Invited Lecture

Louisa Meshi 1,2 Gili Yaniv 1 Asaf Uziel 1 Avraham Bram 1,3 Arnold Kiv 1 Arie Venkert 3 David Fuks 1
1Materials Engineering, Ben-Gurion University of the Negev, Beer-Sheva, Israel
2Ilse Katz Institute for Nanotechnology, Ben-Gurion University of the Negev, Beer-Sheva, Israel
3., Nuclear Research Center Negev, Beer-Sheva, Israel

Strong inter-linkage exists between the physical/chemical properties-chemical composition and crystal structure of the materials. In order to gain improved properties - composition and/or structure should be changed. Such researches are normally done either via theoretical route (i.e. prediction) or experimental (i.e. “trial and error”). Experimental route is time and resource consuming. Prediction (basing on energy landscapes) is not always possible, especially when system of an interest exhibits complex electronic structure. A-T-Al systems (where A-actinide/lanthanide and T-transition metals) are perfect examples of such systems since they contain f-electron elements. In these systems, AT2Al20 alloys were intensively studied with a purpose to find aluminides with possible heavy fermion properties. Our study concentrated on the ThT2Al20 system (where T-3d transition metal) in order to formulate a general “rule of thumb” which will allow to estimate the symmetry of the Al-rich ternary structure forming in the A-T-Al systems. Such prediction will shorten the research time spent on search for the heavy fermion materials with interesting magnetic and electrical properties. We have proved experimentally and theoretically that ternary aluminides structure’s symmetry changes abruptly as a function of atomic number of T (i.e. ZT). At T=Mn, ZT=25, the symmetry decreases from cubic (for ThT2Al20 with ZT<25) to orthorhombic. This change inevitably imposes modification of magnetic and electrical properties. At ZT=28 (i.e. T=Ni) three new structures were formed. Despite the prolonged heat treatment, equilibrium was not attended. Applying novel Electron Diffraction Tomography method (known as EDT) for structure characterization of these phases - geometry of all phases was proposed. EDT datasets were collected manually by tilting the crystal around an arbitrary axis in a tilt step of 1°, reducing the dynamical effects significantly. In order to increase the completeness of data, collection was done in precession illumination (with precession angle of approximately 2°). Solution of atomic structure of the major Th2Ni10Al15 phase was performed. Th2Ni10Al15 phase was found to be orthorhombic, Immm, with lattice parameters a=3.992Å, b=11.172 Å and c=17.343 Å. Although Th, as heavy scatterer, did smeared the Fourier difference map so that finding Al atom positions was not an easy task, coordinated of all 54 atoms in the unit cell were determined. It should be noted that despite the drastic compositional difference between the new Th2Ni10Al15 and studied in this research AT2Al20 stoichiometry, atomic structure of the Th2Ni10Al15 phase can be related to the orthorhombic phase discussed above since they both belong to a family of so called layered structures.

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