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, CeCo₄B type, <i>P</i>6/<i>mmm</i>, <i>a</i> = 0.56167(2) nm, <i>c</i> = 0.74399(3) nm; Eu₃Pt₇B₂, Ca₃Al₇Cu₂ type as an ordered variant of PuNi₃, <i>R</i>3̅<i>m</i>, <i>a</i> = 0.55477(2) nm, <i>c</i> = 2.2896(1) nm; and Eu₅Pt₁₈B_6–<i>x</i>, a new unique structure type, <i>Fmmm</i>, <i>a</i> = 0.55813(3) nm, <i>b</i> = 0.95476(5) nm, <i>c</i> = 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 CeCo₃B₂ type in EuPt₄B; CeCo₃B₂ and Laves MgCu₂ type in Eu₃Pt₇B₂; and CaCu₅-, CeCo₃B₂-, and site-exchange ThCr₂Si₂-type slabs in Eu₅Pt₁₈B_6–<i>x</i>. The striking motif in the Eu₅Pt₁₈B_6–<i>x</i> structure is the boron-centered Pt tetrahedron [BPt₄], which build chains running along the <i>a</i> 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