Crystal structure and ferromagnetism of the new Gd$_{3}$(Co$_{x}$Ni$_{1-x})_{2}$ compounds

The rare-earth binary compound ``Gd$_{3}$Ni$_{2}$,'' reported in current literature with an uncertain composition and unknown crystal structure, has been synthesized. We have found it to be a high temperature and stoichiometric phase; it exists in the range between 660-695 $^{\circ}$C and can be retained at room temperature, as a metastable phase, by mild quenching. Its crystal structure has been studied: it crystallizes in the monoclinic Dy$_{3}$Ni$_{2}$ structure type [\textit{mC}20, space group $C$2/$m$, Z$=$4, the lattice parameters are $a=$13.418(3) {\AA}, $b=$3.720(1) {\AA}, $c=$9.640(2) {\AA}, $\beta =$106.250(3) $^{\circ}$]. It has also been found that Ni can be replaced by substituting Co: up to x $\approx $ 1 without change of the crystal structure . Moreover, such substitution appears to stabilize these new phases down to room temperature. The physical properties of all these phases have been investigated. Gd$_{3}$Ni$_{2}$ shows a ferromagnetic behavior with T$_{C}=$150 K; the transition temperature increases with the Co content. The magnetocaloric properties have also been studied by magnetization and heat capacity measurements. Theoretical calculations have been performed to explain the phase stability, formation, and the ferromagnetic behavior of these compounds.