The resonance theory of incommensurate (IC) phases in insulators studied by Heine and McConnell is reviewed and applied to ammonium hydrogen oxalate hemihydrate, in which an IC phase is stabilized above 3 kbar between the room-temperature paraelastic phase and the low-temperature ferroelastic phase. It is suggested that this IC phase is due to a structural resonance between the ferroelastic ordering mode and an incipient ferroelectric ordering mode. A simple pressure dependence of the latter reproduces the topology of the p-T phase diagram and the pressure dependence of the IC wavevector at T(IC). Specific predictions concerning dielectric behaviour and diffuse scattering round satellite positions under pressure are made.
|Number of pages||9|
|Journal||Journal of Physics: Condensed Matter|
|Publication status||Published - 31 Dec 1990|