We report the temperature evolution of the Dirac band in semiconducting zirconium pentatelluride (${\mathrm{ZrTe}}_5$) using magnetoinfrared spectroscopy. We find that the band gap is temperature independent at low temperatures and increases with temperature at elevated temperatures. Although such an observation seems to support a weak topological insulator phase at all temperatures and defy the previously reported topological phase transition (TPT) at an intermediate temperature in ${\mathrm{ZrTe}}_5$, we show that it is also possible to explain the observation by considering the effect of conduction-valence band mixing and band inversion with a strong topological insulator phase at low temperatures. Our work provides an alternative picture of the band gap evolution across TPT.

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• Landau levels
• Topological insulators
• Dirac semimetal
• Fourier transform infrared spectroscopy