Atomic Site Preference in the Quaternary Phase Eu2Mn1–xCd1+xPt2 (x = 0.11) with LaNiAl type structure

After attempts to synthesize the imaginary ternary phase “EuMnPt” failed, the solid solutions EuMnxCd1–xPt were prepared following speculations that random mixing of the divalent Cd and Mn metals may be achieved in the previ-ously reported analogue EuCdPt (TiNiSi type, Pnma–c3). However, an unexpected atomic ordering resulted in the quater-nary phase Eu2Mn1–xCd1+xPt2 (x = 0.11) which adopts the Pd2(Mn,Pd)Ge2 (or LaNiAl, Pnma–c6) structure type instead, in which two mixed Mn/Cd sites in the structure show pronounced site preference, reminiscent to that of Mn/Pd sites in the prototype. Thus, the crystal structure of Eu2Cd1+xMn1–xPt2 (x = 0.11) represents a unique example of 4f-3d-4d-5d intermetallic system with nearly complete atomic ordering, and it may be best viewed as an intergrowth of hexagonal ZrNiAl type (previously reported for CeCdxMn1–xPt) and orthorhombic TiNiSi type (previously reported for YbCdxMn1–xPt). It is also the first Eu based compound to adopt the LaNiAl type structure, already reported only with the earlier lanthanides (Ln = La, Ce, Pr, Nd), and all noble metals of the Pt group, but Os and Pt itself. The sublattice of paramagnetic Mn atoms consists of linear zigzag chains with Mn–Mn distance of 3.084(1) Å, while triangular magnetic geometrical frustration is realised in the Eu sublattice. Thus, the rather unexpected atomic segregation between two isovalent transition metals Mn (3d5) and Cd (4d10) may be ascribed, at first glance, to chemical bonding preference for homonuclear Mn–Mn over Cd–Cd and heteronuclear Mn–Cd interactions, in agreement with the 18–n rule.