Synergistic Geometrical and Electronic Features in the Intermetallic Phases Ca2AgM2, Ca2MgM2, and Ca2GaM2 (M = Pd, Pt)

Six new analogous phases, Ca2AgM2, Ca2MgM2, and Ca2M2Ga (M = Pd, Pt), were rationally prepared by direct combination of the respective elements at high temperature, allowing us to clarify the factors controlling the structural selection between the orthorhombic W2CoB2‐type structure (Immm) and its distorted monoclinic Gd2AlGe2‐type derivative (C2/c), as well as to investigate the mechanism and the extent of the electronic flexibility of the W2CoB2‐type structure. All crystal structures were refined from single‐crystal X‐ray diffraction data. Two pairs of compounds Ca2AgM2 (5 ve/f.u) and Ca2MgM2 (6 ve/f.u) adopt the orthorhombic W2CoB2 type, whereas the pair of Ga analogues Ca2M2Ga (7 ve/fu) crystallize in the monoclinic Gd2AlGe2‐ or Ca2Ir2Si‐type structure, confirming the valence electron count (vec) as the dominant factor controlling the structural selection in this series. The stability factors and chemical bonding features behind the similarities and differences of these closely related structures were theoretically investigated using first‐principles density functional calculations. They provide additional evidences of the decisive influence of the vec as well as the electronegativity differences between constituent elements, on the structural trends. In addition, the cooperative roles of the geometrical and electronic flexibility associated with the inherent anisotropy of the orthorhombic symmetry of the W2CoB2 type are emphasized.