RCuMg₂ compounds (R = Dy-Tm, Lu): crystal structure, chemical bonding and magnetic properties

The RCuMg₂ intermetallics (R = Dy-Tm, Lu) were found to crystallize in a new structure type (DyCuMg₂, oP32, space group: Pmma, a = 13.5397(4), b = 3.7594(1), c = 13.7985(4) Å). This unique crystal space is characterized by the presence of clusters formed by four Cu-centred trigonal prisms sharing edges to create a cubic-like cavity occupied by a Mg atom; 2D wavy slabs of these star-like fragments and of Mg with a bcc topology are alternated along the c-direction. An architecture based on similar moieties was recognized for the related R₂Cu₂Mg (tP10-Mo₂FeB₂) and RCuMg₄ (oS48-TbCuMg₄) compounds, proposing a generalization scheme. Electronic structure calculations and a chemical bonding analysis in the position space were applied to LuCuMg₂, as a representative. The calculated effective charges indicated that Lu and Cu act as the QTAIM cation and anion, respectively, and the Mg species show a slight positive charge, except that at the centre of the bcc-fragment, showing a tiny negative one. In general, a complex bonding scenario was revealed, dominated by both hetero and homoatomic (within the Mg bcc-slabs) multicentre interactions. The temperature and field dependencies of magnetic susceptibility and specific heat capacity were measured for RCuMg₂. The nonmagnetic LuCuMg₂ phonon reference compound was identified as a Pauli paramagnet. The {Ho, Er, Tm}CuMg₂ compounds ordered antiferromagnetically at critical temperatures between about 5 and 8 K, whereas DyCuMg₂ revealed multiple magnetic transitions. The complexity of the magnetic behaviour of the studied compounds was indicated by the temperature evolution of magnetic entropy.