  The only known compound of sodium and hydrogen is archetypal ionic NaH.
Application of high pressure is known to promote states with higher atomic
coordination, but extensive searches for polyhydrides with unusual
stoichiometry remain unsuccessful in spite of several theoretical predictions.
Here we report the first observation of formation of polyhydrides of Na (NaH3
and NaH7) above 40 GPa and 2000 K. We combined synchrotron x-ray diffraction
and Raman spectroscopy in a laser heated diamond anvil cell and theoretical
Ab-Initio Random Structure search, which both agree in stable structures and
compositions. Our results support the formation of multicenter bonding in a
material with unusual stoichiometry. These results are applicable to the design
of new energetic solids and high-temperature superconductors based on
hydrogenrich materials.
