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

Abstract

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.