Lithium polyhydrides synthesized under high pressure and high temperature

<jats:p>Polyhydrides of alkali metals under high pressures have been attracting great interests owing to the predicted pressure‐stabilized untraditional stoichiometries and potential superconductivity. We have performed Raman scattering measurements of lithium polyhydrides synthesized under high pressure and high temperature conditions up to 182 GPa. Between 30 and 180 GPa, four phases appear, and they are tentatively named <jats:italic>α</jats:italic> (30–140 GPa), <jats:italic>β</jats:italic> (30–90 GPa), <jats:italic>γ</jats:italic> (90–140 GPa), and <jats:italic>δ</jats:italic> (140 GPa) phase. By observing the H<jats:sub>2</jats:sub> vibrons at higher wavenumbers than pure H<jats:sub>2</jats:sub> and their pressure dependence, it was revealed that the <jats:italic>α</jats:italic> phase contains electrically neutral H<jats:sub>2</jats:sub> in its crystal lattice without electron transfer from Li atoms. The comparisons with H<jats:sub>2</jats:sub>‐containing rare gas compounds Ar(H<jats:sub>2</jats:sub>)<jats:sub>2</jats:sub>, Kr(H<jats:sub>2</jats:sub>)<jats:sub>4</jats:sub>, and Xe(H<jats:sub>2</jats:sub>)<jats:sub>8</jats:sub> suggest that a H<jats:sub>2</jats:sub> molecule is surrounded by six to eight neighboring H<jats:sub>2</jats:sub> molecules in the <jats:italic>α</jats:italic> phase. On the other hand, the Raman spectra of the <jats:italic>β</jats:italic>, <jats:italic>γ</jats:italic>, and <jats:italic>δ</jats:italic> phases suggest that they contain the elongated H<jats:sub>2</jats:sub> that is negatively charged by the electron transferred from the surrounding matrices. All the <jats:italic>α</jats:italic>, <jats:italic>β</jats:italic>, <jats:italic>γ</jats:italic>, and <jats:italic>δ</jats:italic> phases were found to remain transparent insulators at pressures below 182 GPa. Copyright © 2017 John Wiley & Sons, Ltd.</jats:p>