Ca2Pd2Ge- and Ca2Ir2Si-type structures are also discussed as defect Laves phases in connection with the newly discovered phase Ca2Pt3Al1–xAgx (x = 0.13(1)) which adopts the TbFe2 type structure (space group R-3m). DFT first principles electronic band structure calculations, conducted on the Ag-free ordered model “Ca2Pt3Al”, suggested that Ag-inclusion is mainly due to geometric factors, while the chemical bonding picture is consistent with a polar intermetallic system. Finally, Eu2Pt2Cd, adopts the W2CoB2 type, suggesting that the valence electron count (vec) has strong influence in the structure selection.