Structural instability in RCu intermetallic compounds

Abstract

Thermal expansion measurements have been carried out on the RCu ( R ≡ Gd, Tb, Dy, Ho and Y) intermetallic compounds in the temperature range between 4 and 700 K. In the case of R ≡ Gd, Tb and Y, large anomalies have been observed, which have been attributed to a martensitic structural transformation. Such a transformation presents a large shape memory, which in order to obtain the initial phase, necessitates annealing of the samples above 600 K. For R ≡ Dy and Ho, only small anomalies in addition to the expected phonon contribution have been observed, and these are ascribed to the onset of antiferromagnetic order. The structural instability in this series has been correlated with the magnitude of the metallic radii of the R ions in the different compounds, which produces small variation in the density of states near the Fermi level. Such a variation can strongly modify the energetic balance produced by a removal of the degeneracy of the d-subband by lowering the point symmetry. This is because the Fermi level lies close to a sharp peak in the density of states.