  We measured the thermal conductivity of an icosahedral quasicrystal
i-Al_{72}Pd_{19.5}Mn_{8.5} in the temperature range between 0.4 K and 300 K.
The analysis of the low-temperature results was based on a Debye-type model.
The results of the analysis for the two temperature regions of 0.4 K <T<40 K
and 0.4 K <T<1 K, are not consistent in the sense that a tunnelling-states
contribution to phonon scattering is verified only for 0.4 K <T<1 K. The same
fitting procedure indicates that structural defects of the stacking-fault type
are an important source of phonon scattering. Their physical presence was
cleary identified by a transmission electron microscopy experiment.
