Specific features of thermal and magnetic properties of YbB50 at low temperatures

Heat capacity, thermal expansion, and magnetization of ytterbium boride $\mathrm{Yb}{\mathrm{B}}_{50}$ were studied at temperatures 0.6--300 K, 5--300 K, and 2--300 K, respectively. We revealed two smooth peaks at about 4.0 and 60 K in the temperature dependence of the heat capacity. A comparison with the heat capacity of the diamagnetic isostructural boride $\mathrm{Lu}{\mathrm{B}}_{50}$ shows that these anomalies can be attributed to excitations in the ytterbium sublattice (Schottky anomalies). A scheme for splitting of the ground $^{2}F_{7/2}$ multiplet of $\mathrm{Y}{\mathrm{b}}^{3+}$ ions in the crystal field is proposed. Reliability of the proposed crystal-field energies of the $\mathrm{Y}{\mathrm{b}}^{3+}$ ions is confirmed by the analysis of temperature dependencies of magnetic susceptibility and magnetization in applied magnetic fields up to 55 kOe. A clear anisotropy of the thermal expansion and a negative expansion within a wide temperature range (40--185 K) were observed. Assuming that this anomaly of the thermal expansion in higher borides is caused by the specific thermal evolution of a crystal lattice observed earlier, in particular, in $\mathrm{Lu}{\mathrm{B}}_{50}$, and the interaction of rare-earth ions with lattice strains, we have determined phenomenological Gr\"uneisen parameters which characterize effects due to thermal transitions of $\mathrm{Y}{\mathrm{b}}^{3+}$ ions between the ground and excited states. A phase transition of $\mathrm{Yb}{\mathrm{B}}_{50}$ to any magnetically ordered state was not observed down to the lowest temperatures of experiments.