Thermal Hysteresis Accompanying the Incommensurate-Commensurate Phase Transitions in Rb2ZnBr4 and Rb2ZnCl4

The dielectric constants of Rb 2 ZnBr 4 and Rb 2 ZnCl 4 exhibit remarkable thermal hysteresis over wide temperature ranges including their incommensurate-commensurate transition points. The most characteristic feature of this thermal hysteresis is that it occurs even when a cooling run is turned to a heating run in the incommensurate phases, i.e., the thermal hysteresis occurs even when the crystal has no experience of phase transition. This phenomenon was studied in detail in this work. If the turning temperature T turn is in the range where the incommensurate wave number is independent of temperature (the range of constant δ), the thermal hysteresis does not occur, while if T turn is in the range of varying incommensurate wave number (the range of varying δ), the thermal hysteresis appears. This fact seems to support our idea that the thermal hysteresis of the dielectric constant is caused by pinning of the incommensurate wave.