Charge Density Wave And Narrow Energy Gap At Room Temperature In 2d Pb3-Xsb1+Xs4te2-Delta With Square Te Sheets

We report a new two-dimensional compound, Pb3-xSb1+xS4Te2-delta, 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 delta = 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 P (1) over bar(alpha beta gamma)0 using (3 + 1)-dimensional crystallography. The resistivity increases with decreasing temperature, suggesting semiconducting behavior. The transition temperature (T-CDW) of the CDW is similar to 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.