Physical properties of rare-earth-based Heusler phases REPdZ and REPd/sub 2/Z (Z = Sb,Bi)

Abstract

Four series of ternary compounds REPdSb (RE = Y, Ho, Er), REPdBi (RE = Nd, Y, Dy, Ho, Er), REPd/sub 2/Sb (RE = Y, Gd-Er) and REPd/sub 2/Bi (RE = Y, Dy-Er) were studied by means of magnetization, magnetic susceptibility, electrical resistivity, and thermoelectric power measurements performed in the temperature range 1.7-300 K and in magnetic fields up to 5 T. Magnetic susceptibility measurements of the Heusler phases REPdZ and REPd/sub 2/Z (RE = Y, Gd-Er; Z = Sb, Bi) have revealed that at low temperatures the compounds studied are either paramagnetic or antiferromagnetic with the Neel points of several Kelvin. For all these compounds but Y-based alloys experimental values of the effective magnetic moment are close to the theoretical ones for respective RE/sup 3+/ ions. Electrical resistivity measurements of the half-Heusler phases REPdX (X = Sb, Bi) have indicated their half-metallic or semimetallic character. At low temperatures the resistivity exhibits a metallic-like behaviour, which is probably due to an impurity band possibly overlapping with the conduction band. At high temperatures the electrical conductivities follow the activation law that accounts for gradual thermal depopulation of impurity levels. The positive sign of the Seebeck coefficient could indicate that holes arising from some acceptor centres are dominant carriers. The energy gaps near the Fermi level are of the order of tens millielectron volts and thus are much smaller than values estimated from the electronic structure calculations made for similar compounds. The Seebeck coefficient is quite large, of the order expected for low carrier density semimetals. The thermoelectric figure of merit evaluated for ErPdSb is ZT /spl ap/ 0.15 at room temperature. While half-Heusler alloys have been characterised as narrow-gap semiconductors all the REPd/sub 2/Sb and REPd/sub 2/Bi ternaries have been found to exhibit metallic-like conductivity. For the REPd/sub 2/Bi series a structural transformation from cubic to lower symmetry structures occurs that manifests itself as distinct anomalies in the transport characteristics. The thermoelectric power for these materials is much lower than that of the REPdX alloys.