X-ray study of structural modulations in Cs2HgCl4

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*.