  From a systematic study of thermal and charge transport in various single
crystals of compensated topological insulators we identify the evolution of a
large low-temperature thermal Hall effect as a characteristic common feature.
In order to separate phononic and electronic contributions in the measured
longitudinal and transverse thermal conductivity, the electronic contributions
are estimated from corresponding electrical resisivity and Hall effect
measurements on the same samples by using the Wiedemann-Franz law. As may be
expected for charge-compensated topological insulators the longitudinal thermal
conductivity is phonon-dominated in all samples. However, we also find a
pronounced field-linear thermal Hall effect that becomes most pronounced in the
low-temperature range, where all samples are good electrical insulators. This
indicates an underlying phononic mechanism of the thermal Hall effect and in
this respect the topological insulators resemble other, mainly ionic,
insulators, which have been reported to show a phonon-induced thermal Hall
effect, but its underlying phononic mechanism remains to be identified. Our
observation of a comparable thermal Hall ratio in topological insulators
supports a theoretical scenario that explains a thermal Hall effect through
skew scattering on charged impurities.
