Tailoring the magnetism of Tb5Si2Ge2compounds by La substitution

Abstract

In order to study the crystal structure, phase relationships, and magnetic properties of the system Tb${}_{5\ensuremath{-}\mathit{x}}$La${}_{\mathit{x}}$Si${}_{2}$Ge${}_{2}$, a series of polycrystalline samples with compositions ranging from $x=0$ to $x=5$ have been synthesized and characterized in detail. At room temperature, two structures have been detected: Compounds in the concentration range $0\ensuremath{\leqslant}x<1$ present a monoclinic Gd${}_{5}$Si${}_{2}$Ge${}_{2}$-type structure, while for $x>1$ a tetragonal Zr${}_{5}$Si${}_{4}$-type structure is observed. The unit cell volume increases linearly with La concentration but with two different slopes: $\ensuremath{\sim}$18 \AA{}${}^{3}/x$ and $\ensuremath{\sim}$33 \AA{}${}^{3}/x$ for $0\ensuremath{\leqslant}x<1$ (monoclinic) and $x>1$ (tetragonal), respectively. In the monoclinic region, an increase of ${T}_{C}$ was observed, reaching a maximum value of ${T}_{C}$ $\ensuremath{\sim}154$ K, at the $x=0.75$ composition. This feature is explained based on specific La occupancy at the $R2$ site which was supported by density functional calculations for low La concentration. The samples that crystallized in the tetragonal structure exhibit a linear decrease of ${T}_{C}(x)$ with a slope of $\ensuremath{\partial}{T}_{C}$/$\ensuremath{\partial}x$ $\ensuremath{\sim}\ensuremath{-}38$ K/$x$, reaching 0 K for the $x=5$ composition. A magnetic and structural $x$-$T$ phase diagram of the Tb${}_{5\ensuremath{-}\mathit{x}}$La${}_{\mathit{x}}$Si${}_{2}$Ge${}_{2}$ system in the temperature range 4--300 K is proposed.