Superionic water

More recent quantum-based MD simulations were performed at temperatures up to 2000 K and pressures up to 30 GPa.73,74 Under these conditions, it was found that the molecular ions H30+ and OH are the major charge carriers in a fluid phase, in contrast to the bcc crystal predicted for the superionic phase. The fluid high-pressure phase has been confirmed by X-ray diffraction results of water melting at ca. 1000 K and up to 40 GPa of pressure.66,75,76 In addition, extrapolations of the proton diffusion constant of ice into the superionic region were found to be far lower than a commonly used criterion for superionic phases of 10 4cm2/s.77 A great need exists for additional work to resolve the apparently conflicting data. 

Given the above definition of a bond distance, we can analyze species lifetimes. The lifetime of all species is less than 12 fs above 2.6g/cc, which is roughly the period of an O-H bond vibration (ca. 10 fs). Hence, water does not contain any molecular states above 75 GPa and at 2000 K but instead forms a collection of short-lived transient states. The L simulations at 2.6g/cc (77 GPa) and 2000 K yield lifetimes nearly identical to that found in the S simulations (within 0.5 fs), which indicates that the amorphous states formed from the L simulations are closely related to the superionic bcc crystal states found in the S simulations. 

Science, 283, 44 (1999). Superionic and Metallic States of Water and Ammonia at Giant Planet Conditions. 

The ionic charge carriers in ionic crystals are the point defects.1 2 23,24 They represent the ionic excitations in the same way as H30+ and OH-ions are the ionic excitations in water (see Fig. 1). They represent the chemical excitation upon the perfect crystallographic structure in the same way as conduction electrons and holes represent electronic excitations upon the perfect valence situation. The fact that the perfect structure, i.e., ground structure, of ionic solids is composed of charged ions, does not mean that it is ionically conductive. In AgCl regular silver and chloride ions sit in deep Coulomb wells and are hence immobile. The occurrence of ionic conductivity requires ions in interstitial sites, which are mobile, or vacant sites in which neighbors can hop. Hence a superionic dissociation is necessary, as, e.g. established by the Frenkel reaction ... 

Counted among these superionic electrolytes are the solutions of strong acids in water and other hydrogen-bonded solvents (e.g., glycerol, hydrogen peroxide) in which the proton has anomalously high mobility due to the Grotthus mechanism... 

C. Cavazzoni, G. L. Chiarotti, S. Scandolo, E. Tosatti, M. Bernasconi, and M. Parrinello (1999) Superionic and metallic states of water and ammonia at giant planet conditions. Science 283, p. 44... 

The arsenate analogue, H3OUO2ASO4.3H2O shows similar features as shown by a NMR study below 170 K the total second moment is characteristic of a rigid network, between 170 and 200 K a reorientation of water and oxonium molecules takes place and above 270 K diffusion of protonic species is observed. The transition to the superionic tetragonal phase occurs at 300 K (see Chapter 31 for a more complete description). Microwave dielectric relaxation study of HUP gives similar conclusions and the phase transition temperatures are almost unchanged by H/D substitution (see p. 398). 

The relaxation time can be deduced from the Debye-like drcular arc. A plot of T values versus the inverse of temperature (10 /r) (Fig. 25.4) allows a measure of activation energy (Fig. 25.5). The separation of domains already discussed above is clearly visible, from fast reorientational motions of dipolar polyatomic ions such as HX04 and HjO" to slow reorientation of NH4 ions. Motions of water molecules cover a broad region they are slow in gel, medium in superionic materials (e.g. HUP) and fast in liquid water. 

J.J. Kweon, R. Fu, E. Steven, C.E. Lee, N.S. Dalai, High field MAS NMR and conductivity study of the superionic conductor LiH2P04 critical role of physisorbed water in its protonic conductivity, J. Phys. Chem. C 118 (2014) 13387—13393. 

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