Mechanism of the Sequence of Phase Transitions [Orthorhombic→Monoclinic Nonferroelectric→Monoclinic Ferroelectric] in NH4HSO4

The nonferroelectric and ferroelectric phases of NH 4 HSO 4 belonging to space groups B 2 1 / a and B a are both recognized as ferroic phases derived from a common prototypic phase which should belong to space group P m m n . The sequence of phase transitions P m m n → B 2 1 / a → B a can be deduced as arising from successive instabilities of a pair of conjugate vibrational modes with wave vectors (±1/4, 0, ±1/2) in the reciprocal space and with a transformation property which is found out. In particular the transition B 2 1 / a → B a is concluded to be due to an instability for variation of the wavephase of the condensed soft modes in the B 2 1 / a phase. The lowest-temperature phase belonging to B 1 is found not to be `homophone' with the B 2 1 / a and B a phases. Concerning the temperature dependences of components of spontaneous polarization and electric susceptibility, the theoretical results are compared with the observations by Pepinsky et al.