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

Spectra of the real and imaginary parts of the pseudo-dielectric permittivity, 〈[epsilon]₁〉(E) and 〈[epsilon]₂〉(E), of ferroelectric ammonium sulfate crystals, (NH₄)₂SO₄, 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, 〈χ₁〉(T) and 〈χ₂〉(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 χ₂(T), together with a simultaneous decrease of the real part of the susceptibility χ₁(T), take place at the phase transition. These anomalies have been ascribed mainly to the SO₄ group of the crystal structure.