Room-temperature polar metal stabilized under high pressure

Abstract

$\mathrm{LiOs}{\mathrm{O}}_{3}$ synthesized under high pressure in recent years is a rare metal since it undergoes a nonpolar to polar phase transition at ${T}_{s}=140\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Forming a polar axis through a phase transition in a metal seems against common sense. It is also not clear whether the transition to a polar phase in the oxide fits the mechanism predicted by Anderson and Blount in 1965. As monitored by an anomaly of resistivity in $\mathrm{LiOs}{\mathrm{O}}_{3}$ at ${T}_{s}$ reported recently, ${T}_{s}$ increases under pressure. The structural study under high pressure could give us a useful clue for understanding how dipoles form in this metallic oxide. Here, we report the identification of a polar phase of $\mathrm{LiOs}{\mathrm{O}}_{3}$ at room temperature under high pressure by using in situ probes of Raman and synchrotron x-ray diffraction. In the Raman study, the pressure-induced modes and their responses to polarized light, the linewidth change, the peak profile change, and mode softening have been directly compared with the corresponding changes of $\mathrm{LiOs}{\mathrm{O}}_{3}$ on cooling through ${T}_{s}$ at ambient pressure. Whereas a complete set of Raman modes from the $R3c$ phase can be found at $P\ensuremath{\ge}15.5\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, a Raman mode of the $R3c$ phase appears in the $R\overline{3}c$ phase at 4.11 GPa. A significant drop in the linewidth occurs at 12.6 GPa that coincides with the critical pressure for the phase transition to the polar phase detected by x-ray diffraction. Fitting the peak profile of a Raman mode to the Fano formula also indicates a clear change of electron-phonon coupling at 16 GPa. In contrast to a sharp structural transition to the polar phase at ${T}_{s}$ under ambient pressure, our results reveal all the structural ingredients to facilitate the polar phase over a broad range of pressure. A bond valence sum analysis has been introduced to reveal the local structural instability under pressure. The transition to the polar phase in metallic $\mathrm{LiOs}{\mathrm{O}}_{3}$ is solely caused by optimizing the local structure in order to make the bond valence sum close to the formal valence of the Li ion.