Magnetic properties and the investigation of possible interstitial occupancy by small atoms in some R3Ni6X2 compounds (R=rare earth; X=Si and Al)

Abstract Polycrystalline samples of R 3 Ni 6 X 2 compounds (R=Gd, Tb, Dy, Ho, Er and Y; X=Si and Al) were synthesized by arc melting followed by a long annealing at 800 °C. Powder X-ray diffraction spectra of these compounds showed that the R 3 Ni 6 X 2 compounds crystallize in the cubic Ca 3 Ag 8 -type structure with a large unit cell parameter a ∼ 8.9 A as reported previously in the literature. Various physical properties like magnetization, electrical resistivity, and heat capacity of R 3 Ni 6 Al 2 and Gd 3 Ni 6 Si 2 were measured. The aluminides order ferromagnetically and the Curie temperature, which is the highest (∼120 K) in the Gd compound, decreases in the heavier analogs qualitatively, in accordance with the deGenness scaling. The data show evidence of two magnetic transitions in some compounds. We have also investigated the possibility of introducing small atoms like boron and carbon as interstitial atoms in these compounds by melting alloys with compositions R 3 Ni 6 X 2 T y (T=B and C; 0 y ⩽ 1.5 ). We find that while boron and carbon cannot be alloyed with R 3 Ni 6 Al 2 compounds to form single-phase alloys, a limited solubility of carbon exists in Gd 3 Ni 6 Si 2 . Both Gd 3 Ni 6 Si 2 and Gd 3 Ni 6 Si 2 C 0.5 are antiferromagnetic but the transition temperature of the carbide is enhanced compared to the parent compound. The resistivity of the carbide shows an upturn at the magnetic transition as the temperature is reduced, arising most probably due to the additional energy gaps introduced by incommensurate antiferromagnetic order.