Inverse Isotope Effect in the Ternary Perovskite Hydride SrPdH/D${}_{2.9}$: A Signature of Quantum Zero-Point Fluctuations

Guided by first-principles calculations, we demonstrate superconductivity in the ternary perovskite hydride SrPdH${}_{3-x}$, synthesized at low pressure. Structural characterization via neutron diffraction reveals the near-stoichiometric composition SrPdD${}_{2.9(2)}$ with 96% deuterium site occupancy. Subsequent transport and magnetic susceptibility measurements establish onset superconducting transitions at $T_{\text{c}}=\text{SI}2.1K$ (H) and $T_{\text{c}}=\text{SI}2.2K$ (D), exhibiting an inverse isotope effect that our first-principles calculations attribute predominantly to quantum zero-point motion. The excellent agreement between theory and experiment with respect to thermodynamic stability and superconducting properties provides important validation for theory-guided superconductor discovery. This work establishes superconductivity in the perovskite hydride structural prototype – expanding the limited family of experimentally realized ternary hydride superconductors – and demonstrates the importance of quantum nuclear motion on the accurate theoretical treatment of low-pressure hydride superconductors.