Dielectric dispersion of (NH4)2SO4in the near-millimetre and far-infrared range: manifestations of disorder

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Abstract
Dielectric dispersion of ammonium sulphate in the 4-400 cm-1 range has been investigated in a broad temperature region around the pseudo-proper ferroelectric transition at Tc=223 K. In the 4-18 cm-1 range the backward-wave-oscillator transmission technique and in the 20-400 cm-1 range polarised reflectivity with a grating spectrometer were used to determine the dielectric response function. All external lattice mode parameters have been evaluated as a function of temperature. Near the phase transition a pronounced increase of damping of all modes has been observed. In addition to phonon mode dispersion, broad relaxation-type dispersions occur in all polarisations with characteristic frequencies 20-60 cm-1 and increasing strength on approaching Tc from below. In the ferroelectric E//c polarisation, this absorption appears in addition to the critical soft relaxation of lower frequency. The absence of any anomaly in mode frequencies near Tc indicates that the dynamic origin of the phase transition consists of ordering of NH4+ ions. Higher-frequency relaxations are assigned to anharmonic rotations of partially disordered SO42- ions. Consequences for thermodynamic theories of the phase transition are discussed.