We studied the Nb-H system over extended pressure and temperature ranges to establish the highest level of hydrogen abundance we could achieve from the resulting alloy. We probed the Nb-H system with laser heating and X-ray diffraction complemented by numerical density functional theory-based simulations. New quenched double hcp NbH 2.5 appears under 46 GPa, and above 56 GPa cubic NbH 3 is formed as theoretically predicted. Nb atoms are arranged in closedpacked lattices which are

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... transformed in the sequence: fcc → hcp → dhcp → distorted bcc as pressure increases. The appearance of fcc NbH 2.5-3 and dhcp NbH 2.5 cannot be understood in terms of enthalpical stability, but can be rationalized when finite temperatures are taken into account. The structural and compressional behavior of NbH x>2 is similar to that of NbH. Nevertheless, a direct H-H interaction emerges with hydrogen concentration increases, which manifests itself via a reduction in the lattice expansion induced by hydrogen dissolution. Keywords: metal polyhydride, high pressure, laser heating and first principles 3 / 21 Significance Statement To date, very few of the stable metal polyhydride predictions have been fully examined or confirmed due to current experimental limitations. Here, we systematically studied the Nb-H system both experimentally and theoretically up to one megabar. Using DAC, laser heating and insitu synchrotron XRD, we successfully synthesized and measured several new phases of NbH x . NbH 2-2.5 underwent a phase transition from an fcc phase to an irregular hexagonal phase at 39 GPa. Interestingly, we observed hcp/dhcp NbH 2.5 and distorted bcc NbH 3 above 56 GPa along a different thermodynamic path, which was supported by our first principles calculations. Our findings provide new insights into the formation of metal polyhydrides at extreme conditions, which may have energy storage potential in the future.