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) & Aring;). 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.