The effect of varying the crystal structure on the magnetism, electronic structure and thermodynamics in the Gd5(SixGe1-x)4 system near x=0.5

The crystal structure, magnetic and other physical properties of the intermetallic Gd5(SixGe1-x)4 phases are strongly dependent on the Si:Ge ratio (x). Especially intriguing behavior is observed when the chemical composition in this system is near x congruent with 0.5, where small changes in the stoichiometry result in drastic variations in the chemical bonding, electronic structure, crystal structure, and magnetism. Furthermore, the fully reversible magnetic/crystallographic (TC congruent with 270 K) and the irreversible thermoelastic crystallographic (between ∼500 and ∼750 K) transformations exist near this critical chemical composition. Both of these transformations involve the same two crystallographic modifications: the monoclinic Gd5(Si2Ge2)-type (β) and the orthorhombic Gd5Si4-type structures (α and γ). First principle calculations of the electronic structure and exchange coupling of these materials are in nearly quantitative agreement with the experiment. It appears that the unusual behavior observed in the near critical Gd5(SixGe1-x)4 phases is closely related to the stability of the well-defined sub-nanometer thick atomic slabs coupled with the flexibility of their arrangements