Synthesis and stability of high- T c LaH 10 plus or minus delta films at high pressures

High-pressure hydrides hold the record for the highest superconducting critical temperatures across all classes of superconductors. Currently lanthanum decahydride, LaH 10, exhibits the highest critical temperature among binaries, with T c about 250 K at pressures between 140-180 GPa. Here, we report the synthesis of LaH 10 plus or minusδ films in two DACs at pressures of 168 GPa and 176 GPa via in situ laser heating of elemental lanthanum films with ammonia borane (NH 3 BH 3) as the hydrogen donor. The high-symmetry fcc lanthanum sublattice (space group Fm 3m) is resolved using synchrotron X-ray diffraction, with unit cell parameters in excellent agreement with previous studies on bulk samples. We provide confirmation of high- T c superconductivity in LaH 10 plus or minusδ with highest T c of 247 K at 176 GPa evidenced in electrical measurements. The characteristic suppression of superconductivity is observed in magnetic fields. Furthermore, combined diffraction and electrical measurements reveal remarkable temporal stability of both the crystal structure and the high- T c superconductivity over the full measurement period of about 300 days post laser heating. Our work establishes film precursors using physical vapour deposition (PVD) techniques as a practical route to hydride formation, opening a pathway toward the controlled synthesis of promising ternary hydrides and the integration of micro-fabricated device geometries in diamond anvil cells.