N.V. Mitroshenkov

Negative thermal expansion and anomalies of heat capacity of LuB50 at low temperatures

Heat capacity and thermal expansion of LuB50 boride were experimentally studied in the 2-300 K temperature range. The data reveal an anomalous contribution to the heat capacity at low temperatures. The value of this contribution is proportional to the first degree of temperature. It was identified that this anomaly in heat capacity is caused by the effect of disorder in the LuB50 crystalline structure and it can be described in the soft atomic potential model (SAP). The parameters of the approximation were determined. The temperature dependence of LuB50 heat capacity in the whole temperature range was approximated by the sum of SAP contribution, Debye and two Einstein components. The parameters of SAP contribution for LuB50 were compared to the corresponding values for LuB66, which was studied earlier. Negative thermal expansion at low temperatures was experimentally observed for LuB50. The analysis of the experimental temperature dependence for the Gruneisen parameter of LuB50 suggested that the low-frequency oscillations, described in SAP mode, are responsible for the negative thermal expansion. Thus, the glasslike character of the behavior of LuB50 thermal characteristics at low temperatures was confirmed.

Anomalies in thermal expansion and heat capacity of TmB50 at low temperatures: magnetic phase transition and crystal electric field effect

We experimentally study the heat capacity and thermal expansion of thulium boride (TmB50) at temperatures of 2-300 K. The wide temperature range (2-180 K) of boride negative expansion was revealed. We found the anomalies in C(T) heat capacity temperature dependence, attributed to the Schottky contribution (i.e. the influence of the crystal electric field: CEF), as well as the magnetic phase transition. CEF-splitting of the f-levels of the Tm3+ ion was described by the Schottky function of heat capacity with a quasi-quartet in the ground state. Excited multiplets are separated from the ground state by energy gaps δ1 = 100 K, and δ2 ≈ 350 K. The heat capacity maximum at Tmax ≈ 2.4 K may be attributed to the possible magnetic transition in TmB50. Other possible causes of the low-temperature maximum of C(T) dependence are the nonspherical surroundings of rare earth atoms due to the boron atoms in the crystal lattice of the boride and the emergence of two-level systems, as well as the splitting of the ground multiplet due to local magnetic fields of the neighboring ions of thulium. Anomalies in heat capacity are mapped with the thermal expansion features of boride. It is found that the TmB50 thermal expansion characteristic features are due to the influence of the CEF, as well as the asymmetry of the spatial arrangement of boron atoms around the rare earth atoms in the crystal lattice of RB50. The Grüneisen parameters, corresponding to the excitation of different multiplets of CEF-splitting, were determined. A satisfactory accordance between the experimental and estimated temperature dependencies of the boride thermal expansion coefficient was achieved.