Salt-flux growth of HoCuMg₄ single crystals

Polycrystalline samples of the magnesium-rich intermetallic compounds RECuMg₄ (RE = Dy, Ho, Er, Tm) were synthesized by reaction of the elements in sealed tantalum ampoules heated in a high-frequency induction furnace. Phase purity of the RECuMg₄ phases was ascertained by powder X-ray diffraction patterns. Well-shaped single crystals of HoCuMg₄ could be grown in a NaCl/KCl salt flux and the crystal structure was refined from single crystal X-ray diffraction data: TbCuMg₄ structure-type, space group Cmmm, a = 1361.4(2), b = 2039.3(4), c = 384.62(6) pm. The crystal structure of the RECuMg₄ phases can be understood as a complex intergrowth variant of CsCl and AlB₂ related slabs. The remarkable crystal chemical motif concerns the orthorhombically distorted bcc-like magnesium cubes with Mg-Mg distances ranging from 306 to 334 pm. At high temperatures DyCuMg₄ and ErCuMg₄ are Curie-Weiss paramagnets with paramagnetic Curie-Weiss temperatures of -15 K and -2 K for RE = Dy and Er, respectively. The effective magnetic moments, 10.66μ_B for RE = Dy and 9.65μ_B for RE = Er prove stable trivalent ground states for the rare earth cations. Magnetic susceptibility and heat capacity measurements reveal long-range antiferromagnetic ordering at low temperatures (<21 K). Whereas DyCuMg₄ exhibits two subsequent antiferromagnetic transitions at T_N = 21 and 7.9 K which successively remove half of the entropy of a doublet crystal field ground state of Dy, ErCuMg₄ shows a single, possibly broadened, antiferromagnetic transition at 8.6 K. The successive antiferromagnetic transitions are discussed with respect to magnetic frustration in the tetrameric units present in the crystal structure.