Gd_5−xY_xTt₄ (Tt = Si or Ge): Effect of Metal Substitution on Structure, Bonding, and Magnetism

A crystallographic study and theoretical assessment of the Gd/Y site preferences in the Gd_5−xY_xTt₄ (Tt = Si, Ge) series prepared by high-temperature methods is presented. All structures for the Gd_5−xY_xSi₄ system belong to the orthorhombic, Gd₅Si₄-type (space group Pnma). For the Gd_5−xY_xGe₄ system, phases with x < 3.6 and x ≥ 4.4 adopt the orthorhombic, Sm₅Ge₄-type structure. For the composition range of 3.6 ≤ x ≤ 4.2, a monoclinic, U₂Mo₃Si₄-type structure (space group P2₁/c) occurs as the majority phase. This structure type has not been previously observed in the RE₅T₄ (T = Si, Ge, Ga) system and differs from the known monoclinic structure of Gd₅Si₂Ge₂-type (space group P2₁/a) because all Ge···Ge contacts between slabs are equivalent. The structural relationships between the Sm₅Ge₄-type and the U₂Mo₃Si₄-type structures are discussed. Single crystal refinements of the occupancies for the three sites for Gd/Y atoms in the asymmetric unit reveal a partially ordered arrangement of Gd and Y atoms. TB-LMTO-ASA calculations were performed to study these atomic distributions as well as to elucidate possible electronic forces that might drive the structural variation. These results illustrate the importance of one of the Gd/Y-sites in shaping the magnetic and structural features observed in Gd_5−xY_xTt₄ system. The magnetic properties of some of the Gd_5−xY_xTt₄ phases are also reported. Germanides with x ≤ 2 show a metamagnetic-type transition similar to Gd₅Ge₄ from 57−92(2) K. As the Y concentration increases (3 ≤ x ≤ 4), these phases exhibit at least ferrimagnetic ordering with transition temperatures ranging from 15−31(2) K to the paramagnetic state.