Neutral and Ionized Hydrogen Species in Oxides

However, it is important to note a major difference between the considerations in semiconductor physics and in high temperature chemistry In semiconductors. 

The equilibrium concentrations of hydrogen atoms and molecules in oxides are perhaps not widely different between different oxides. As a first estimate, the entropy of the reaction from one mole of H2 gas to (H2)i or 2Hj in the solid may be expected to be -120 J moH K 1, implying that at very high temperatures (entropy controlled) we would have occupied site fractions of the order of 1 ppm of the interstitial sites for (H2)i and 0.1% for Hj. The enthalpy of dissolution, which determines how the concentrations develop with decreasing temperature, contains the bonding of the H species in the lattice or to existing defects and - in the case of atoms H - the breaking of the bond (435 kj mohi) of the H2 molecule. 

All in all, the equilibrium concentration of neutral hydrogen species will depend on nd temperature, but not on Fermi level, doping or oxygen activity (at constant p fj. The concentrations will probably not be very high, but also not very low either. Their temperature dependences are probably not very strong. 

The neutral hydrogen species may give rise to considerable hydrogen permeability, a possibility considered by some scientists, as we shall see towards the end of this chapter, but in general not in the gas separation membrane community. 

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