Protonic defects mobility

From the thermodynamics of such dynamical hydrogen bonds , one may actually expect an activation enthalpy of long-range proton diffusion of not more than 0.15 eV, provided that the configuration O—H "0 is linear, for which the proton-transfer barrier vanishes at 0/0 distances of less than 250 pm. However, the mobility of protonic defects in cubic perovskite-type oxides has activation enthalpies on the order of 0.4—0.6 eV. This raises the question as to which interactions control the activation enthalpy of proton transfer. 

At elevated temperatures, where the electron lifetime was much shorter than the pulse lengths of a few nanoseconds used, a second mobile species could be observed as a slowly decaying after-pulse conductivity component for large pulses. This was attributed to proton conduction with a proton mobility of 6.4 x 10 cm /Vs in H,0 ice and a somewhat lower value in D2O ice. ° In the case of the proton, the mobility was found to have an apreciable negative activation energy of 0.22 eV. The motion and trapping of protons was tentatively explained in terms of an equilibrium between free protons and a proton complexed with an orientational L-defect. °... 

For more simple systems, the predictive character of ab initio and quantum MD simulations has already made possible the directed identification of improved proton- conducting materials. A prominent example is yttria-doped BaZr03, an oxide with the perovskite structure forming highly mobile protonic defects in the presence of water vapor [22,23]. Quantum MD simulations [24,25] have revealed details of the proton-conduction mechanism, including the critical interactions, and electronic structure calculation have helped identify the best possible dopant [26]. 

The elementary reactions underlying the mobility of protonic defects have been investigated in great detail, experimentally as well as by computer simulations, and their contributions to the total activation enthalpy have been estimated. 

Except for perovskite-type oxides with small lattice constants, that is, short oxygen separations, reorientation of protonic defects (rotational diffusion) is generally fast, and proton transfer is the rate limiting process. According to the above described interactions, the activation enthalpy of the latter exhibits contributions from the compression of the OH/O separation and elongation of the B/O and O/H bond. Not only symmetry reduction of the average structure, but also local symmetry perturbations, for example, by acceptor dopants, may significantly reduce the mobility of protonic defects [212]. 

Another key feature is the availability of a nearly perfect acceptor dopant (i.e. a dopant which does not change the electronic structure of the oxygen). While in all other reported cases, the increase of the acceptor dopant concentration leads to a reduction of the proton mobility and an entropic destabilization of protonic defects [222], both the proton mobility and the thermodynamics of hydration are practically unchanged for dopant levels up to 20% Y in BaZr03 (Fig. 3.2.13). High proton mobility and entropically stabilized protonic defects even at high dopant concentrations and the high-solubility limit lead to the enormous proton... 

All in all, proton conductivity in oxides is a matter of compromise in composition and temperature between high concentration of protons (favorable hydration kinetics), high proton mobility, and chemical robustness. In this contribution, we concentrate on a description of protonic defects and their thermodynamics in various perovskite-related oxides, give an overview of the resulting proton... 

Exactly the opposite occurs, namely the conversion of an ex situ parameter to an in situ one, if foreign components become sufficiently mobile. The corresponding incorporation reaction then becomes reversible. Under such conditions it is natiurally better to speak of solubility equihbria. Important examples are segregation equilibria of impurities at very high temperatures, another refers to the incorporation of protonic defects in oxides by the dissolution of H2O. Materials interesting in this respect are CaO-doped Z1O2 [196] or acceptor-doped perovskites [197], such as the Fe-doped SrTiOs discussed above. (As before we regard the acceptor dopant... 

Jacobs et al. [59,925,926] (Fig. 17). While this scheme conveniently summarizes many features of the observed behaviour, a number of variations or modifications of the mechanisms indicated have been proposed. Maycock and Pai Vemeker [924,933] emphasize the possible role of point defects and suggest, on the evidence of conductivity measurements, that the initial step may be the transfer of either a proton or an electron. Boldyrev et al. [46] suggest that proton conduction permits rapid migration of HC104 within the reactant and this undergoes preferential decomposition in distorted regions. More recently, the ease of proton transfer and the mobilities of other species in or on AP crystals have been investigated by a.c. [360] and d.c. [934] conductivity measurements. Owen et al. [934] could detect no surface proton conductivity and concluded that electron transfer was the initial step in decomposition. At the present time, these inconsistencies remain unresolved. 

An alternative possibility cannot be excluded. Since the time scale for proton mobility and infrared vibration are greatly different, infrared is not the pertinent technique for studying proton mobility. Therefore, the exchange with D2 can occur with only a small fraction of the hydroxyls, at impurity centers, or at a limited number of defects. The isotopes should mix then by a rapid diffusion. This possibility has been envisaged by Cant and Hall (13) for the exchange reaction of surface hydroxyls of... 

We remember that minority point defect concentrations in compounds depend on the activity of their components. This may be illustrated by the solubility of hydrogen in olivine since it depends on the oxygen potential in a way explained by the association of the dissolved protons with O" and O- as minority defects [Q. Bai, D. L. Kohlstedt (1993)]. Similarly, tracer diffusion coefficients and mobilities of Si and O are expected to depend on the activity of Si02. The value (0 lnDf/0 In aSio2)> = Si and O, should give information on the disorder type as discussed in Section 2.3. 

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