Structure and magnetic properties of RCu5-xMx (R = rare earth; M = Pd, Mn and Be) compounds

Full text: The rare earths form a number of binary compounds with the d-transition metals at the composition RT5, where T = Cu, Ni, Co and Pt. An interesting variation of the crystal symmetry is observed in RCu5 compounds where the hexagonal CaCu5-type phase forms for R = La to Eu and Y, while for R = Dy to Tm and Lu, the cubic AuBe5-type structure is adopted. GdCu5 and TbCu5 are dimorphic, the structure adopted depending upon the heat treatment. The effect of partial substitution of Cu with some other elements, M, such as M = Al, Ag, Au, In and Pd has been reported in the literature. In some cases it leads to RCu4M compounds with the cubic MgCu4Sn-type (an ordered ternary derivative of AuBe5) structure. Interesting physical properties, such as semi-metallic behaviour, geometrically frustrated exchange antiferromagnetism and strongly correlated electron behaviour in Ce and Yb analogs has been reported in some of these compounds. We have investigated the pseudo-ternary alloys RCu5-xPdx (R = La to Eu), RCu4Mn (R = La to Gd) and more recently, RCu4Be using the techniques of powder X-ray diffraction, magnetization, electrical resistivity, heat capacity and 151Eu Moessbauer spectroscopy. While CeCu4Pd, PrCu4Pd and EuCu5-xPdx (x = 1 and 2) retain the hexagonal symmetry of the parent compounds, interestingly additional replacement of Cu by Pd leads to cubic, disordered AuBe5-type structure in CeCu3Pd2 and PrCu3Pd2. The transformation to the cubic symmetry takes place at RCu4Pd for R = Nd and Sm. CeCu3Pd2 exhibits dense Kondo lattice behaviour with a magnetically ordered ground state. PrCu4Pd and PrCu3Pd2 show ferromagnetic-like behaviour at low temperatures in contrast to PrCu5 which is a van-Vleck paramagnet with a singlet ground state. The Mn ions in the hexagonal RCu4Mn substitute randomly on the Cu sublattice, giving rise to a complex, inhomogeneous magnetic state due to the combined effect of Cu-Mn disorder, 3d-3d and 3d-4f interactions. In RCu4Be, the as cast state of the compounds for R = Nd, Sm, Gd, Tb, Dy and Ho exhibits the hexagonal CaCu5-type symmetry, though the parent DyCu5 and HoCu5 are cubic. The cubic symmetry is adopted in ErCu4Be and TmCu4Be. Appropriate heat treatment of as cast alloys results in switching of the symmetry between the hexagonal and cubic in some of the analogues. With the exception of TmCu4Be, the compounds order magnetically. The nature of the magnetic ordering is predominantly ferromagnetic though the corresponding RCu5 compounds are mostly antiferromagnetic with relatively low magnetic ordering temperatures