Crystal structure and physical properties of Yb2In and Eu2−xYbxIn alloys

While binary $R{E}_{2}\mathrm{In}$, where $RE=\mathrm{rare}\phantom{\rule{0.16em}{0ex}}\mathrm{earth}$, have been reported a few decades ago, recent investigations revealed intriguing new physical insights. For instance, the discovery of a nearly ideal first-order ferromagnetic transition in ${\mathrm{Eu}}_{2}\mathrm{In}$ calls for further exploration of structures and properties of $R{E}_{2}\mathrm{In}$, in particular for the least-documented $RE=\mathrm{Eu}$ and Yb cases. Here, we investigate ${\mathrm{Eu}}_{2\text{\ensuremath{-}}x}{\mathrm{Yb}}_{x}\mathrm{In}$ pseudobinaries with nominal values of $x=0.25$, 0.5, 0.75, 1, 1.5, 2 by powder x-ray diffraction (including as function of temperature from 100 to 375 K for ${\mathrm{Yb}}_{2}\mathrm{In}$), magnetization (5--300 K), as well as electrical resistivity (5--300 K) and calorimetric (2--150 K) measurements for ${\mathrm{Yb}}_{2}\mathrm{In}$. Compared to other RE, Yb or Eu always raise challenging questions linked to their valence states. From average atomic volume, Yb is anticipated to be divalent in ${\mathrm{Yb}}_{2}\mathrm{In}$, at least between 100 and 375 K, which is in line with the absence of $4f$ magnetism. In agreement with x-ray diffraction and magnetization data, the resistivity of ${\mathrm{Yb}}_{2}\mathrm{In}$ is rather featureless and typical of a metal. Establishing ${\mathrm{Yb}}_{2}\mathrm{In}$ as a nonmagnetic isostructural reference for ${\mathrm{Eu}}_{2}\mathrm{In}$ allows one to use its heat capacity to revisit that of the latter, and get experimental insights into the exceptional magnetocaloric effect of the compound with Eu. In particular, we show that a third of the total magnetic entropy (${S}_{\mathrm{m}}\ensuremath{\approx}35.6\phantom{\rule{0.16em}{0ex}}\mathrm{J}\phantom{\rule{0.16em}{0ex}}\mathrm{mo}{\mathrm{l}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at $T=100\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) is concentrated in a 3 K temperature window around the ${T}_{\mathrm{C}}$ of ${\mathrm{Eu}}_{2}\mathrm{In}$. Starting from the ferromagnetic compound ${\mathrm{Eu}}_{2}\mathrm{In}$ $[{T}_{\mathrm{C}}=55.2(5)\phantom{\rule{0.16em}{0ex}}\mathrm{K}]$, we show that Yb substitutions in ${\mathrm{Eu}}_{2\text{\ensuremath{-}}x}{\mathrm{Yb}}_{x}\mathrm{In}$ lead to a decrease in both the Curie temperature [${T}_{\mathrm{C}}=41(2)$ and 32(2) K for $x=0.25$ and 0.5] and magnetic saturation, while weakening the first-order character of the transition as $x$ increases. A significant isothermal entropy change of $5.1(4)\phantom{\rule{0.16em}{0ex}}\mathrm{J}\phantom{\rule{0.16em}{0ex}}\mathrm{mo}{\mathrm{l}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ for $\mathrm{\ensuremath{\Delta}}B=2\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ is found at 44 K in ${\mathrm{Eu}}_{1.75}{\mathrm{Yb}}_{0.25}\mathrm{In}$, demonstrating that the giant magnetocaloric effect of ${\mathrm{Eu}}_{2}\mathrm{In}$ can be tuned to lower temperatures by Yb substitutions.