Room-Temperature Polar Metal Stabilized Under High Pressure

LiOsO3 synthesized under high pressure in recent years is a rare metal since it undergoes a nonpolar to polar phase transition at T-s = 140 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 LiOsO3 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 LiOsO3 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 LiOsO3 on cooling through T-s at ambient pressure. Whereas a complete set of Raman modes from the R3c phase can be found at P >= 15.5 GPa, a Raman mode of the R3c phase appears in the R (3) over barc 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 LiOsO3 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.