Using Landau-level spectroscopy, we determine the temperature dependence of the energy band gap in zirconium pentatelluride (${\mathrm{ZrTe}}_5$). We find that the band gap reaches $E_g=(5\pm 1)\mathrm{meV}$ at low temperatures and increases monotonically when the temperature is raised. This implies that ${\mathrm{ZrTe}}_5$ is a weak topological insulator, with noninverted ordering of electronic bands in the center of the Brillouin zone. Our magnetotransport experiments performed in parallel show that the resistivity anomaly in ${\mathrm{ZrTe}}_5$ is not connected with the temperature dependence of the band gap.

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• Landau levels
• Topological insulators
• Dirac semimetal
• Infrared spectroscopy