Gd_(5-x)Y_xTt_4 (Tt = Si or Ge): Effect of Metal Substitution on Structure, Bonding, and Magnetism

摘要

A crystallographic study and theoretical assessment of the Gd/Y site preferences in the Gd_(5-x)Y_xTt_4, (Tt = Si, Ge) series prepared by high-temperature methods is presented. All structures for the Gd_(5-x)Y_xSi_4 system belong to the orthorhombic, Gd_5Si_4-type (space group Pnma). For the Gd_(5-x)Y_xGe_4 system, phases with x < 3.6 and x ≥ 4.4 adopt the orthorhombic, Sm_5Ge_4-type structure. For the composition range of 3.6 ≤ x ≤ 4.2, a monoclinic, U_2Mo_3Si_4-type structure (space group P2_1/c) occurs as the majority phase. This structure type has not been previously observed in the RE_5T_4 (T= Si, Ge, Ga) system and differs from the known monoclinic structure of Gd_5Si_2Ge_2-type (space group P2_1/a) because all Ge…Ge contacts between slabs are equivalent. The structural relationships between the Sm_5Ge_4-type and the U_2Mo_3Si_4-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-x)Y_xTt_4 system. The magnetic properties of some of the Gd_(5-x)Y_XTt_4 phases are also reported. Germanides with x ≤ 2 show a metamagnetic-type transition similar to Gd_5Ge_4 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.