Charge Density Wave and Narrow Energy Gap at Room Temperature in 2D Pb_3-xSb_1+xS₄Te_2-δ with Square Te Sheets

We report a new two-dimensional compound, Pb_3-xSb_1+xS₄Te_2-δ, that has a charge density wave (CDW) at room temperature. The CDW is incommensurate with q-vector of 0.248(6)a∗ + 0.246(8)b∗ + 0.387(9)c∗ for x = 0.29(2) and δ = 0.37(3) due to positional and occupational long-range ordering of Te atoms in the sheets. The modulated structure was refined from the single-crystal X-ray diffraction data with a superspace group P1(αβγ)0 using (3 + 1)-dimensional crystallography. The resistivity increases with decreasing temperature, suggesting semiconducting behavior. The transition temperature (T_CDW) of the CDW is ∼345 K, above which the Te square sheets become disordered with no q-vector. First-principles density functional theory calculations on the undistorted structure and an approximate commensurate supercell reveal that the gap is due to the structure modulation.