Successive Phase Transitions in (NH4)2ZnCl4

Dielectric, thermal and X-ray diffraction measurements are performed for (NH 4 ) 2 ZnCl 4 . Ferroelectric activity is confirmed below 270 K and two other phase transitions are found at 319 K and 406 K. Phase I ( T >406 K) is the normal phase. Phase II (319< T <406 K) and Phase III (270< T <319 K) are paraelectric and antiferroelectric, respectively, with c =4 c 0 , where c 0 is a unit cell parameter of the normal phase. Phase IV ( T <270 K) is the ferroelectric phase having c =3 c 0 .