Low-temperature thermal properties of yttrium and lutetium dodecaborides

The heat capacity (C_p) and dilatation (α) of YB₁₂ and LuB₁₂ are studied. C_p of the zone-melted YB₁₂ tricrystal is measured in the range 2.5-70 K, of the zone-melted LuB₁₂ single crystal in the range 0.6-70 K, and of the LuB₁₂ powder sample in the range 4.3-300 K α of the zone-melted YB₁₂ tricrystal and LuB₁₂ single crystals is measured in the range 5-200 K. At low temperatures a negative thermal expansion (NTE) is revealed for both compounds: for YB₁₂ at 50-70 K, for LuB₁₂ at 10-20 K and 60-130 K. Their high-temperature NTE is a consequence of nearly non-interacting freely oscillating metal ions (Einstein oscillators) in cavities of a simple cubic rigid Debye lattice formed by B₁₂ cage units. The Einstein temperatures are ~254 and ~164 K, and the Debye temperatures are ~1040 K and ~1190 K for YB₁₂ and LuB₁₂ respectively. The LuB₁₂ low-temperature NTE is connected with an induced low-energy defect mode. The YB₁₂ superconducting transition has not been detected up to 2.5 K.