Neutron scattering studies of phase transitions in protonated and deuterated ammonium hydrogen sulphate

Abstract

Ammonium hydrogen sulphate, (NH₄)HSO₄, is ferroelectric [1] in the temperature range between 270 K and 154 K. With increasing hydrostatic pressure the ferroelectric properties of this crystal disappear [2] and proton superionic conductivity is observed at temperatures above 460 K [3]. The results of neutron powder diffraction (NPD) and inelastic incoherent neutron scattering (IINS) studies of normal and deuterated ammonium hydrogen sulphate in different solid phases are presented. The measurements were performed on the NERA inverted geometry time-of-flight spectrometer [4] of the IBR-2 high-flux pulsed reactor of the JINR at Dubna.

No change is found in the NPD and IINS spectra for the I–II phase transition at 270 K, while the II–III phase transition at 154 K is clearly seen. The latter phase transition is marked by a change in structure and proton dynamics. The behaviour of the deuterated ammonium hydrogen sulphate at cooling is analogous to that of a protonated substance. Upon heating at about 230 K a new structural transition was observed. The new phase thus obtained may be overcooled below the temperature of the II–III phase transition. Our investigations show that the NPD spectra of this new phase correspond to the high pressure phase IV of a normal substance. The phase IV below ca. 200 K becomes metastable and exists even at normal pressure within the temperature region of the phases II and III. With increasing temperature the phase IV transforms directly to the phase I at about 270 K. The IINS spectra of the phases I, II and IV indicate disorder of hydrogen bonds in the crystal and fast reorientations of ammonium ions above the temperature of the II–III phase transition. Below this temperature difference in vibrational spectra of the phase III and IV is caused by different crystal structures and the ordering of hydrogen bonds.