Crystal and magnetic structure of the GdNiAl intermetallic compound

Full text: The RNiAl ternary equiatomic intermetallic compounds with R = Rare Earth (except Eu) are a series of interest that exhibits spin-glass behaviour and frustrated magnetism. The first study of the crystal structure of these compounds was reported by Oesterreicher et al. in 1967 and 1973. These were followed by Dwight et al. in 1968 and Ferro et al. in 1974. Within this series, an abrupt change in the unit cell parameters upon cooling was reported for GdNiAl at around 205 K. Furthermore, the electric and magnetic behaviours were found to be quite sensitive to this change. GdNiAl orders ferromagnetically around 60 K with further magnetic transitions at 30 K and 14 K being reported. In the present work, the GdNiAl compound is examined using X-ray and neutron diffraction. The Rietveld refinement of the XRD patterns revealed that the change at ∼205 K is actually a transition between two different forms of the ZrNiAl-type structure; with these two forms coexisting over a temperature range of approximately 250 K down to 125 K. The refinement of the GdNiAl neutron diffraction patterns collected from 80 K down to 5 K indicate only the low temperature structural variant, with two different commensurate magnetic contributions: A ferromagnetic component from ∼60 K down to 32.5 ± 1.7 K, and a mixed Ferro/Antiferro-magnetic structure from 32.5 ± 1.7 K down to 5 K. Moreover, no additional transition was observed near 14 K in contrast to the earlier reports. (author)