Measurements of the energy band gap and valence band structure of AgSbTe₂

The de Haas-van Alphen effect, galvanomagnetic and thermomagnetic properties of high-quality crystals of AgSbTe₂ are measured and analyzed. The transport properties reveal the material studied here to be a very narrow-gap semiconductor (E_g≈7.6±3 meV) with ∼5×10¹⁹ cm⁻³ holes in a valence band with a high density of states and thermally excited ∼10¹⁷ cm⁻³ high-mobility (2200 cm²/Vs) electrons at 300 K. The quantum oscillations are measured with the magnetic field oriented along the ⟨111⟩ axis. Taken together with the Fermi energy derived from the transport properties, the oscillations confirm the calculated valence band structure composed of 12 half-pockets located at the X-points of the Brillouin zone, six with a density-of-states effective mass m_da^∗⪢0.21m_e and six with m_db^∗⪢0.55m_e, giving a total density-of-states effective mass, including Fermi pocket degeneracy, of m_d^∗≈1.7±0.2m_e (m_e is the free electron mass). The lattice term dominates the thermal conductivity, and the electronic contribution in samples with both electrons and holes present is in turn dominated by the ambipolar term. The low thermal conductivity and very large hole mass of AgSbTe₂ make it a most promising p-type thermoelectric material.