PHASE TRANSITIONS IN PROTON CONDUCTING Me3H(AO4)2. R. Melzer, Institut fur Mineralogie, TU Berlin, D-10587 Berlin
The different types of temperature-dependent phase transitions, which can be observed in the proton conductor family Me3H(AO4)2 with Me = NH4, K, Rb, Cs and A= S, Se, will be discussed with special emphasis on their behaviour, origin and symmetry reduction.
Two-dimensional rhombohedral proton conducting phases of this family are stable at moderate temperatures above room temperature (see e.g. Pawlowski et al., 1990). They are characterised by disordered hydrogen bridges accompanied by strong librations of the AO4 tetrahedra (see e.g. Bohn et al., 1995). On lowering the temperature all family members show one or more (up to 6) phase transitions into lower-symmetry phases. Different ordering mechanisms are responsible for the, generally ferroic, transitions, which can be classified as follows:
1. freezing of one type of AO4 libration
2. ordering of the two-dimensional hydrogen bridge system
3. AO4 dimer switching
4. proton ordering within a hydrogen bridge
A complete survey of all known transitions will be presented and examples of the different ordering mechanisms will be analysed in view of experiment and theory.
References:
Pawlowski, A., Pawlaczyk, Cz., Hilczer, B.: Solid State Ionics 44 (1990) 17-19.
Bohn, A., Melzer, R., Sonntag, R., Lechner, R.E., Schuck, G., Langer, K.( 1 995). Solid State Ionics 77, 111 - 117.