X-Ray Structural Study of the Successive Phase Transitions in Ammonium Tetrachlorozincate, (NH4)2ZnCl4. I. Crystal Structure Determination

The crystal structures of (NH 4 ) 2 ZnCl 4 at 418 K (Phase I), 333 K (Phase II) and 298 K (Phase III, antiferroelectric) were determined from X-ray diffraction data, and were compared with that at 223 K (Phase IV, ferroelectric). The space groups and final discrepancy factors are P m c n and 0.052 at 418 K, P c 2 1 n and 0.046 at 333 K and P c 11 and 0.050 at 298 K, respectively. The structure of Phase I belongs to the β-K 2 SO 4 type structure and those of Phases II and III are modulated structures with the quadruple c axis of Phase I in contrast to that of Phase IV with the triple c axis. These modulated structures are characterized by slight rotation of the ZnCl 4 groups from the positions in Phase I.