The point charge model is found to be not enough to explain the anomalous temperature dependence of the spontaneous polarization in the ammonium sulfate crystal on the basis of the calculations of the dipole moments of $NH_4^+$(I) $NH_4^+$(II) and $SO_4^{2-}$ as a function of temperature where the X-ray diffraction data of atomic positions are available.
To explore the reorientational ordering effects of $NH_4^+$(I) and $NH_4^+$(II) ions on the spontaneous polarization we have studied in detail the temperature dependence of the Roman bands associated with the relaxation dynamics of two nonequivalent ammonium ions-$NH_4^+$(I) and $NH_4^+$(II).
Temperature dependences of polarized Raman spectra were studied in $(NH_4)_2SO_4$ crystal in the temperature range of 300K to 40K.
The temperature dependence of the $NH_4^+$-traslational bands of Z(YY)X Raman spectrum in the region of 200$cm^{-1}$ seems to confirm that the peculiar temperature dependence of the spontaneous polarization in $(NH_4)_2SO_4$ crystal may be ascribed to the polaization contribution from the $NH_4^+$(I) and $NH_4^+$(II) dipoles of opposite directions with different reorientational relaxation dependence on temperature.
