In most proton-conducting oxides, proton conductivity is mediated by a Grotthuss-like mechanism, i.e. the succession of transfers, in which the proton jumps from an oxygen onto another (breaking, and then reforming an ionocovalent hydroxyl bond OH), and reorientations, in which the OH bond simply rotates without breaking. We focus in this work on this second step, i.e. the reorientation of the protonic defect, in barium zirconate, one of the most promising proton-conducting oxides. Considered alone, the rotations of a proton are responsible for a rotational diffusion of this proton around a given oxygen atom. The reorientation of the protonic defect, like transfer, is strongly affected by the quantum effects associated with the protonic motions, and can only take place thanks to two specific lattice vibrations: (i) a reorganization, that symmetrizes the proton potential and puts the proton levels in the initial and final wells in coincidence, and (ii) the increase of the separation between the protonated oxygen and the nearest neighbor barium atom (that forms an obstacle to the reorientation of the protonic defect). The former (necessary step) mostly consists in rotations of the two oxygen octahedra sharing the protonated oxygen, while the latter (facilitating step) helps to overcome the proton‑barium electrostatic repulsion and lowers the proton barrier. We show that the reorientation of the protonic defect in barium zirconate undergoes a transition at about 220 K between a low-temperature and a high-temperature regime. Below ∼ 220 K, it is governed by tunneling, with a reorientation rate having an activation energy of 0.09 eV and a prefactor of ∼ 0.4 THz, due to quasi-non-adiabatic transitions between coincident protonic ground states (GS → GS), and involving an effective proton coupling of 5.7 meV. Above 220 K, two additional contributions, corresponding to transitions between the first excited protonic state (1st) in one of the two wells, and the protonic ground state (GS) in the other well, become dominant. These asymmetric transitions (1st → GS and GS → 1st) are quasi-adiabatic, and their rates are associated with an activation energy of 0.14 eV and a prefactor of 1.7 THz. The contribution due to adiabatic symmetric transitions between first excited protonic states (1st → 1st) should become equivalent to each of the asymmetric ones only at high temperature (above 900 K).

在大多数质子传导氧化物中，质子传导性由类 Grotthuss 机制介导，即连续转移，其中质子从一个氧跃迁到另一个氧（破坏，然后重新形成离子共价羟基键 OH）和重新定向，其中 OH 键简单地旋转而不会断裂。我们在这项工作中专注于第二步，即重新定向锆酸钡中的质子缺陷，这是最有希望的质子传导氧化物之一。单独考虑，质子的旋转负责旋转扩散这个质子围绕给定的氧原子。质子缺陷的重新定向，如转移，受到与质子运动相关的量子效应的强烈影响，并且只能由于两种特定的晶格振动而发生：（i）重组，使质子势对称并使质子初始和最终井中的水平重合，以及（ii）质子化氧与最近邻钡原子之间的分离增加（这对质子缺陷的重新定向形成了障碍）。前者（必要步骤）主要包括共享质子化氧的两个氧八面体的旋转，而后者（促进步骤）有助于克服质子-钡静电排斥并降低质子势垒。我们表明，锆酸钡中质子缺陷的重新定向在大约 220 K 处经历了低温和高温状态之间的转变。在~ 220 K 以下，由于重合质子基态之间的准非绝热跃迁（GS  →  GS)，并涉及 5.7 meV 的有效质子耦合。在 220 K 以上，两个额外的贡献，对应于两个井中的一个井中的第一个激发质子态(1 st ) 和另一个井中的质子基态 ( GS ) 之间的跃迁，成为主导。这些不对称跃迁（1 st  →  GS和GS  → 1 st）是准绝热的，它们的速率与 0.14 eV 的活化能和 1.7 THz 的前因数相关。由于第一激发质子态之间的绝热对称跃迁（1 st  → 1 st) 仅在高温（高于 900 K）下才应与每个不对称的等效。