RNi8Si3 (R=Gd,Tb): Novel ternary ordered derivatives of the BaCd11 type

Abstract

The title compounds have been synthesized and characterized both from the structural and magnetic point of view. Both crystallize in a new monoclinic structure strictly related to the tetragonal BaCd11 type. The structure was solved by means of X-ray single-crystal techniques for GdNi8Si3 and confirmed for TbNi8Si3 on powder data; the corresponding lattice parameters (obtained from Guinier powder patterns) are a=6.3259(2), b=13.7245(5), c=7.4949(3)Å, β=113.522(3)°, Vcell=596.64(3)Å3 and a=6.3200(2), b=13.6987(4), c=7.4923(2)Å, β=113.494(2)°, Vcell=594.88(2)Å3. The symmetry relationship between the tI48-I41/amd BaCd11 aristotype and the new ordered mS48-C2/c GdNi8Si3 derivative is described via the Bärnighausen formalism within the group theory.The large Gd–Gd (Tb–Tb) distances, mediated via Ni–Si network, likely lead to weak magnetic interactions. Low-field magnetization vs temperature measurements indicate weak and field-sensitive antiferromagnetic ground state, with ordering temperatures of 3K in GdNi8Si3 and about 2–3K in TbNi8Si3. On the other hand, the isothermal field-dependent magnetization data show the presence of competing interactions in both compounds, with a field-induced ferromagnetic behavior for GdNi8Si3 and a ferrimagnetic-like behavior in TbNi8Si3 at the ordering temperature TC/N of about (or slightly higher than) 3K. The magnetocaloric effect, quantified in terms of isothermal magnetic entropy change ΔSm, has the maximum values of –19.8J(kgK)−1 (at 4K for 140kOe field change) and −12.1J(kgK)−1 (at 12K for 140kOe field change) in GdNi8Si3 and TbNi8Si3, respectively.