Commensurate-commensurate phase transitions in (NH4)2ZnCl4

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Abstract
New commensurate phases of (NH4)2ZnCl4 are found at low temperatures using X-ray diffraction. With decreasing temperature the cell dimension c of the pseudo-hexagonal axis changes as c 0, 4c 0, 3.5c 0, 3c 0, where c 0 is the fundamental period of the structure. The super-period 3.5c 0 takes place only in coexistence with the superperiod 4c 0 in the temperature range 272 to 268.3 K. A first-order phase transition from the superstructure with 3.5c 0 to the superstructure with 3c 0 occurs at 268.3 K. Coexistence of two super-periods 4c 0 and 3c 0 takes place between 268.3 K and about 266 K. The temperature-dependence of the lattice parameters and the modulation parameter from 293 K to 113 K are presented.