Vacancy Ordering in SmGe₂_-<i>_x</i> and GdGe₂<i>_-</i><i>_x</i> (<i>x</i> = 0.33): Structure and Properties of Two Sm₃Ge₅ Polymorphs and of Gd₃Ge₅

The crystal structures and the magnetic properties of three new binary rare-earth intermetallic phases are reported. α-Sm3Ge5 and β-Sm3Ge5 and Gd3Ge5 have been prepared from the corresponding elements through high-temperature reactions using the flux-growth method. The structures of the three compounds have been established using single-crystal X-ray diffraction: α-Sm3Ge5 crystallizes with its own type in the hexagonal space group P6̄2c (No. 190) with cell parameters a = 6.9238(11) Å, c = 8.491(3) Å, and Z = 2, whereas β-Sm3Ge5 adopts the face-centered orthorhombic Y3Ge5 type with space group Fdd2 (No. 43) and with cell parameters a = 5.8281(6) Å, b = 17.476(2) Å, c = 13.785(2) Å, and Z = 8. The orthorhombic Gd3Ge5 with cell parameters a = 5.784(2) Å, b = 17.355(6) Å, and c = 13.785(5) Å is isostructural with β-Sm3Ge5. The structures of the title compounds can be described as AlB2 and α-ThSi2 derivatives with long-range ordering of the germanium vacancies. Temperature-dependent DC magnetization (5−300 K) measurements show evidence of antiferromagnetic ordering below ca. 30 and 10 K for α-Sm3Ge5 and β-Sm3Ge5, respectively. Gd3Ge5 undergoes two successive magnetic transitions below ca. 15 and 11 K. The temperature dependence of the resistivity and heat capacity of Gd3Ge5 are discussed as well.