  The pentatellurides, ZrTe5 and HfTe5 are layered compounds with one
dimensional transition-metal chains that show a never understood temperature
dependent transition in transport properties as well as recently discovered
properties suggesting topological semimetallic behavior. Here we show that
these materials are semiconductors and that the electronic transition is due to
a combination of bipolar effects and different anisotropies for electrons and
holes. We report magneto-transport properties for two kinds of ZrTe5 single
crystals grown with the chemical vapor transport (S1) and the flux method (S2),
respectively. These have distinct transport properties at zero field: the S1
displays a metallic behavior with a pronounced resistance peak and a sudden
sign reversal in thermopower at approximately 130 K, consistent with previous
observations of the electronic transition; in strikingly contrast, the S2
exhibits a semiconducting-like behavior at low temperatures and a positive
thermopower over the whole temperature range. Refinements on the single-crystal
X-ray diffraction and the energy dispersive spectroscopy analysis revealed the
presence of noticeable Te-vacancies in the sample S1, confirming that the
widely observed anomalous transport behaviors in pentatellurides actually take
place in the Te-deficient samples. Electronic structure calculations show
narrow gap semiconducting behavior, with different transport anisotropies for
holes and electrons. For the degenerately doped n-type samples, our transport
calculations can result in a resistivity peak and crossover in thermopower from
negative to positive at temperatures close to that observed experimentally. Our
present work resolves the longstanding puzzle regarding the anomalous transport
behaviors of pentatellurides, and also resolves the electronic structure in
favor of a semiconducting state.
