Crystal structures and magnetic properties of NdCoAl4, Nd2Co3Al9 and Sm2Co3Al9

The new compounds, NdCoAl4, Nd2Co3Al9 and Sm2Co3Al9 have been prepared by arc-melting of the elemental components. The chemical and structural characterizations were carried out on both as-cast and annealed samples using X-ray diffractometry, metallographic analysis and EDS-microanalysis. The crystal structures of NdCoAl4 and Nd2Co3Al9 were refined from the single-crystal X-ray diffraction data, collected at room temperature. NdCoAl4 adopts the LaCoAl4 type of structure, crystallizing with the orthorhombic space group Pmma (no. 51), in a unit cell of dimensions: a = 7.6574(3) Å, b = 4.0401(1) Å, c = 6.8499(3) Å. Nd2Co3Al9 crystallizes in the orthorhombic space group Cmcm (n°63), with the lattice parameters: a = 12.8911(4) Å, b = 7.5594(2) Å, c = 9.4703(2) Å. Nd2Co3Al9 and Sm2Co3Al9 are isostructural to the Y2Co3Ga9-type. Magnetic susceptibility measurements, recorded in the temperature range 1.7-300 K, have revealed that all phases orders antiferromagnetically at low temperature. The Néel temperature are 2.8, 3.0 and 6 K for NdCoAl4, Nd2Co3Al9 and Sm2Co3Al9, respectively. At high temperature, the paramagnetic domain is dominated by the contribution of the rare-earth ions.