U2Ru2Sn has been classified as the first tetragonal U-based Kondo insulator. Here, we present measurements of the thermal conductivity κ and thermopower S of high-quality single-crystalline U2Ru2Sn along and perpendicular to the tetragonal c-axis, in the temperature range between 100 mK and 1 K, in zero field and in a magnetic field of 6 T. Below 400 mK, the phonon contribution to κ(T) shows a T2 behaviour for both directions that can be attributed to phonons scattered by electrons. S(T) presents a linear behaviour in the whole temperature range. S is positive along the c-axis and negative perpendicular to the c-axis. Using a one-band model the effective mass m* is estimated to be 2m0 along and 16m0 perpendicular to the c-axis, where m0 is the free-electron mass. This indicates that U2Ru2Sn has a highly anisotropic residual density of states within the pseudogap.

We present low-temperature heat and charge transport as well as caloric properties of a ThAsSe single crystal. An extra -AT1/2 term in the electrical resistivity, independent of magnetic fields as high as 14 T, provides evidence for an unusual scattering of conduction electrons. Additionally, both the thermal conductivity and specific heat show a glass-type temperature dependence which signifies the presence of tunneling states. These observations apparently point to an experimantal realization of a two-channel Kondo effect derived from structural two-level systems.

Transport properties of Ba8Ga16Ge30 single-crystalline samples prepared with an excess of Ga are presented. The excess Ga does not only produces p-type transport properties but also a thermal conductivity (kappa) that, unlike in any other Ba8Ga16Ge30 samples reported in the literature, is similar to that of the iso-structural Eu8Ga16Ge30 and Sr8Ga16Ge30. These observations disagree with the commonly made assumption that kappa of Eu8Ga16Ge30 and Sr8Ga16Ge30 at very low temperatures is determined by phonons scattered from guest atom tunneling states, since such states are believed to be absent in Ba8Ga16Ge30. Instead we propose that phonon charge carrier scattering must be considered in order to explain kappa at the low temperatures. The transport data also suggest that the resonant scattering, which dominates at intermediate temperatures, strongly depends on the charge carrier concentration.

The thermal conductivity κ of Yb4(As1−xPx)3 (x=0 and 0.3) was measured in the temperature range between 0.1 and 7K in applied magnetic fields up to 8T. The fact that κ(T) follows, between 0.4 and 3K, the relation aT+bT2 confirms our earlier interpretation of the dominating role of magnons both as heat carriers and as scatterers for the phonons. Above 1K, κ decreases with increasing magnetic field, the opening of a gap in the magnon-excitation spectrum being a possible explanation of this behavior.

We discuss to which extent the concepts of Zintl phases and of 'phonon glasses and electron single crystals' apply to type-I clathrates. In (β-) Eu8Ga16Ge30 the presence of residual charge carriers appears to be related to a slight off-stoichiometry of the samples pointing to the validity of the Zintl concept in stoichiometric samples. The low and almost stoichiometry independent mobilities of (β-) Eu8Ga16Ge30, Sr8Ga16Ge30, and Ba8Ga16Ge30 seriously question the validity of the 'electron single crystal' concept for type-I clathrates. The temperature dependence of the thermal conductivity of a Ba8Ga16Ge30 single crystal indicates that tunneling states play a central role in producing 'phonon glass'-like thermal conductivities.