Gd5-xYxTt4 (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- x Y x Tt 4 ( Tt = Si, Ge) series prepared by high-temperature methods is presented. All structures for the Gd 5- x Y x Si 4 system belong to the orthorhombic, Gd 5Si 4-type (space group Pnma). For the Gd 5- x Y x Ge 4 system, phases with x < 3.6 and x >or= 4.4 adopt the orthorhombic, Sm 5Ge 4-type structure. For the composition range of 3.6 <or= x <or= 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 5 T 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 x Tt 4 system. The magnetic properties of some of the Gd 5- x Y x Tt 4 phases are also reported. Germanides with x <or= 2 show a metamagnetic-type transition similar to Gd 5Ge 4 from 57-92(2) K. As the Y concentration increases (3 <or= x <or= 4), these phases exhibit at least ferrimagnetic ordering with transition temperatures ranging from 15-31(2) K to the paramagnetic state.