Structural and physical properties diversity of new CaCu5-type related europium platinum borides

Abstract

Three novel europium platinum borides have been synthesized by arc melting of constituent elements and subsequent annealing. They were characterized by X-ray powder and single-crystal diffraction: EuPt4B, CeCo 4B type, P6/mmm, a = 0.56167(2) nm, c = 0.74399(3) nm; Eu 3Pt7B2, Ca3Al7Cu 2 type as an ordered variant of PuNi3, R3Ì...m, a = 0.55477(2) nm, c = 2.2896(1) nm; and Eu5Pt18B 6-x, a new unique structure type, Fmmm, a = 0.55813(3) nm, b = 0.95476(5) nm, c = 3.51578(2) nm. These compounds belong to the CaCu5 family of structures, revealing a stacking sequence of CaCu5-type slabs with different structural units: CaCu5 and CeCo 3B2 type in EuPt4B; CeCo3B 2 and Laves MgCu2 type in Eu3Pt 7B2; and CaCu5-, CeCo3B 2-, and site-exchange ThCr2Si2-type slabs in Eu5Pt18B6-x. The striking motif in the Eu 5Pt18B6-x structure is the boron-centered Pt tetrahedron [BPt4], which build chains running along the a axis and plays a decisive role in the structure arrangement by linking the terminal fragments of repeating blocks of fused Eu polyhedra. Physical properties of two compounds, EuPt4B and Eu3Pt7B2, were studied. Both compounds were found to order magnetically at 36 and 57 K, respectively. For EuPt4B a mixed-valence state of the Eu atom was confirmed via magnetic and specific heat measurements. Moreover, the Sommerfeld value of the specific heat of Eu3Pt7B2 was found to be extraordinarily large, on the order of 0.2 J/mol K2. © 2013 American Chemical Society.