Protonic defects formation

From the formation reaction of protonic defects in oxides (eq 23), it is evident that protonic defects coexist with oxide ion vacancies, where the ratio of their concentrations is dependent on temperature and water partial pressure. The formation of protonic defects actually requires the uptake of water from the environment and the transport of water within the oxide lattice. Of course, water does not diffuse as such, but rather, as a result of the ambipolar diffusion of protonic defects (OH and oxide ion vacancies (V ). Assuming ideal behavior of the involved defects (an activity coefficient of unity) the chemical (Tick s) diffusion coefficient of water is... 

Ab initio methods provide elegant solutions to the problem of simulating proton diffusion and conduction with the vehicular and Grotthuss mechanism. Modeling of water and representative Nation clusters has been readily performed. Notable findings include the formation of a defect structure in the ordered liquid water cluster. The activation energy for the defect formation is similar to that for conduction of proton in Nafion membrane. Classical MD methods can only account for physical diffusion of proton but can create very realistic model... 

Irradiation of the ZnO - Li samples by protons leads to the appearance of a photosensitive EPR signal (f- signal) which corresponds to the center with an axial symmetry and g-factor gy = 1.9948 and gi= 1.9963. This signal is very important for understanding the processes of defect formation, since it leads to gives rise to two alternative interpretations for the microscopic structure of the intrinsic defects that determine the deviation from the stoichiometry in ZnO. ... 

The dominant intermolecular interaction in vater is hydrogen bonding. The introduction of an excess proton (i.e. the formation of a protonic defect) leads to the contraction of hydrogen bonds in the vicinity of such a defect. This corresponds to the vell-kno vn structure forming properties of excess protons in water (see for example Ref. [26]). Thus the isolated dimer H5O2+ finds its energetic minimum at an O / O separation of only 240 pm [27, 28[ with an almost symmetrical single well potential for the excess proton in the center of the complex. 

The variation in n- or p-conductivity as a function of water pressure at constant oxygen activity appears to be a good method for determining the proton contribution to defect formation. When protons correspond to the majority defects, they can dominate the conductivity and the transport number may be determined. 

Potier and Rousselet proposed the following mechanism derived from Bjerrum there is a rotation of the H30 ion followed by proton transfer to a CIO4 anion, then a rotation of the resulting HCIO4 species and subsequently a proton transfer to H2O. Such a mechanism implies the presence of proton defects the formation of which can be expressed by two possible equilibria... 

FIGURE 3.2.12 Equilibrium constant for the formation reaction of protonic defects (dissociative absorption of water) in different perovskite-type oxides (calculated from literature data [212]). The standard reaction enthalpies and entropies are given in the insert. 

Higher hydrates of triflic acid were studied by Hayes, Paddison, and Tuckerman using path integral Car-Parrinello MD (Hayes et al., 2009, 2011). The larger amount of water available per acidic proton led to the formation of larger proton complexes and more versatile proton defect structures. Formation of such defects involves local proton transfer events. The observed defects correspond to local structures. They are energetically expensive to form, and it seems unlikely that they could propagate in the crystal. 

In some metal oxides such as barium cerate-based perovskites, water can be incorporated at oxygen vacancies as well as oxygen atoms, leading to the formation of singly charged protonic defects ... 

J.H. Yu, J.-S. Lee, J. Maier, Formation of protonic defects in perovskite-type oxides with redox-active acceptors case study on Fe-doped SrTiOs. Phys Chem Chem Phys, 7, 3560-3564 (2005)... 

The mobihty of protons is not only very sensitive toward reduction of the crystallographic symmetry but also toward local structural and chemical perturbations induced by the aeeeptor dopant or by mixed occupaney on the B site. To allow for the formation of protonic defects by dissociative absorption of water, perovskite-type oxides are eommonly doped with aliovalent ions (aeeeptor dopants) matching the ionie radius of the B-site cation (e.g., In for Zr" ). Indeed, this simple concept has been proven successful, e.g., for oxide ion conductors (Sc-doped zirconia shows higher oxide ion eonduetivity than... 

In an attempt to calculate association energies for the formation of clusters of protonic defects and different kinds of acceptor dopants in SrCe03-based proton conductors [44], the lowest energies have been found for Y and Yb, the most commonly used dopants for this type of compound. For CaZrOs, the most favorable dopants are anticipated to be Ga, Sc, and In [45] Even interaction of protonic defects with residual oxide ion vacancies has been anticipated to influence the local proton dynamics in SrTiOs [20]. 

Nuclear fusion processes derive energy from the formation of low-mass nuclei, which have a different binding energy. Fusion of two nuclear particles produces a new nucleus that is lighter in mass than the masses of the two fusing particles. This mass defect is then interchangeable in energy via Einstein s equation E = me2. Specifically, the formation of an He nucleus from two protons and two neutrons would be expected to have mass ... 

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