What is it about?
High resolution x-ray diffraction at temperatures from 7 to 300 K shows that at room temperature caesium tetrachloromercurate Cs2HgCl4 has the β-K2SO4 structure but on cooling exhibits a sequence of incommensurate and commensurate modulations along a∗ and c∗ axes. Two modulation phases, one of which is incommensurate, are found between 184 and 221 K with wavevectors along a* and, below 184 K, six further modulation phases, one of which is ∗ incommensurate, are found with wavevectors along c* .
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Why is it important?
In spite of several structural investigations of Cs2HgCl4, the nature of the rich collection of low temperature phases is poorly understood. We have therefore carried out an x-ray study in the range 7–300 K, which confirms the existence of both incommensurate and commensurate modulated phases that occur as modulation waves directed along both the a and c axes.
Perspectives
Our measurements show that Cs2HgCl4 has a complex phase diagram in which some phases can coexist depending on the thermal history of the crystal. Nine phases have been identified, labelled in order of decreasing temperature as A, B, C, D, E, F, G, H and I. Phase A is the high temperature disordered form with the Pnma β-K2SO4 structure, phases B and C show modulation along the a∗ axis and phases D to I show modulation along the c∗ axis. The modulations of phases B and F are incommensurate.
Dr Bagautdin Bagautdinov
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This page is a summary of: X-ray study of structural modulations in Cs2HgCl4, Journal of Physics Condensed Matter, August 2000, Institute of Physics Publishing,
DOI: 10.1088/0953-8984/12/37/310.
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