Crystal lattice disorder and characteristic features of the low-temperature thermal properties of the higher borides

Heat capacity CP(T) and lattice parameters a(T), b(T) and c(T) of LuB44Si3,5 borosilicide are experimentally studied as function of temperature in the range of 2–300 K. The results are compared with those for the pseudo-isostructural LuB50 boride. At the lowest temperatures, it is shown that the CP(T) dependence of borosilicide changes linearly with temperature. This is attributed to the effect of glass-like character of the heat capacity due to disorder in the sublattice of non-metals. The presence of defects in the B-C sublattice and the irregular form of the cages in the B-C matrix, which are occupied by Lu3+ ions, lead to the formation of two-level systems (TLS) in the Lu3+ subsystem. The TLS lead to a characteristic bell-like low-temperature contribution to the heat capacity of borosilicide. We show that there is a wide temperature range (5–150 K) of negative thermal expansion of borocilicide, which is attributed to the influence of quasi-independent vibrations of Lu3+ ions in the cages of the borocilicide crystal structure.