Metastable phase of UTe2 formed under high pressure above 5 GPa

Abstract

Uranium ditelluride $({\mathrm{UTe}}_{2})$ has attracted recent interest due to its unique superconducting properties, which include the potential for a topological odd-parity superconducting state. Recently, ac-calorimetry measurements under pressure indicate a change in the ground state of ${\mathrm{UTe}}_{2}$ from superconducting to antiferromagnetic at 1.4 GPa. Here, we investigate the effect of pressure on the crystal structure of ${\mathrm{UTe}}_{2}$ up to 25 GPa at room temperature using x-ray diffraction. We find that ${\mathrm{UTe}}_{2}$, which at ambient conditions has an orthorhombic (Immm) structure, transforms to a body-centered tetragonal $(I4/mmm)$ structure at 5 GPa in a quasihydrostatic neon (Ne) pressure-transmitting medium. In the absence of a pressure-transmitting medium, this transformation occurs between 5 and 8 GPa. The data were fit with a third-order Birch-Murnaghan equation of state resulting in values of ${B}_{0}=46.0\ifmmode\pm\else\textpm\fi{}0.6\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, ${B}^{\ensuremath{'}}=9.3\ifmmode\pm\else\textpm\fi{}0.5$ (no pressure medium), and ${B}_{0}=42.5\ifmmode\pm\else\textpm\fi{}2.0\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, ${B}^{\ensuremath{'}}=9.3$ (fixed) (neon pressure medium) for the Immm phase. For the $I4/mmm$ phase, ${B}_{0}=78.9\ifmmode\pm\else\textpm\fi{}0.5\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ GPa and ${B}^{\ensuremath{'}}=4.2\ifmmode\pm\else\textpm\fi{}0.1$ (no pressure-transmitting medium), and ${B}_{0}=70.0\ifmmode\pm\else\textpm\fi{}1.1\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ and ${B}^{\ensuremath{'}}=4.1\ifmmode\pm\else\textpm\fi{}0.2$ (neon pressure medium). The high-pressure tetragonal phase is retained after decompression to ambient pressure, with approximately $30%$ remaining after 2 days. We argue that the observed phase transition into a higher-symmetry structure at $P\ensuremath{\sim}5\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ (orthorhombic to tetragonal) is accompanied by an increase in the shortest distance between uranium atoms from 3.6 \AA{} (orthorhombic) to 3.9 \AA{} (tetragonal), which suggests localization of the $5f$ electrons, albeit with a 10.7% decrease in volume.