Dielectric properties of (NH4)2SO4 crystals in the range of electronic excitations

Spectra of the real and imaginary parts of the pseudo-dielectric permittivity, (E) and (E), of ferroelectric ammonium sulfate crystals, (NH4)(2)SO4, have been measured in the range of electronic excitations 4.0 to 9.5 eV by ellipsometry using synchrotron radiation. Temperature dependences of the corresponding susceptibilities, (T) and (T), obtained for the photon energy E = 8.5 eV, related to excitations of oxygen p-electrons, reveal sharp peak-like temperature changes near the Curie point T-C = 223 K. The large temperature-dependent increase of the imaginary part of the susceptibility chi(2)(T), together with a simultaneous decrease of the real part of the susceptibility chi(1)(T), take place at the phase transition. These anomalies have been ascribed mainly to the SO4 group of the crystal structure.