Study of NH4 motion in some antiferroelectric crystals by inelastic scattering of cold neutrons

Abstract

AbstractThe spectra of the inelastic scattering of cold neutrons by antiferroelectric NH4H2PO4, NH4H2AsO4, and (NH4)2H3IO6 have been measured above and below their transition point, using a rotating crystal spectrometer together with a cold source. The low temperature neutron measurements show very little change as compared with the room temperature results. The torsional bands are narrow in both phases. The broadening of the bands is inversely related to the hindrance, and therefore the measurements suggest that the rotation of the NH+4 ion is strongly hindered, which is supported by the angular distribution of elastic scattering. The deuteron magnetic resonance of ND4D2AsO4 indicates that in addition to rotational motion of the ND4 group this group might oscillate in the paraelectric phase. The oscillational mode condenses at Tc. In order to check the assignments obtained from the inelastic neutron scattering studies the ir absorption spectra of NH4H2PO4, NH4H2AsO4, and (NH4)2H3IO6 and their deuterated analogues were measured at room temperature.