Dielectric properties of (NH 4 ) 2 SO 4 crystals in the range of electronic excitations

Spectra of the real and imaginary parts of the pseudo‐dielectric permittivity, 〈ɛ 1 〉( E ) and 〈ɛ 2 〉( E ), of ferroelectric ammonium sulfate crystals, (NH 4 ) 2 SO 4 , 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, 〈χ 1 〉( T ) and 〈χ 2 〉( 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 χ 2 ( T ), together with a simultaneous decrease of the real part of the susceptibility χ 1 ( T ), take place at the phase transition. These anomalies have been ascribed mainly to the SO 4 group of the crystal structure.