Impurities, ions and defects

Whilst most of the impurities in liquid water remain preferentially in the liquid phase when it begins to freeze, it is possible to include small amounts of some materials in true solid solution in an ice crystal. Two types of impurity, which we may call proton donors and proton acceptors, are of particular importance since they can change the balance of ion states and of orientational defects in the ice crystal. In this way they are very similar to the electron donor and electron acceptor impurities which are familiar in normal semiconductors and, as we shall see later, the analogy is quite far reaching. 

The important thing about a proton donor is that it enters the ice structure substitutionally, replacing a water molecule, and possesses an extra proton. For this to occur the impurity molecule must be very nearly the same size as the water molecule and the optimum case is that of ammonia, NHj. The incorporation of an ammonia molecule into the ice lattice is shown in a simplified way in fig. 7.4(a), where the real ice structure has been replaced for clarity by a two-dimensional four-bonded net. If all other molecular 

Another possible modification of the donor site can take place as shown in fig. 7.4(c). There is always a finite possibility of a proton jump along a bond at the impurity site creating a pair of ion states. Between two water molecules the process is symmetrical but at an NH3 impurity a jump in one sense will yield NH H3O+ and in 

In an exactly similar way, which is illustrated in the second part of fig. 7.4, a proton acceptor impurity such as hydrogen fluoride, HF, can be incorporated substitutionally in the ice structure and liberates L-defects and positive ion states, depressing the concentrations of D-defects and negative ion states. 

Details of ionization and dissociation energies and related quantities are found from dielectric and conductivity studies, as we shall see in chapter 9, but for convenience the experimental information is assembled in table 7.4. Statistical considerations relate the concentration n of dissociated defects to the total concentration N of impurity centres by the mass action relation 

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