Magnesium-rich intermetallic compounds Gd5Cu5Mg13 and Tb5Cu5Mg13 – intergrowth variants with CsCl and AlB2 related slabs

Abstract The magnesium-rich intermetallic compounds Gd5Cu5Mg13 and Tb5Cu5Mg13 were obtained from direct reactions of the elements (induction melting) in sealed tantalum ampoules. Both compounds crystallize with the orthorhombic Y5Cu5Mg13 type structure, space group Cmcm and Z = 4. The polycrystalline samples were characterized by powder X-ray diffraction. The structure of the gadolinium compound was refined from single crystal X-ray diffraction data: a = 414.78(2), b = 1921.87(12), c = 2573.89(16) pm, wR2 = 0.0492, 1611 F2 values and 77 variables. Refinement of the occupancy parameters revealed a small degree of Gd/Mg mixing for the Gd3 site, leading to the composition Gd4.93(1)Cu5Mg13.07(1) for the studied crystal. The Gd5Cu5Mg13 structure contains slabs of equiatomic GdCuMg, which are embedded in a magnesium matrix. From a geometrical point of view, one can describe the Gd5Cu5Mg13 and Tb5Cu5Mg13 structures as intergrowth variants of distorted W/CsCl and AlB2 related slabs. The most remarkable crystal chemical feature concerns the bcc like magnesium slabs with short Mg–Mg distances ranging from 300 to 342 pm. Temperature dependent magnetic susceptibility measurements show Curie-Weiss paramagnetism for Tb5Cu5Mg13 (10.5(1) μ B Tb atom−1 and Θ P = −11.6(1) K). Antiferromagnetic ordering was detected below the Néel temperatures of T N = 30.5(3) K.