Superconducting Zirconium Polyhydrides at Moderate Pressures.

Highly compressed hydrides have been at the forefront of the search for high-Tc superconductivity. Especially, recent discovery of record-high Tc's in H3S and LaH10±x under high pressure fuels the enthusiasm finding good superconductors in similar hydride groups. Guided by first principles structure prediction, we successfully synthesized ZrH3 and Zr4H15 at modest pressures (30-50 GPa) in diamond anvil cells by two different reaction routes: ZrH2 + H2 at room temperature and Zr + H2 at ~ 1500 K by laser heating. From the synchrotron X-ray diffraction patterns, ZrH3 is found to have a Pm-3n structure corresponding to the familiar A15 structure, and Zr4H15 has a I-43d structure isostructural to Th4H15. Electrical resistance measurement and the dependence of Tc on applied magnetic field of the sample showed the emergence of two superconducting transitions at 6.4 and 4.0 K at 40 GPa, which correspond to the two Tc's for ZrH3 and Zr4H15.