  We report a detailed study of specific heat, electrical resistivity and
optical spectroscopy in the superconducting boride LuB12 (Tc = 0.4 K) and
compare it to the higher Tc compound ZrB12 (Tc = 6 K). Both compounds have the
same structure based on enclosed metallic Lu or Zr ions in oversized boron
cages. The infrared reflectivity and ellipsometry in the visible range allow us
to extract the optical conductivity from 6 meV to 4 eV in the normal state from
20 to 280 K. By extracting the superconducting properties, phonon density of
states and electron-phonon coupling function from these measurements we discuss
the important factors governing Tc and explain the difference between the two
compounds. The phonon density of states seems to be insignificantly modified by
substitution of Zr with Lu. However, the soft vibrations of the metal ions in
boron cages, responsible for the relatively high Tc in ZrB12, have almost no
contribution to the electron-phonon coupling in LuB12.
