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Magnetism in Giant Unit Cells – Crystal Structure and Magnetic Properties of R117Co52+δSn112+γ (R = Sm, Tb, Dy)


  • Dedicated to Professor John D. Corbett on the occasion of his 85th birthday


Ternary polar intermetallics Sm117Co55.6Sn116, Tb117Co59Sn111, and Dy117Co58Sn111 were prepared by the arc-melting technique. They belong to the Tb117Fe52Ge112 structure type and are characterized by a giant cubic unit cell (V > 25000 Å3). The build-up of the structure can be understood by identifying three basic metal-centered polyhedra, a 14-vertex Co@Co6Sn8 at the edge centers and at the body center of the unit cell, a 14-vertex Sm@Co8Sn6 at the origin of the unit cell, and a 22-vertex Sm@Co12Sn10 at the center of each octant. The rest of the atoms in the crystal structure can be considered as forming concentric polyhedra around these basic units. Such an approach results in three types of large multishell polyhedra that share their outermost faces and provide complete space filling. The arrangement of these polyhedra follows the arrangement of atoms in the structure of Heusler alloy Cu2MnAl. The description of this complex structure in terms of multishell clusters allows its separation into regions of different polarity, as each individual shell is composed of either Sm or Co/Sn atoms. Sm117Co55.6Sn116 exhibits ferromagnetic ordering in the Sm sublattice at 86 K. Tb117Co59Sn111 and Dy117Co58Sn111 undergo consecutive paramagnet–ferromagnet and ferromagnet-canted–antiferromagnet phase transitions when the temperature is lowered. The transitions take place at 38 K and 20 K for Tb117Co59Sn111 and at 30 K and 11 K for Dy117Co58Sn111.