Low-temperature thermal properties of yttrium and lutetium dodecaborides

The heat capacity (C-p) and dilatation (alpha) 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; alpha 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 B-12 cage units. The Einstein temperatures are similar to 254 and similar to 164 K, and the Debye temperatures are similar to 1040 K and similar to 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.