  The present controversy over the origin of glasslike thermal conductivity
observed in certain crystalline materials is addressed by studies on
single-crystal x-ray diffraction, thermal conductivity k(T) and specific heat
Cp(T) of carrier-tuned Ba8Ga16X30 (X = Ge, Sn) clathrates. These crystals show
radically different low-temperature k(T) behaviors depending on whether their
charge carriers are electrons or holes, displaying the usual crystalline peak
in the former case and an anomalous glasslike plateau in the latter. In
contrast, Cp(T) above 4 K and the general structural properties are essentially
insensitive to carrier tuning. We analyze these combined results within the
framework of a Tunneling/Resonant/Rayleigh scatterings model, and conclude that
the evolution from crystalline to glasslike k(T) is accompanied by an increase
both in the effective density of tunnelling states and in the resonant
scattering level, while neither one of these contributions can solely account
for the observed changes in the full temperature range. This suggests that the
most relevant factor which determines crystalline or glasslike behavior is the
coupling strength between the guest vibrational modes and the frameworks with
different charge carriers.
