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 Eu2Pd2Al, Eu2Pd2Ga, and Eu2Pd2Ge 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 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, attributed to stronger relativistic influence on Pt atom. In addition, Eu2Pt2Cd, adopts the W2CoB2 type, suggesting that the valence electron count (vec) has strong influence in the structure selection. Finally, 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.