Superconducting Zirconium Polyhydrides at Moderate Pressures

Highly compressed hydrides have been at the forefront of the search for high-T-c superconductivity. The recent discovery of record-high T-c's in H3S and LaH10 +/- x under high pressure fuels the enthusiasm for 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 + H-2 at room temperature and Zr + H-2 at similar to 1500 K by laser heating. From the synchrotron X-ray diffraction patterns, ZrH3 is found to have a Pm (3) over barn structure corresponding to the familiar A15 structure, and Zr4H15 has an I (4) over bar 3d structure isostructural to Th4H15. Electrical resistance measurement and the dependence of T-c on the 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 T-c's for ZrH3 and Zr4H15.