ABSTRACT
The first-order monoclinic-to-orthorhombic (beta-->gamma) phase transition of the giant magnetocaloric material Gd(5)Si(2)Ge(2) was studied using in situ high-temperature single-crystal X-ray diffraction. A special crystal mounting procedure was developed to avoid crystal contamination by oxygen or nitrogen at high temperatures. The elastic beta-->gamma transformation occurs at 300-320 degrees C during heating, and it is reversible during fast and slow heating and slow cooling but irreversible during rapid cooling. Contrary to theoretical predictions, the macroscopic distribution of the Si and Ge atoms remains the same in both the orthorhombic gamma-polymorph and the monoclinic beta-phase. It appears that interstitial impurities may affect stability of both the monoclinic and orthorhombic phases. In the presence of small amounts of air, the beta-->gamma transformation is complete only at 600 degrees C. The interslab voids, which can accommodate impurity atoms, have been located in the structure, and an effect of partially filling these voids with oxygen or nitrogen atoms on the beta-gamma transition is discussed.
ABSTRACT
X-ray single crystal and powder diffraction studies on the Gd(5)Ga(x)()Ge(4)(-)(x)() system with 0 < or = x < or = 2.2 reveal dependence of interslab T-T dimer distances and crystal structures themselves on valence electron concentration (T is a mixture of Ga and Ge atoms). While the Gd(5)Ga(x)()Ge(4)(-)(x)() phases with 0 < or = x < or = 0.6 and valence electron concentration of 30.4-31 e(-)/formula crystallize with the Sm(5)Ge(4)-type structure, in which all interslab T-T dimers are broken (distances exceeding 3.4 A), the phases with 1 < or = x < or = 2.2 and valence electron concentration of 28.8-30 e-/formula adopt the Pu(5)Rh(4)- or Gd(5)Si(4)-type structures with T-T dimers between the slabs. An orthorhombic Pu(5)Rh(4)-type structure, which is intermediate between the Gd(5)Si(4)- and Sm(5)Ge(4)-type structures, has been identified for the Gd(5)GaGe(3) composition. Tight-binding linear-muffin-tin-orbital calculations show that substitution of three-valent Ga by four-valent Ge leads to larger population of the antibonding states within the dimers and, thus, to dimer stretching and eventually to dimer cleavage.
ABSTRACT
Stoichiometric intermetallic compounds have always been touted for their attractive chemical, physical, electrical, magnetic and mechanical properties, but few practical uses have materialized because they are brittle at room temperature. Here we report on a large family of fully ordered, stoichiometric binary rare-earth intermetallic compounds with high ductility at room temperature. Although conventional wisdom calls for special conditions, such as non-stoichiometry, metastable disorder or doping to achieve some ductility in intermetallic compounds at room temperature, none of these is required in these unique B2 rare-earth compounds. Ab initio calculations of YAg, YCu and NiAl crystal defect energies support the observed deformation modes of these intermetallics.
