Ternary Rare Earth Platinum Germanides RE 2 Pt 9 Ge 3 ( RE = Y, Tb–Lu) Crystallizing with a Monoclinic Structure Closely Related to Orthorhombic Y 2 Co 3 Ga 9

A new series of isotypic germanides RE 2 Pt 9 Ge 3 ( RE = Y, Tb‐Lu) was prepared by arc‐melting of the elemental components or in the case of the ytterbium compound via induction heating in a high‐frequency furnace. Subsequently, all samples were annealed at temperatures between 900 and 1000 °C. The investigated germanides crystallize in a new monoclinic structure type which for Tb 2 Pt 9 Ge 3 was determined from single‐crystal X‐ray data: C 2/ c (no. 15), mC 56, Z = 4, a = 13.373(1) Å, b = 7.742(1) Å, c = 10.029(1) Å, β = 116.73(1)°, R F = 0.030 for 973 structure factors and 67 variable parameters. The structures of Dy 2 Pt 9 Ge 3 ( a = 13.346(1) Å, b = 7.735(1) Å, c = 10.015(1) Å, β = 116.72(1)°, R F = 0.039, 1240 F 's, 67 variables) and Ho 2 Pt 9 Ge 3 ( a = 13.332(1) Å, b = 7.723(1) Å, c = 9.999(1) Å, β = 116.67(1)°, R F = 0.047, 864 F 's, 67 variables) were refined from single‐crystal data, too. The structure may be described as an ABAB stacking of two different pseudo‐trigonal layers with compositions RE 2 Pt 3 (designated A ) and Pt 2 Ge (designated B ). Layers B may be considered as distorted hexagonal close packed, similar to a compression of three adjacent layers I–Cd–I of the CdI 2 type structure. Structural relationships among the structures of Tb 2 Pt 9 Ge 3 , Y 2 Co 3 Ga 9 , Yb 2 Pd 3 Ga 9 , Er 4 Pt 9 Al 24 , Sc 1.2 Fe 4 Si 9.8 , and ErNi 3 Al 9 are discussed. The temperature dependence of magnetic susceptibility, electrical resistivity, and specific heat capacity of the compounds RE 2 Pt 9 Ge 3 ( RE = Y, Tb‐Er, Yb, Lu) were investigated. All of them are metallic conductors. Y 2 Pt 9 Ge 3 and Lu 2 Pt 9 Ge 3 are Pauli paramagnets, while the others show Curie paramagnetic behavior with antiferromagnetic ordering temperatures of 5 K (for the Dy compound) or less. Ytterbium in Yb 2 Pt 9 Ge 3 takes the trivalent oxidation state. Thermodynamic standard values at 298.15 K are derived and crystal electric field contributions are discussed.