TEMPERATURE BEHAVIOR OF THE LIBRATIONAL M ODE OF NH4 THE TETRAHEDRON IN MIXED K-1-X(NH4)(X)H2PO4 CRYSTALS

The temperature dependences of the Raman spectra are analysed in the r egion of ammonium ion librational vibration for the mixed K-1-x(NH4)(x )H2PO4(KADP) crystals having various ammonium content (x = 0.22; 0.32; 0.53; 0.74; 0.82; 1.00). The behavior of the intensity, frequency and line halfwidth of the vibration suggests that due to the high-frequen cy reorientational processes the ammonium ions are regular tetrahedra like in shape at room temperature. As the temperature decreases, the f requency of these hopping processes also decreases slid at the tempera tures of about 120-110 K the ammonium tons become fixed in the lattice . This is true for ail KADP crystals Irrespective of the ammonium cont ent. Fixation of ammonium ions rakes place at the temperatures 30-50 K above that of proton subsystem ordering over the O - H...O bonds. The activation energies of reorientational processes have been calculated for the KADP crystals with a various ammonium concentration.