Defect mode and crystal-electric-field effects on the thermal expansion and heat capacity of RB50 boride

Abstract

Specific heat and thermal expansion in LuB50 and TbB50 borides at 2–300 K were experimentally studied. In both compounds, for a large range of low temperatures a negative thermal expansion caused by defects in the crystal lattice was observed. The effect of the crystal electric field on the thermal characteristics of TbB50 was observed as a Schottky anomaly in the temperature dependence of the heat capacity and as an additional negative contribution to the thermal expansion. The analysis of the heat capacity data suggests that the ground state of the ion Tb3+ is a quasi-quintet, consisting of close-located singlet and two doublets. The phase transition of TbB50 to the antiferromagnetic state appears as a very weak broad anomaly in the temperature dependence of the heat capacity; within the scattering of the experimental data, it is not visible in the temperature dependence of the unit cell volume.