Magnetic, transport, and thermal properties of Yb2T3X9 compounds (T = Rh, Ir; X = Al, Ga)

The low-temperature properties of a family of ytterbium-based compounds with the composition ${\mathrm{Yb}}_{2}{T}_{3}{X}_{9}$ $(T=\mathrm{Rh}, \mathrm{Ir}; X=\mathrm{Al}, \mathrm{Ga})$ are studied by means of magnetic susceptibility, electrical resistivity, thermoelectric power, and specific-heat ${C}_{P}$ measurements. ${\mathrm{Yb}}_{2}{\mathrm{Rh}}_{3}{\mathrm{Al}}_{9}$ and ${\mathrm{Yb}}_{2}{\mathrm{Ir}}_{3}{\mathrm{Al}}_{9}$ show the typical properties of heavy-fermion antiferromagnets, namely they order at ${T}_{N}=3.5 \mathrm{K}$ and ${T}_{N}=5.5 \mathrm{K},$ respectively, their ${C}_{P}/T$ data extrapolate to ${\ensuremath{\gamma}}_{o}=230 \mathrm{mJ}/\mathrm{mol} \mathrm{Yb} {\mathrm{K}}^{2}$ below 0.5 K, and their entropy gain at ${T}_{N}$ reaches $0.6R\mathrm{ln}2$ only. The transport data show evidence of low-lying crystal-electric-field levels separated by ${\ensuremath{\Delta}}_{\mathrm{CEF}}\ensuremath{\simeq}30 \mathrm{K}$ from the ground-state doublet. On the contrary, ${\mathrm{Yb}}_{2}{\mathrm{Rh}}_{3}{\mathrm{Ga}}_{9}$ and ${\mathrm{Yb}}_{2}{\mathrm{Ir}}_{3}{\mathrm{Ga}}_{9}$ show mixed-valent behavior, as observed in most Yb-based intermetallics. The effect of the ligands on the ground-state properties is discussed and compared with other lanthanide- and actinide-based isostructural ${R}_{2}{T}_{3}{X}_{9}$ homologues.