Low-temperature phase transitions and dynamics of ammonium in (NH4)3H(SO4)2 and [(NH4)1-xRbx]3H(SO4)2 crystals

The (NH4)(3)H(SO4)(2) and [(NH4)(0.82)Rb-0.18](3)H(SO4)(2) crystals are investigated by dielectric spectroscopy, inelastic incoherent neutron scattering (IINS), and neutron powder diffraction. A comparative analysis of the data obtained is given. It is shown that the phase transitions II <-> III, III <-> IV, IV <-> V, and V <-> VII in the (NH4)(3)H(SO4)(2) crystal are accompanied by changes in the orientation ordering of the NH4+ ions. In the [(NH4)(0.82)Rb-0.18](3)H(SO4)(2) crystal, these phase transitions are completely suppressed and the long-range order inherent in the II phase is retained over the entire temperature range covered (6-300 K). It is revealed that this crystal at the temperature T-g approximate to 70 K undergoes a transition to the dipole glass phase, which is attended by "freezing" the orientation disordering of the ammonium ions. (C) 2001 MAIK "Nauka/Interperiodica".