Digging into the Intrinsic Negative Thermal Expansion of Uranium Tetrafluoride.

The intrinsic negative thermal expansion of UF below room temperature was examined. A redetermination of the structure of UF by single-crystal X-ray diffraction at 100, 200, and 300 K accompanied by an evaluation of the atomic displacement parameters (ADPs) of the F atoms was performed. The structure of UF was described as the stacking of two subnetworks, respectively, constituted by the U(1) and U(2) atoms. The subnetwork formed by the U(2) atoms consists of infinite layers that run parallel to the (, ) plan. The layers are composed of infinite zigzag chains of corner-sharing U(2)F polyhedra running along the -axis. An increase of temperature from 100 to 300 K leads to a decrease of the unit cell volume and the and lattice parameters and an increase of the lattice parameter. As the temperature increases, the intrachain, interchain, and interlayer U-U distances decrease. It is proposed that the decrease of the intrachain and interlayer U-U distances causes a contraction of the U(2) subnetwork along the - and -axis and that a translation of the chains along the -axis causes an expansion along the -axis. Analysis of the ADPs of the F atoms indicates that a guitar string effect in the U-F-U units is possibly the origin of the decrease in the U-U distances. A correlation between the U-U distances and the magnitude of the ADPs of the F atoms was established.