Superionic transition temperature

The ionic conductivity of fluorite-structured halides such as SrF2, CaF, BaF, (3- Pb F 2, and SrCh, shows a rapid but continuous increase on heating, reaching values very dose to those shown by the liquid state [67-69]. The transition to the superionic state is also associated with peak in the specific heat Cp (the maximum value of which is generally taken to define the superionic transition temperature, f), anomalous behavior ofthe elastic constants and changes in the lattice expansivity (for details and references, see Refs. [38, 70]). 

Of all the fluorite-structured halides, P-PbF2 has attracted the most attention, since it has the lowest superionic transition temperature (Tc = 711 K). The ionic conductivity and structural properties of a number of compounds based on PbF2 have been studied, with the ternary compound PbSnF4 possessing one of the highest values of ionic conductivity of any F ion conductor at ambient temperature (<7, = 10 S cm- [76]). 

A word about the so-called normal-superionic conducting state transitions would be in order. It seems to be wrong to refer to these transitions as superionic transitions by analogy with the electronic superconducting transitions. While Agl related materials show a marked jump in ionic conductivity at a transition temperature, there are many other superionic materials that do not.60... 

Sum rule for oscillator strengths, 102, 11 If Superconductivity high-temperature, 455 transition temperature, 399 Superionic conductors, 316 Surface dipoles, 399ff, 425ff Surface energy ionic compounds, 336 semiconductors, 230ff simple metals, 401f Surface tension, 233 Surface states... 

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). 

Sprik et al, 1993 Signorini et al, 1990), a typical example being the orientational disorder associated with NH in NH Br. Detailed simulations have been reported on (NaCN),, t(KCN),t and other mixed alkali halides and alkali cyanides. Other systems studied include potassium and calcium nitrate crystals and their mixtures. The transition from the crystalline to the superionic conductor phase in solid electrolytes has also been successfully investigated. Molecular dynamics studies of Agl were carried out by Parrinello, Rahman Vashishta (1983). LijSO has been investigated by molecular dynamics by Impey et al. (1985). Here, the Li ions become mobile at high temperatures. The ions exhibit orientational disorder and the orientational... 

Figure 20. Sketch of the real structure of a Frenkel disordered crystal (only the affected sublattice is shown). The increase in temperature corresponds to an increase in defect concentration. Interaction leads to a narrowing of the spacing of the energy levels (electrochemical potentials minus configurational term) and eventually to a transition into the superionic state.12 (Reprinted from J. Maier and W. Munch, Z. Anorg. Allg. Chem., 626,264-269, Copyright 2000 with permission from WILEY-VCH Verlag GmbH.)...

The possibility of inherent difficulties in the accurate MD simulation of slowly diffusing systems is also to be borne in mind following the finding of discrepancies between experiment and simulation observed in the nonliquid but still very relevant case of the ciystalline superionic conductor CaFj. In this system, Rahman has reported MD diffusivities of F that approach zero, on linear D scale plots of the Fig. 3 type, more rapidly than expected from the observed Arrhenius behavior of the measured diffusivities. (Rahman compared his results with extrapolations of data measured below the weak lambda transition at 1200°C, but the electrical data have validated the extrapolation to the accuracy of the comparison.) Rahman s high-temperature data have been accurately reproduced in one of the authors laboratories using a rather different form of pair potential. ... 

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