  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){\AA}, b = 4.5119(2){\AA}, c = 15.6747(10){\AA} 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 (TEM) measurements reveal strong structural distortions in
the low temperature phase, showing a 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 reveals 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 2D semiconducting mechanisms, which would shed a new light on
exploring novel 2D semiconductors with collective electronic states.
