Pressure-induced structural and electronic phase transitions of uranium trioxide

The crystal structures, phonon spectra, and electronic properties of uranium trioxide (${\mathrm{UO}}_3$) under high pressure have been systematically explored using a particle swarm optimization structure prediction method in conjunction with first-principles calculations. Our calculated lattice constants and the transition pressure of the two experimentally reported phases of $\gamma$- and $\eta -{\mathrm{UO}}_3$ are consistent with previous experiments. At pressures of 13, 62, 220 GPa, three new structures of $P{6}_3$/$mmc$, $Pm\overline{3}n$, and $Fm\overline{3}m$ are predicted in sequence to be thermodynamically stable. Based on our calculated elastic constants and phonon spectra, we indicate that these three phases are mechanically and dynamically stable. Interestingly, upon phase transition from $P{6}_3$/$mmc$ to $Pm\overline{3}n$, ${\mathrm{UO}}_3$ undergoes a semiconductor-to-metal electronic transition. In addition, we report results of specific heat, entropy, bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and Debye temperature. Our results provide key insights into understanding the structural as well as the electronic behaviors of ${\mathrm{UO}}_3$ under the condition of external pressure.

Figure 1

The relative enthalpy curves as a function of pressure for ${\mathrm{UO}}_3$ from 0 to 300 GPa. The enthalpy of the $Pm\overline{3}m$ phase is set as zero for reference.

Figure 2

Structural features of stable ${\mathrm{UO}}_3$ phases at different pressures: (a) $\mathit{Fddd}$, $I{4}_1/amd$ at 0 GPa, (b) $P{2}_1{2}_1{2}_1$ at 3 GPa, (c) $P{6}_3$/$mmc$ at 50 GPa, (d) $Pm\overline{3}n$ at 200 GPa, and (e) $Fm\overline{3}m$ at 250 GPa. In all these structures, green and orange spheres represent O and U atoms, respectively.

Figure 3

Calculated phonon dispersions as well as PhDOSs for (a) $P{6}_3$/$mmc$ phase at 50 GPa, (b) $Pm\overline{3}n$ phase at 200 GPa, and (c) $Fm\overline{3}m$ phase at 250 GPa.

Figure 4

Temperature dependencies of (a) specific heat at constant volume and (b) entropy for $P{6}_3$/$mmc$, $Pm\overline{3}n$, and $Fm\overline{3}m$ phases of ${\mathrm{UO}}_3$ at 50, 200, and 250 GPa, respectively. Results of the $Fm\overline{3}m{\mathrm{UO}}_2$ from DFT [13] and experiments [70, 71] are presented for comparison.

Figure 5

Electronic band structures for (a) $P{6}_3$/$mmc$ phase at 50 GPa, (b) $Pm\overline{3}n$ phase at 200 GPa, and (c) $Fm\overline{3}m$ phase at 250 GPa. The size of the circles of the electronic band is proportional to the contribution of U-$5f$, U-$6d$, and O-$2p$ electrons.

Figure 6

TDOSs and PDOSs for (a) $I{4}_1$/$amd$ phase at 0 GPa, (b) $P{2}_1{2}_1{2}_1$ phase at 5 GPa, (c) $P{6}_3$/$mmc$ phase at 50 GPa, (d) $Pm\overline{3}n$ phase at 200 GPa, and (e) $Fm\overline{3}m$ phase at 250 GPa. The Fermi energy level is set as zero and is denoted by the vertical dotted line.

Figure 7

The band gaps of (a) $\alpha$-, (b) $\beta$-, (c) $\delta$-, and (d) $\gamma -{\mathrm{UO}}_3$ calculated by the $\mathrm{GGA}+U$ formalism with $U$ in the range of 0–6 eV. For comparison, experimental [4, 49] and other theoretical [5, 35, 81] results are also presented.

Figure 8

The total DOS for the (a) $P{6}_3$/$mmc$, (b) $Pm\overline{3}n$, and (c) $Fm\overline{3}m$ phase calculated within $\mathrm{GGA}+U$ formalism with $U=0$, 3, and 6 eV. The projected DOSs for the U-$5f$ and O-$2p$ orbitals are also shown. The Fermi energy level is set at zero.