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

The (NH4)(3)H(SO4)(2) and [(NH4)(0.82)Rb-0.18](3)H(SO4)(2) crystals are inv estigated by dielectric spectroscopy, inelastic incoherent neutron scatteri ng (IINS), and neutron powder diffraction. A comparative analysis of the da ta obtained is given. It is shown that the phase transitions II <-> III, II I <-> IV, IV <-> V, and V <-> VII in the (NH4)(3)H(SO4)(2) crystal are acco mpanied by changes in the orientation ordering of the NH4+ ions. In the [(N H4)(0.82)Rb-0.18](3)H(SO4)(2) crystal, these phase transitions are complete ly 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 t his crystal at the temperature T-g approximate to 70 K undergoes a transiti on to the dipole glass phase, which is attended by "freezing" the orientati on disordering of the ammonium ions. (C) 2001 MAIK "Nauka/Interperiodica".