What is it about?
A rich sequence of structural modulations in Cs2HgCl4 as a function of temperature was studied by means of X-ray diffraction. Accurate satellite-position measurements on the cooling and heating paths of the crystal revealed abnormal thermal hystereses for incommensurate phases and coexistences of neighboring commensurate phases. A well-defined X-ray picture of the a-axis modulated phases in the range of 221184 K were observed on the heating path, while the c-axis modulated phases existing below 184 K were definitely detected on the cooling path. The proper conditions for a precise phase diagram of Cs2HgCl4 can be correlated with relatively defect-free transformations of a-axis modulations at heating and of c-axis modulations at cooling. The peculiarity of Cs2HgCl4 to switch modulation direction among the a- and c-axes at 184 K allows us deliberately accumulate and thus control a majority of mobile defects on the mutually perpendicular (100) or (001) planes by possessing crystal within temperature domain of a- or c-axes modulations, respectively.
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Why is it important?
In view of the assorted data related to the Cs2HgCl4 phases, and in face of continuing interest on the nature of modulation under external stimuli, we felt that a systematic X-ray diffraction investigation of the thermal variation of the struc- tural modulations would be worthwhile. With the intention to gain a better understanding of the thermal hysteresis features within the incommensurately and commensurately modulated regions and revealing a particular responses of the a - and c -axes modulations to the cooling or heating cycles of the crystal, we carried out the present experimental X-ray study.
Perspectives
A complete picture of the phase transitions of Cs2HgCl4 is obtainable by performing a combination of analyses on the cooling and heat- ing paths. The crystal’s peculiarity of switching the direction of the structural modulation between mutually perpendicular a- to c-axes can be used to control redistributions of the structural mobile defects rather on the (100) or (001) planes by proceeding the cooling or heating paths, respectively. It allows detection of a clear picture of the structural phases with pronounced physical properties for relatively defect-free direction: a-axis modula- tion upon heating and c-axis modulation upon cooling. A proper way to obtain a clear pic- ture of the different structural phases of Cs2HgCl4 will be useful in understanding of the underlying physical properties of the crystal. Likely, for systems with a directionally switching mode of modulated phases, the method of combining of the heating and cooling cycles will be useful to obtain exact phases with specific properties
Dr Bagautdin Bagautdinov
Read the Original
This page is a summary of: X-ray study of the anomalous thermal hystereses of the modulation wavevectors in Cs2HgCl4, Phase Transitions, December 2015, Taylor & Francis,
DOI: 10.1080/01411594.2015.1080258.
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