Topological conical intersections govern proton transport in solids

A unified theoretical framework for proton transport in hydrated perovskite oxides, based on the concept of a protonic conical intersection (PCI) between two protonic diabatic states, is presented: a locally bound O–H configuration and a delocalized, conducting proton-polaron state. Vibronic coupling of these states to selected lattice coordinates generates a conical intersection that governs non-adiabatic proton transfer in the solid state. The PCI framework provides a coherent physical interpretation of key experimental phenomena observed across BaSnO 3 , BaZrO 3 , and BaCeO 3 , including hydration-induced Raman shifts, pronounced isotope effects between H ₂ 2 O and D ₂ 2 O, the emergence of soft and imaginary phonon modes under acceptor doping, and the conductivity maximum at intermediate Y concentrations. Within this picture, non-Arrhenius transport, entropy-enthalpy compensation, the Meyer–Neldel relationship, and the appearance of an isokinetic temperature arise naturally from the topology of the coupled proton-lattice energy landscape. More broadly, the protonic conical intersection establishes a conceptual bridge between non-adiabatic molecular physics and solid-state protonics, revealing that proton transport in functional oxides is governed by topological principles analogous to those controlling ultrafast electronic relaxation in polyatomic molecules. Impact statement This article introduces the protonic conical-intersection (PCI) framework, fundamentally reshaping the understanding of ion transport by replacing traditional static, single-barrier migration models with dynamic, multi-dimensional landscapes. While demonstrated here for proton conduction, the true impact of this framework extends far beyond: it provides a universal physical blueprint for lowering activation barriers across solid-state ionics and heterogeneous catalysis. By resorting to concepts of non-adiabatic molecular dynamics into the solid state, the PCI framework reveals how collective lattice fluctuations (phonons) can be tailored to lower or at best eliminate energy barriers. This unified theory directly addresses long-standing anomalies in energy materials, shifting the design paradigm from static chemical doping to materials architecture by dynamic lattice engineering. The implications are transformative: this framework establishes the predictive tools needed to accelerate the discovery of fast ion conductors and solid electrolytes, optimize electrocatalytic interfaces, and lower activation energies for chemical conversion. Ultimately, by bridging molecular photo-chemistry, condensed matter physics, and materials engineering, this work delivers a roadmap for designing the next generation of functional materials required for high-efficiency energy storage, conversion, and sustainable chemical synthesis.