Cu_xUTe₃: Stabilization of UTe₃ in the ZrSe₃ structure type via copper insertion. The artifact of Te-Te chains and evidence for distortions due to long range modulations

The ternary phase, Cu_xUTe₃ (x = 0.25 and 0.33), was synthesized from a 3/1/4 mixture of Cu/U/Te that was heated to 800 °C for 6 days and cooled at a rate of -4 °C h^-1. It adopts the monoclinic space group P2₁/m with a = 6.0838(12) Å, b = 4.2140(8) Å, c = 10.361(2) Å, β = 98.83(3)°, and V = 262.47(9) ų (for x ∼ 0.25). The structure is built from UTe₃ layers of ZrSe₃-type that are connected in the [001] direction by Cu atoms. The Cu atoms stabilize α-UTe₃ by inserting between the layers. Cu_xUTe₃ can be prepared rationally via a soft chemistry route by reaction of Cu with α-UTe₃. The structural analysis suggests the presence of straight chains of Te atoms (∼3.0 Å apart) along the a-axis but this is an artifact as shown by electron diffraction studies of Cu_xUTe₃ that indicate the existence of a supercell along the a-axis. Pair distribution function analysis (PDF) was used to show that the Te-Te chains contain Te-Te dimers at 2.74 Å. Charge transport measurements suggest a narrow gap semiconductor but they also indicate anomalous behavior as a function of temperature with a n-type to p-type transition at ∼40 K.