Structural and Magnetocaloric Properties of the New Ternary Silicides Gd₆M_5/3Si₃ with M = Co and Ni

Unlike Gd₆Co_5/3Si₃, which is obtained as a single phase after melting of the constituents, annealing under vacuum at 1073 K for 1 month is necessary to prepare the new ternary silicide Gd₆Ni_5/3Si₃. Indeed, after melting, this alloy is a mixture containing mainly Gd₃NiSi₂ and Gd₅Si₃. These two compounds, Gd₆M_5/3Si₃ (M = Co or Ni), crystallize with the hexagonal Ce₆Ni_2−<i>x</i>Si₃-type structure. The crystal structure of Gd₆Ni_5/3Si₃, refined on a single crystal with space group <i>P</i>6₃/<i>m</i> and unit cell parameters <i>a</i> = 11.7433(1) Å and <i>c</i> = 4.1857(1) Å, is characterized by infinite chains of face-shared trigonal prisms [Gd₆] filled by silicon or nickel atoms. These chains run along the <i>c</i>-axis and extend as triangular columns by sharing rectangular faces in the <i>a</i><i>b</i>-plane. Between these columns, infinite chains of face-shared octahedra of Gd atoms partially are filled by Ni atoms. The strong delocalization of the electron density of Ni observed in these latter chains has been attributed to high steric strains. No diffuse scattering nor superstructure were observed from electron diffraction experiments. The magnetization measurements reveal that (i) Gd₆Ni_5/3Si₃ orders ferromagnetically at 310 K, a Curie temperature higher than that observed for Gd₆Co_5/3Si₃ (<i>T</i>_C = 294 K) and pure gadolinium (<i>T</i>_C = 294 K); (ii) Ni as well as Co carries no magnetic moment; and (iii) these ternary silicides exhibit interesting magnetocaloric properties; for instance, the magnetic entropy (Δ<i>S</i>_m determined by magnetization measurements) of Gd₆Ni_5/3Si₃ is at a maximum around 312 K with values of Δ<i>S</i>_m = −2.93 and –5.72 J/K kg at applied magnetic fields of 2 and 4.8 T, respectively. These properties are compared to those existing for the most famous magnetocaloric materials as Gd or Gd₅Ge₂Si₂.