Layered semiconductor EuTe4 with charge density wave order in square tellurium sheets

We report a novel quasi-two-dimensional compound of EuTe4 hosting charge density waves (CDW) instability. The compound has a crystallographic structure in a orthorhombic space group Pmmn (No. 59) with cell parameters a = 4.6347(2) angstrom, b = 4.5119(2) angstrom, c = 15.6747(10) angstrom at room temperature. The pristine structure contains consecutive near-square Te sheets separated by corrugated Eu-Te slabs. Upon cooling, the compound experiences a phase transition near 255 K. X-ray crystallographic analysis and transmission electron microscopy measurements reveal strong structural distortions in the low-temperature phase, showing a 1a x 3b x 2c superstructure with a periodic formation of Te trimers in the monolayer Te sheets, yielding evidence for the formation of CDW order. The charge transport properties show a semiconducting behavior in the CDW state. Density functional theory calculations reveal a Fermi surface nesting-driven instability with a nesting vector in good agreement with the one observed experimentally. Our finding provides a promising system for the study of CDW driven two-dimensional (2D) semiconducting mechanisms, which would shed a light on exploring 2D semiconductors with collective electronic states.