Structural and Physical Properties Diversity of New CaCu(5)-Type Related Europium Platinum Borides

[Image: see text] 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: EuPt(4)B, CeCo(4)B type, P6/mmm, a = 0.56167(2) nm, c = 0.74399(3) nm; Eu(3)Pt(7)B(2), Ca(3)Al(7)Cu(2) type as an ordered variant of PuNi(3), R3̅m, a = 0.55477(2) nm, c = 2.2896(1) nm; and Eu(5)Pt(18)B(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 CaCu(5) family of structures, revealing a stacking sequence of CaCu(5)-type slabs with different structural units: CaCu(5) and CeCo(3)B(2) type in EuPt(4)B; CeCo(3)B(2) and Laves MgCu(2) type in Eu(3)Pt(7)B(2); and CaCu(5)-, CeCo(3)B(2)-, and site-exchange ThCr(2)Si(2)-type slabs in Eu(5)Pt(18)B(6–x). The striking motif in the Eu(5)Pt(18)B(6–x) structure is the boron-centered Pt tetrahedron [BPt(4)], 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, EuPt(4)B and Eu(3)Pt(7)B(2), were studied. Both compounds were found to order magnetically at 36 and 57 K, respectively. For EuPt(4)B 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 Eu(3)Pt(7)B(2) was found to be extraordinarily large, on the order of 0.2 J/mol K(2).